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Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  

Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

IP Routing

The following ICND1 exam topics are covered in this chapter:
Routing Technologies

Interpret the components of routing table

Network mask

Next hop

Routing protocol code

Administrative distance


Gateway of last resort Describe how a routing table is populated by different routing information source Admin distance Compare and contrast static routing and dynamic routing Configure, verify, and troubleshoot IPv4 and IPv6 static routing
EIGRP Dynamic Routing or Summary Static RoutesDefault route

Network route
Host route
Floating stat
Configure, verify, and troubleshoot RIPv2 for IPv4 (excluding authentication, filtering, manual summarization, redistribution)
It’s time now to turn our focus toward the core topic of the ubiquitous IP routing process. It’s integral to networking because it pertains to all routers and configurations that use it, which is easily the lion’s share. IP routing is basically the process of moving packets from one network to another network using routers. And by routers, I mean Cisco routers, of course! However, the terms router and layer 3 device are interchangeable, and throughout this chapter when I use the term router, I am referring to any layer 3 device.
Before jumping into this chapter, I want to make sure you understand the difference between a routing protocol and a routed protocol. Routers use routing protocols to dynamically find all networks within the greater internetwork and to ensure that all routers have the same routing table. Routing protocols are also employed to determine the best path a packet should take through an internetwork to get to its destination most efficiently. RIP, RIPv2, EIGRP, and OSPF are great examples of the most common routing protocols. Once all routers know about all networks, a routed protocol can be used to send user data (packets) through the established enterprise. Routed protocols are assigned to an interface and determine the method of packet delivery. Examples of routed protocols are IP and IPv6. I’m pretty confident I don’t have to underscore how crucial it is for you to have this chapter’s material down to a near instinctive level. IP routing is innately what Cisco routers do, and they do it very well, so having a firm grasp of the fundamentals and basics of this topic is vital if you want to excel during the exam and in a real-world networking environment as well! In this chapter, I’m going to show you how to configure and verify IP routing with Cisco routers and guide you through these five key subjects:
Routing basics

The IP routing process Static routing Default routing Dynamic routing I want to start by nailing down the basics of how packets actually move through an internetwork, so let’s get started!

Routing Basics

Once you create an internetwork by connecting your WANs and LANs to a router, you’ll need to configure logical network addresses, like IP addresses, to all hosts on that internetwork for them to communicate successfully throughout it. The term routing refers to taking a packet from one device and sending it through the network to another device on a different network. Routers don’t really care about hosts—they only care about networks and the best path to each one of them. The logical network address of the destination host is key to getting packets through a routed network. It’s the hardware address of the host that’s used to deliver the packet from a router and ensure it arrives at the correct destination host. Routing is irrelevant if your network has no routers because their job is to route traffic to all the networks in your internetwork, but this is rarely the case! So here’s an important list of the minimum factors a router must know to be able to effectively route packets: Destination address Neighbor routers from which it can learn about remote networks Possible routes to all remote networks The best route to each remote network How to maintain and verify routing information The router learns about remote networks from neighboring routers or from an administrator. The router then builds a routing table, which is basically a map of the internetwork, and it describes how to find remote networks. If a network is directly connected, then the router already knows how to get to it. But if a network isn’t directly connected to the router, the router must use one of two ways to learn how to get to the remote network. The static routing method requires someone to hand-type all network locations into the routing table, which can be a pretty daunting task when used on all but the smallest of networks! Conversely, when dynamic routing is used, a protocol on one router communicates with the same protocol running on neighboring routers. The routers then update each other about all the networks they know about and place this information into the routing table. If a change occurs in the network, the dynamic routing protocols automatically inform all routers about the event. If static routing is used, the administrator is responsible for updating all changes by hand onto all routers. Most people usually use a combination of dynamic and static routing to administer a large network. Before we jump into the IP routing process, let’s take a look at a very simple example that demonstrates how a router uses the routing table to route packets out of an interface. We’ll be going into a more detailed study of the process soon, but I want to show you something called the “longest match rule” first. With it, IP will scan a routing table to find the longest match as compared to the destination address of a packet. Let’s take a look at Figure 9.1 to get a picture of this process.

 A simple routing example
a simple network. Lab_A has four interfaces. Can you see which interface will be used to forward an IP datagram to a host with a destination IP address of
By using the command show ip route on a router, we can see the routing table (map of the internetwork) that Lab_A has used to make its forwarding decisions:
Lab_A#sh ip route
Codes: L - local, C - connected, S - static, [output cut] is variably subnetted, 6 subnets, 4 masks C is directly connected, FastEthernet0/3
L           is directly connected, FastEthernet0/3 C is directly connected, FastEthernet0/2
L           is directly connected, FastEthernet0/2 C is directly connected, FastEthernet0/1 L is directly connected, FastEthernet0/1 S* is directly connected, FastEthernet0/0 The C in the routing table output means that the networks listed are “directly connected,” and until we add a routing protocol like RIPv2, OSPF, etc. to the routers in our internetwork, or enter static routes, only directly connected networks will show up in our routing table. But wait— what about that L in the routing table—that’s new, isn’t it? Yes it is, because in the new Cisco IOS 15 code, Cisco defines a different route, called a local host route. Each local route has a /32 prefix, defining a route just for the one address. So in this example, the router has relied upon these routes that list their own local IP addresses to more efficiently forward packets to the router itself.
So let’s get back to the original question: By looking at the figure and the output of the routing table, can you determine what IP will do with a received packet that has a destination IP address of The answer is that the router will packet-switch the packet to interface FastEthernet 0/1, which will frame the packet and then send it out on the network segment. This is referred to as frame rewrite. Based upon the longest match rule, IP would look for, and if that isn’t found in the table, then IP would search for, then, and so on until a route is discovered.
Here’s another example: Based on the output of the next routing table, which interface will a packet with a destination address of be forwarded from?
Lab_A#sh ip route
[output cut]
Gateway of last resort is not set
C        is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 C is directly connected, FastEthernet0/1 L is directly connected, FastEthernet0/1 C is directly connected, FastEthernet0/2 L is directly connected, FastEthernet0/2 C is directly connected, Serial 0/0
L        is directly connected, Serial0/0

To figure this out, look closely at the output until you see that the network is subnetted and each interface has a different mask. And I have to tell you—you just can’t answer this question if you can’t subnet! would be a host in the subnet that’s connected to the FastEthernet0/1 interface. Don’t freak if you’re struggling and don’t get this! Instead, just go back and reread Chapter 4, “Easy Subnetting,” until it becomes clear to you.

The IP Routing Process

The IP routing process is fairly simple and doesn’t change, regardless of the size of your network. For a good example of this fact, I’ll use Figure
9.2 to describe step-by-step what happens when Host A wants to communicate with Host B on a different network.

 IP routing example using two hosts and one router
In Figure 9.2 a user on Host_A pinged Host_B’s IP address. Routing doesn’t get any simpler than this, but it still involves a lot of steps, so let’s work through them now:
1.  Internet Control Message Protocol (ICMP) creates an echo request payload, which is simply the alphabet in the data field.
2. ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a minimum, this packet contains an IP source address, an IP destination address, and a Protocol field with 01h. Don’t forget that Cisco likes to use 0x in front of hex characters, so this could also look like 0x01. This tells the receiving host to whom it should hand the payload when the destination is reached—in this example, ICMP.
3. Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.
4. Since IP has determined that this is a remote request, the packet must be sent to the default gateway so it can be routed to the remote
5 network. The Registry in Windows is parsed to find the configured default gateway.           
The default gateway of Host_A is configured to For this packet to be sent to the default gateway, the hardware address of the router’s interface Ethernet 0, which is configured with the IP address of, must be known. Why? So the packet can be handed down to the Data Link layer, framed, and sent to the router’s interface that’s connected to the network. Because hosts communicate only via hardware addresses on the local LAN, it’s important to recognize that for Host_A to communicate to Host_B, it has to send packets to the Media Access Control (MAC) address of the default gateway on the local network.
6. Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the IP address of the default gateway has already been resolved to a hardware address. If it has, the packet is then free to be handed to the Data Link layer for framing. Remember that the hardware destination address is also handed down with that packet. To view the ARP cache on your host, use the following command:
C:\>arp -a
Interface: --- 0x3
Internet Address                           Physical Address                                                             Type                                                          00-15-05-06-31-b0                                                     dynamic
If the hardware address isn’t already in the ARP cache of the host, an ARP broadcast will be sent out onto the local network to search for the hardware address. The router then responds to the request and provides the hardware address of Ethernet 0, and the host caches this address.
7. Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used, which is Ethernet in this case. A frame is then generated, encapsulating the packet with control information.
Within that frame are the hardware destination and source addresses plus, in this case, an Ether-Type field, which identifies the specific Network layer protocol that handed the packet to the Data Link layer. In this instance, it’s IP. At the end of the frame is something called a Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC). The frame would look something like what I’ve detailed in Figure 9.3. It contains Host A’s hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host’s MAC address—remember that!
FIGURE 9.3 Frame used from Host A to the Lab_A router when Host B is pinged
8. Once the frame is completed, it’s handed down to the Physical layer to be put on the physical medium (in this example, twisted-pair wire) one bit at a time.
9. Every device in the collision domain receives these bits and builds the frame. They each run a CRC and check the answer in the FCS field. If the answers don’t match, the frame is discarded.
If the CRC matches, then the hardware destination address is checked to see if it matches (which, in this example, is the router’s interface Ethernet 0).
If it’s a match, then the Ether-Type field is checked to find the protocol used at the Network layer.
10. The packet is pulled from the frame, and what is left of the frame is discarded. The packet is handed to the protocol listed in the Ether- Type field—it’s given to IP.
11.  IP receives the packet and checks the IP destination address. Since the packet’s destination address doesn’t match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table.
12. The routing table must have an entry for the network or the packet will be discarded immediately and an ICMP message will be sent back to the originating device with a destination network unreachable message.
3. If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1. The following output displays the Lab_A router’s routing table. The C means “directly connected.” No routing protocols are needed in this network since all networks (all two of them) are directly connected.
Lab_A>sh ip route
C is directly connected,    Ethernet0 L is directly connected, Ethernet0 C is directly connected,    Ethernet1 L is directly connected, Ethernet1
4. The router packet-switches the packet to the Ethernet 1 buffer.
5.  The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache.
If the hardware address of Host_B has already been resolved and is in the router’s ARP cache, then the packet and the hardware address will be handed down to the Data Link layer to be framed. Let’s take a look at the ARP cache on the Lab_A router by using the show ip arp command:
Lab_A#sh ip arp
Protocol           Address                 Age(min) Hardware Addr
                           Type Interface
Internet                   -               00d0.58ad.05f4                            ARPA
Internet                   3              0030.9492.a5dd                            ARPA
Internet                    -                            00d0.58ad.06aa     ARPA Ethernet0
Internet                 12               0030.9492.a4ac                            ARPA
The dash (-) signifies that this is the physical interface on the router. This output shows us that the router knows the (Host_A) and (Host_B) hardware addresses. Cisco routers will keep an entry in the ARP table for 4 hours.
Now if the hardware address hasn’t already been resolved, the router will send an ARP request out E1 looking for the hardware address. Host_B responds with its hardware address, and the packet and destination hardware addresses are then both sent to the Data Link layer for framing.
6. The Data Link layer creates a frame with the destination and source hardware addresses, Ether-Type field, and FCS field at the end. The frame is then handed to the Physical layer to be sent out on the physical medium one bit at a time.
7. Host_B receives the frame and immediately runs a CRC. If the result matches the information in the FCS field, the hardware destination address will then be checked next. If the host finds a match, the Ether- Type field is then checked to determine the protocol that the packet should be handed to at the Network layer—IP in this example.
8. At the Network layer, IP receives the packet and runs a CRC on the IP header. If that passes, IP then checks the destination address. Since a match has finally been made, the Protocol field is checked to find out to whom the payload should be given.
9. The payload is handed to ICMP, which understands that this is an echo request. ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply.
0.  A packet is then created including the source and destination addresses, Protocol field, and payload. The destination device is now Host_A.
1.  IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network. Since the destination device is on a remote network, the packet needs to be sent to the default gateway.
2. The default gateway IP address is found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address.
3.         Once the hardware address of the default gateway is found, the packet and destination hardware addresses are handed down to the Data Link layer for framing.
4.         The Data Link layer frames the packet of information and includes the following in the header:
The destination and source hardware addresses
The Ether-Type field with 0x0800 (IP) in it The FCS field with the CRC result in tow
5.         The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time.
6.         The router’s Ethernet 1 interface receives the bits and builds a frame. The CRC is run, and the FCS field is checked to make sure the answers match.
7.         Once the CRC is found to be okay, the hardware destination address is checked. Since the router’s interface is a match, the packet is pulled from the frame and the Ether-Type field is checked to determine which protocol the packet should be delivered to at the Network layer. 
The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address.
Since the IP destination address doesn’t match any of the router’s interfaces, the routing table is checked to see whether it has a route to If it doesn’t have a route over to the destination network, the packet will be discarded immediately. I want to take a minute to point out that this is exactly where the source of confusion begins for a lot of administrators because when a ping fails, most people think the packet never reached the destination host. But as we see here, that’s not always the case. All it takes for this to happen is for even just one of the remote routers to lack a route back to the originating host’s network and— poof!—the packet is dropped on the return trip, not on its way to the host!
9.         In this case, the router happens to know how to get to network—the exit interface is Ethernet 0—so the packet is switched to interface Ethernet 0.
0.         The router then checks the ARP cache to determine whether the hardware address for has already been resolved.
1.         Since the hardware address to is already cached from the originating trip to Host_B, the hardware address and packet are then handed to the Data Link layer.
2.         The Data Link layer builds a frame with the destination hardware address and source hardware address and then puts IP in the Ether- Type field. A CRC is run on the frame and the result is placed in the FCS field.
3.         The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
4.         The destination host receives the frame, runs a CRC, checks the destination hardware address, then looks into the Ether-Type field to find out to whom to hand the packet.
5.         IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the Protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.
6.         ICMP acknowledges that it has received the reply by sending an exclamation point (!) to the user interface. ICMP then attempts to send four more echo requests to the destination host.
You’ve just experienced Todd’s 36 easy steps to understanding IP routing. The key point here is that if you had a much larger network, the process would be the same. It’s just that the larger the internetwork, the more hops the packet goes through before it finds the destination host.
It’s super-important to remember that when Host_A sends a packet to Host_B, the destination hardware address used is the default gateway’s Ethernet interface. Why? Because frames can’t be placed on remote networks—only local networks. So packets destined for remote networks must go through the default gateway.
Let’s take a look at Host_A’s ARP cache now:
C:\ >arp -a
Interface: --- 0x3
Internet Address
Physical Address
Did you notice that the hardware (MAC) address that Host_A uses to get to Host_B is the Lab_A E0 interface? Hardware addresses are always local, and they never pass through a router’s interface. Understanding this process is as important as air to you, so carve this into your memory!

The Cisco Router Internal Process

One more thing before we get to testing your understanding of my 36 steps of IP routing. I think it’s important to explain how a router forwards packets internally. For IP to look up a destination address in a routing table on a router, processing in the router must take place, and if there are tens of thousands of routes in that table, the amount of CPU time would be enormous. It results in a potentially overwhelming amount of overhead—think about a router at your ISP that has to calculate millions of packets per second and even subnet to find the correct exit interface! Even with the little network I’m using in this book, lots of processing would need to be done if there were actual hosts connected and sending data. Cisco uses three types of packet-forwarding techniques. Process switching This is actually how many people see routers to this day, because it’s true that routers actually did perform this type of bare- bones packet switching back in 1990 when Cisco released their very first router. But those days when traffic demands were unimaginably light are long gone—not in today’s networks! This process is now extremely complex and involves looking up every destination in the routing table and finding the exit interface for every packet. This is pretty much how I just explained the process in my 36 steps. But even though what I wrote was absolutely true in concept, the internal process requires much more than packet-switching technology today because of the millions of packets per second that must now be processed. So Cisco came up with some other technologies to help with the “big process problem.”

Fast switching This solution was created to make the slow performance of process switching faster and more efficient. Fast switching uses a cache to store the most recently used destinations so that look ups are not required for every packet. By caching the exit interface of the destination device, as well as the layer 2 header, performance was dramatically improved, but as our networks evolved with the need for even more speed, Cisco created yet another technology! Cisco Express Forwarding (CEF) This is Cisco’s newer creation, and it’s the default packet-forwarding method used on all the latest Cisco routers. CEF makes many different cache tables to help improve performance and is change triggered, not packet triggered. Translated, this means that when the network topology changes, the cache changes along with it.

Testing Your IP Routing Understanding

Since understanding IP routing is super-important, it’s time for that little test I talked about earlier on how well you’ve got the IP routing process down so far. I’m going to do that by having you look at a couple of figures and answer some very basic IP routing questions based upon them. LAN connected to RouterA that’s connected via a WAN link to RouterB. RouterB has a LAN connected with an HTTP server attached.

 IP routing example 1

The critical information you want to obtain by looking at this figure is exactly how IP routing will occur in this example. Let’s determine the characteristics of a frame as it leaves HostA. Okay—we’ll cheat a bit. I’ll give you the answer, but then you should go back over the figure and see if you can answer example 2 without looking at my three-step answer!

1.            The destination address of a frame from HostA would be the MAC address of Router A’s Fa0/0 interface.

2.            The destination address of a packet would be the IP address of the HTTP server’s network interface card (NIC).

3.            The destination port number in the segment header would be 80. That was a pretty simple, straightforward scenario. One thing to remember is that when multiple hosts are communicating to a server using HTTP, they must all use a different source port number. The source and destination IP addresses and port numbers are how the server keeps the data separated at the Transport layer.

Let’s complicate matters by adding another device into the network and then see if you can find the answers. Figure 9.5 shows a network with only one router but two switches.

 IP routing example 2
The key thing to understand about the IP routing process in this scenario is what happens when HostA sends data to the HTTPS server? Here’s your answer:
1.            The destination address of a frame from HostA would be the MAC address of RouterA’s Fa0/0 interface.
2.            The destination address of a packet is the IP address of the HTTPS server’s network interface card (NIC).
3.            The destination port number in the segment header will have a value of 443. Did you notice that the switches weren’t used as either a default gateway or any other destination? That’s because switches have nothing to do with routing. I wonder how many of you chose the switch as the default gateway (destination) MAC address for HostA? If you did, don’t feel bad
—just take another look to see where you went wrong and why. It’s very important to remember that the destination MAC address will always be the router’s interface—if your packets are destined for outside the LAN, as they were in these last two examples! Before moving on into some of the more advanced aspects of IP routing, let’s look at another issue. Take a look at the output of this router’s routing table:
Corp#sh ip route
[output cut]
R  [120/2] via, 00:00:23, Serial0/0
R  [120/1] via, 00:00:23, Serial0/0
R  [120/1] via, 00:00:23, Serial0/0
C  is directly connected, Serial0/0
L  is directly connected, Serial0/0
C  is directly connected, FastEthernet0/0
L  is directly connected, FastEthernet0/0

What do we see here? If I were to tell you that the corporate router received an IP packet with a source IP address of and a destination address of, what do you think the Corp router will do with this packet?
If you said, “The packet came in on the FastEthernet 0/0 interface, but because the routing table doesn’t show a route to network (or a default route), the router will discard the packet and send an ICMP destination unreachable message back out to interface FastEthernet 0/0,” you’re a genius! The reason that’s the correct answer is because that’s the source LAN where the packet originated from.
Now, let’s check out the next figure and talk about the frames and packets in detail. We’re not really going over anything new here; I’m just making sure you totally, completely, thoroughly, fully understand basic IP routing! It is the crux of this book, and the topic the exam objectives are geared toward. It’s all about IP routing, which means you need to be all over this stuff! We’ll use Figure 9.6 for the next few scenarios.

Basic IP routing using MAC and IP addresses
Referring to Figure 9.6, here’s a list of all the answers to questions you need inscribed in your brain:
1.            In order to begin communicating with the Sales server, Host 4 sends out an ARP request. How will the devices exhibited in the topology respond to this request?
2.            Host 4 has received an ARP reply. Host 4 will now build a packet, then place this packet in the frame. What information will be placed in the header of the packet that leaves Host 4 if Host 4 is going to communicate to the Sales server?
3.            The Lab_A router has received the packet and will send it out Fa0/0 onto the LAN toward the server. What will the frame have in the header as the source and destination addresses?
4.            Host 4 is displaying two web documents from the Sales server in two browser windows at the same time. How did the data find its way to the correct browser windows?
The following should probably be written in a teensy font and put upside down in another part of the book so it would be really hard for you to cheat and peek, but since I’m not that mean and you really need to have this down, here are your answers in the same order that the scenarios were just presented:
1.            In order to begin communicating with the server, Host 4 sends out an ARP request. How will the devices exhibited in the topology respond to this request? Since MAC addresses must stay on the local network, the Lab_B router will respond with the MAC address of the Fa0/0 interface and Host 4 will send all frames to the MAC address of the Lab_B Fa0/0 interface when sending packets to the Sales server.
2.            Host 4 has received an ARP reply. Host 4 will now build a packet, then place this packet in the frame. What information will be placed in the header of the packet that leaves Host 4 if Host 4 is going to communicate to the Sales server? Since we’re now talking about packets, not frames, the source address will be the IP address of Host 4 and the destination address will be the IP address of the Sales server.
3.            Finally, the Lab_A router has received the packet and will send it out Fa0/0 onto the LAN toward the server. What will the frame have in the header as the source and destination addresses? The source MAC address will be the Lab_A router’s Fa0/0 interface, and the destination MAC address will be the Sales server’s MAC address because all MAC addresses must be local on the LAN.
4.            Host 4 is displaying two web documents from the Sales server in two different browser windows at the same time. How did the data find its way to the correct browser windows? TCP port numbers are used to direct the data to the correct application window.
Great! But we’re not quite done yet. I’ve got a few more questions for you before you actually get to configure routing in a real network. Ready?
Figure 9.7 shows a basic network, and Host 4 needs to get email. Which address will be placed in the destination address field of the frame when it leaves Host 4?

 Testing basic routing knowledge
The answer is that Host 4 will use the destination MAC address of the Fa0/0 interface on the Lab_B router—you knew that, right? Look at Figure 9.7 again: What if Host 4 needs to communicate with Host 1—not the server, but with Host 1. Which OSI layer 3 source address will be found in the packet header when it reaches Host 1?
Hopefully you’ve got this: At layer 3, the source IP address will be Host 4 and the destination address in the packet will be the IP address of Host 1. Of course, the destination MAC address from Host 4 will always be the Fa0/0 address of the Lab_B router, right? And since we have more than one router, we’ll need a routing protocol that communicates between both of them so that traffic can be forwarded in the right direction to reach the network that Host 1 is connected to. Okay—one more scenario and you’re on your way to being an IP routing machine! Again, using Figure 9.7, Host 4 is transferring a file to the email server connected to the Lab_A router. What would be the layer 2 destination address leaving Host 4? Yes, I’ve asked this question more than once. But not this one: What will be the source MAC address when the frame is received at the email server?
Hopefully, you answered that the layer 2 destination address leaving Host 4 is the MAC address of the Fa0/0 interface on the Lab_B router and that the source layer 2 address that the email server will receive is the Fa0/0 interface of the Lab_A router. If you did, you’re ready to discover how IP routing is handled in a larger network environment!

Configuring IP Routing
It’s time to get serious and configure a real network. Figure 9.8 shows three routers: Corp, SF, and LA. Remember that, by default, these routers only know about networks that are directly connected to them. I’ll continue to use this figure and network throughout the rest of the chapters in this book. As I progress through this book, I’ll add more routers and switches as needed.

 Configuring IP routing
As you might guess, I’ve got quite a nice collection of routers for us to play with. But you don’t need a closet full of devices to perform most, if not all, of the commands we’ll use in this book. You can get by nicely with pretty much any router or even with a good router simulator.
Getting back to business, the Corp router has two serial interfaces, which will provide a WAN connection to the SF and LA router and two Fast Ethernet interfaces as well. The two remote routers have two serial interfaces and two Fast Ethernet interfaces.
The first step for this project is to correctly configure each router with an IP address on each interface. The following list shows the IP address scheme I’m going to use to configure the network. After we go over how the network is configured, I’ll cover how to configure IP routing. Pay attention to the subnet masks—they’re important! The LANs all use a /24 mask, but the WANs are using a /30.
Serial 0/0: Serial 0/1: Fa0/0:
S0/0/0: Fa0/0:


S0/0/0: Fa0/0:

The router configuration is really a pretty straightforward process since you just need to add IP addresses to your interfaces and then perform a no shutdown on those same interfaces. It gets a tad more complex later on, but for right now, let’s configure the IP addresses in the network.

Corp Configuration

We need to configure three interfaces to configure the Corp router. And configuring the hostnames of each router will make identification much easier. While we’re at it, let’s set the interface descriptions, banner, and router passwords too because it’s a really good idea to make a habit of configuring these commands on every router! To get started, I performed an erase startup-config on the router and reloaded, so we’ll start in setup mode. I chose no when prompted to enter setup mode, which will get us straight to the username prompt of the console. I’m going to configure all my routers this same way. Here’s how what I just did looks:
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]:


Press RETURN to get started! Router>en
Router#config t Router(config)#hostname Corp Corp(config)#enable secret GlobalNet Corp(config)#no ip domain-lookup Corp(config)#int f0/0
Corp(config-if)#desc Connection to LAN BackBone Corp(config-if)#ip address Corp(config-if)#no shut
Corp(config-if)#int s0/0
Corp(config-if)#desc WAN connection to SF
Corp(config-if)#ip address
Corp(config-if)#no shut
Corp(config-if)#int s0/1
Corp(config-if)#desc WAN connection to LA
Corp(config-if)#ip address
Corp(config-if)#no shut Corp(config-if)#line con 0 Corp(config-line)#password console Corp(config-line)#logging Corp(config-line)#logging sync Corp(config-line)#exit Corp(config)#line vty 0 ?
<1-181>          Last Line number
Corp(config)#line vty 0 181 Corp(config-line)#password telnet Corp(config-line)#login Corp(config-line)#exit
Corp(config)#banner motd # This is my Corp Router #
Corp(config)#^Z Corp#copy run start
Destination filename [startup-config]? Building configuration...
Corp# [OK]

Let’s talk about the configuration of the Corp router. First, I set the hostname and enable secret, but what is that no ip domain-lookup command? That command stops the router from trying to resolve hostnames, which is an annoying feature unless you’ve configured a host table or DNS. Next, I configured the three interfaces with descriptions and IP addresses and enabled them with the no shutdown command. The console and VTY passwords came next, but what is that logging sync command under the console line? The logging synchronous command stops console messages from writing over what you are typing in, meaning it’s a sanity-saving command that you’ll come to love! Last, I set my banner and then saved my configs.

To view the IP routing tables created on a Cisco router, use the command
show ip route. Here’s the command’s output:
Corp#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override Gateway of last resort is not set is subnetted, 1 subnets
C          is directly connected, FastEthernet0/0
L           is directly connected, FastEthernet0/0 Corp#

It’s important to remember that only configured, directly connected networks are going to show up in the routing table. So why is it that only the FastEthernet 0/0 interface shows up in the table? No worries—that’s just because you won’t see the serial interfaces come up until the other side of the links are operational. As soon as we configure our SF and LA routers, those interfaces should pop right up!
But did you notice the C on the left side of the output of the routing table? When you see that there, it means that the network is directly connected. The codes for each type of connection are listed at the top of the show ip route command, along with their descriptions.

SF Configuration
Now we’re ready to configure the next router—SF. To make that happen correctly, keep in mind that we have two interfaces to deal with: Serial 0/0/0 and FastEthernet 0/0. So let’s make sure we don’t forget to add the hostname, passwords, interface descriptions, and banners to the router configuration. As I did with the Corp router, I erased the configuration and reloaded since this router had already been configured before.
Here’s the configuration I used:
R1#erase start
% Incomplete command. R1#erase startup-config
Erasing the nvram filesystem will remove all configuration files!
Continue? [confirm][enter] [OK]
Erase of nvram: complete R1#reload
Proceed with reload? [confirm][enter] [output cut]
%Error opening tftp:// (Timed out)
%Error opening tftp:// (Timed out)

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

Before we move on, let’s talk about this output for a second. First, notice that beginning with IOS 12.4, ISR routers will no longer take the command erase start. The router has only one command after erase that starts with s, as shown here:
Router#erase s?
I know, you’d think that the IOS would continue to accept the command, but nope—sorry! The second thing I want to point out is that the output tells us the router is looking for a TFTP host to see if it can download a configuration. When that fails, it goes straight into setup mode. This gives you a great picture of the Cisco router default boot sequence we talked about in Chapter 7, “Managing a Cisco Internetwork.”

Let’s get back to configuring our router:
Press RETURN to get started! Router#config t Router(config)#hostname SF SF(config)#enable secret GlobalNet SF(config)#no ip domain-lookup SF(config)#int s0/0/0
SF(config-if)#desc WAN Connection to Corp
SF(config-if)#ip address
SF(config-if)#no shut
SF(config-if)#clock rate 1000000 SF(config-if)#int f0/0 SF(config-if)#desc SF LAN
SF(config-if)#ip address
SF(config-if)#no shut
SF(config-if)#line con 0 SF(config-line)#password console SF(config-line)#login
SF(config-line)#logging sync SF(config-line)#exit SF(config)#line vty 0 ?
<1-1180>          Last Line number
SF(config)#line vty 0 1180 SF(config-line)#password telnet SF(config-line)#login
SF(config-line)#banner motd #This is the SF Branch router#
SF(config)#exit SF#copy run start
Destination filename [startup-config]? Building configuration...
Let’s take a look at our configuration of the interfaces with the following two commands:
SF#sh run | begin int
interface FastEthernet0/0 description SF LAN
ip address
duplex auto speed auto

interface FastEthernet0/1 no ip address
shutdown duplex auto speed auto
interface Serial0/0/0
description WAN Connection to Corp
ip address
clock rate 1000000
SF#sh ip int brief

down down


down down


Now that both ends of the serial link are configured, the link comes up. Remember, the up/up status for the interfaces are Physical/Data Link layer status indicators that don’t reflect the layer 3 status! I ask students in my classes, “If the link shows up/up, can you ping the directly connected network?” And they say, “Yes!” The correct answer is, “I don’t know,” because we can’t see the layer 3 status with this command. We only see layers 1 and 2 and verify that the IP addresses don’t have a typo. This is really important to understand!
The show ip route command for the SF router reveals the following:
SF#sh ip route
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 is subnetted, 1 subnets
C          is directly connected, Serial0/0/0
L           is directly connected, Serial0/0/0

Notice that router SF knows how to get to networks and; we can now ping to the Corp router from SF:

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Now let’s head back to the Corp router and check out the routing table:
Corp>sh ip route is subnetted, 1 subnets
C          is directly connected, Serial0/0
L           is directly connected, Serial0/0 is subnetted, 1 subnets
C          is directly connected, FastEthernet0/0
L           is directly connected, FastEthernet0/0

On the SF router’s serial interface 0/0/0 is a DCE connection, which means a clock rate needs to be set on the interface. Remember that you don’t need to use the clock rate command in production. While true, it’s still imperative that you know how/when you can use it and that you understand it really well when studying for your CCNA exam!
We can see our clocking with the show controllers command:
SF#sh controllers s0/0/0 Interface Serial0/0/0 Hardware is GT96K
DCE V.35, clock rate 1000000
Corp>sh controllers s0/0
Interface Serial0/0
Hardware is PowerQUICC MPC860
DTE V.35 TX and RX clocks detected.

Since the SF router has a DCE cable connection, I needed to add clock rate to this interface because DTE receives clock. Keep in mind that the new ISR routers will autodetect this and set the clock rate to 2000000. And you still need to make sure you’re able to find an interface that is DCE and set clocking to meet the objectives.
Since the serial links are showing up, we can now see both networks in the Corp routing table. And once we configure LA, we’ll see one more network in the routing table of the Corp router. The Corp router can’t see the network because we don’t have any routing configured yet—routers see only directly connected networks by default.

LA Configuration

To configure LA, we’re going to do pretty much the same thing we did with the other two routers. There are two interfaces to deal with, Serial 0/0/1 and FastEthernet 0/0, and again, we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the router configuration:
Router(config)#hostname LA LA(config)#enable secret GlobalNet LA(config)#no ip domain-lookup LA(config)#int s0/0/1
LA(config-if)#ip address
LA(config-if)#no shut
LA(config-if)#clock rate 1000000 LA(config-if)#description WAN To Corporate LA(config-if)#int f0/0
LA(config-if)#ip address
LA(config-if)#no shut
LA(config-if)#description LA LAN LA(config-if)#line con 0 LA(config-line)#password console LA(config-line)#login
LA(config-line)#logging sync LA(config-line)#exit LA(config)#line vty 0 ?
<1-1180>          Last Line number
LA(config)#line vty 0 1180 LA(config-line)#password telnet LA(config-line)#login LA(config-line)#exit
LA(config)#banner motd #This is my LA Router#
LA(config)#exit LA#copy run start
Destination filename [startup-config]? Building configuration...
Nice—everything was pretty straightforward. The following output, which I gained via the show ip route command, displays the directly connected networks of and
LA#sh ip route is subnetted, 1 subnets

C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1  C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0
So now that we’ve configured all three routers with IP addresses and administrative functions, we can move on to deal with routing. But I want to do one more thing on the SF and LA routers—since this is a very small network, let’s build a DHCP server on the Corp router for each LAN.

Configuring DHCP on Our Corp Router

While it’s true that I could approach this task by going to each remote router and creating a pool, why bother with all that when I can easily create two pools on the Corp router and have the remote routers forward requests to the Corp router? Of course, you remember how to do this from Chapter 7!
Let’s give it a shot:
Corp#config t
Corp(config)#ip dhcp excluded-address Corp(config)#ip dhcp excluded-address Corp(config)#ip dhcp pool SF_LAN
Corp(dhcp-config)#dns-server Corp(dhcp-config)#exit Corp(config)#ip dhcp pool LA_LAN
Corp(dhcp-config)#dns-server Corp(dhcp-config)#exit Corp(config)#exit
Corp#copy run start
Destination filename [startup-config]? Building configuration...
Creating DHCP pools on a router is actually a simple process, and you would go about the configuration the same way on any router you wish to add a DHCP pool to. To designate a router as a DHCP server, you just create the pool name, add the network/subnet and the default gateway, and then exclude any addresses that you don’t want handed out. You definitely want to make sure you’ve excluded the default gateway address, and you’d usually add a DNS server as well. I always add any exclusions first, and remember that you can conveniently exclude a range of addresses on a single line. Soon, I’ll demonstrate those verification commands I promised I’d show you back in Chapter 7, but first, we need to figure out why the Corp router still can’t get to the remote networks by default!
Now I’m pretty sure I configured DHCP correctly, but I just have this nagging feeling I forgot something important. What could that be? Well, the hosts are remote across a router, so what would I need to do that would allow them to get an address from a DHCP server? If you concluded that I’ve got to configure the SF and LA F0/0 interfaces to forward the DHCP client requests to the server, you got it!
Here’s how we’d go about doing that:
LA#config t
LA(config)#int f0/0
LA(config-if)#ip helper-address

SF#config t
SF(config)#int f0/0
SF(config-if)#ip helper-address

I’m pretty sure I did this correctly, but we won’t know until I have some type of routing configured and working. So let’s get to that next!

Configuring IP Routing in Our Network

So is our network really good to go? After all, I’ve configured it with IP addressing, administrative functions, and even clocking that will automatically occur with the ISR routers. But how will our routers send packets to remote networks when they get their destination information by looking into their tables that only include directions about directly connected networks? And you know routers promptly discard packets they receive with addresses for networks that aren’t listed in their routing table!
So we’re not exactly ready to rock after all. But we will be soon because there are several ways to configure the routing tables to include all the networks in our little internetwork so that packets will be properly forwarded. As usual, one size fits all rarely fits at all, and what’s best for one network isn’t necessarily what’s best for another. That’s why understanding the different types of routing will be really helpful when choosing the best solution for your specific environment and business requirements.
These are the three routing methods I’m going to cover with you: Static routing

Default routing Dynamic routing

We’re going to start with the first way and implement static routing on our network, because if you can implement static routing and make it work, you’ve demonstrated that you definitely have a solid understanding of the internetwork. So let’s get started.

Static Routing

Static routing is the process that ensues when you manually add routes in each router’s routing table. Predictably, there are pros and cons to static routing, but that’s true for all routing approaches.
Here are the pros:
There is no overhead on the router CPU, which means you could probably make do with a cheaper router than you would need for dynamic routing.
There is no bandwidth usage between routers, saving you money on WAN links as well as minimizing overhead on the router since you’re not using a routing protocol. It adds security because you, the administrator, can be very exclusive and choose to allow routing access to certain networks only. And here are the cons:

Whoever the administrator is must have a vault-tight knowledge of the internetwork and how each router is connected in order to configure routes correctly. If you don’t have a good, accurate map of your internetwork, things will get very messy quickly! If you add a network to the internetwork, you have to tediously add a route to it on all routers by hand, which only gets increasingly insane as the network grows.
Due to the last point, it’s just not feasible to use it in most large networks because maintaining it would be a full-time job in itself.
But that list of cons doesn’t mean you get to skip learning all about it mainly because of that first disadvantage I listed—the fact that you must have such a solid understanding of a network to configure it properly and that your administrative knowledge has to practically verge on the supernatural! So let’s dive in and develop those skills. Starting at the beginning, here’s the command syntax you use to add a static route to a routing table from global config: ip route [destination_network] [mask] [next-hop_address or exitinterface] [administrative_distance] [permanent]
This list describes each command in the string:
ip route The command used to create the static route. destination_network The network you’re placing in the routing table. mask The subnet mask being used on the network. next-hop_address This is the IP address of the next-hop router that will receive packets and forward them to the remote network, which must signify a router interface that’s on a directly connected network. You must be able to successfully ping the router interface before you can add the route. Important note to self is that if you type in the wrong next-hop address or the interface to the correct router is down, the static route will show up in the router’s configuration but not in the routing table. exitinterface Used in place of the next-hop address if you want, and shows up as a directly connected route. administrative_distance By default, static routes have an administrative distance of 1 or 0 if you use an exit interface instead of a next-hop address. You can change the default value by adding an administrative weight at the end of the command. I’ll talk a lot more about this later in the chapter when we get to the section on dynamic routing. permanent If the interface is shut down or the router can’t communicate to the next-hop router, the route will automatically be discarded from the routing table by default. Choosing the permanent option keeps the entry in the routing table no matter what happens.
Before I guide you through configuring static routes, let’s take a look at a sample static route to see what we can find out about it:
Router(config)#ip route

The ip route command tells us simply that it’s a static route. is the remote network we want to send packets to. is the mask of the remote network. is the next hop, or router, that packets will be sent to.
But what if the static route looked like this instead?
Router(config)#ip route 150
That 150 at the end changes the default administrative distance (AD) of 1 to 150. As I said, I’ll talk much more about AD when we get into dynamic routing, but for now, just remember that the AD is the trustworthiness of a route, where 0 is best and 255 is worst.
One more example, then we’ll start configuring:
Router(config)#ip route s0/0/0
Instead of using a next-hop address, we can use an exit interface that will make the route show up as a directly connected network. Functionally, the next hop and exit interface work exactly the same.
To help you understand how static routes work, I’ll demonstrate the configuration on the internetwork shown previously in Figure 9.8. Here it is again in Figure 9.9 to save you the trouble of having to go back and forth to view the same figure.

FIGURE 9.9 Our internetwork


Each routing table automatically includes directly connected networks. To be able to route to all indirectly connected networks within the internetwork, the routing table must include information that describes where these other networks are located and how to get to them.
The Corp router is connected to three networks. For the Corp router to be able to route to all networks, the following networks have to be configured into its routing table:

The following router output shows the static routes on the Corp router and the routing table after the configuration. For the Corp router to find the remote networks, I had to place an entry into the routing table describing the remote network, the remote mask, and where to send the packets. I am going to add a 150 at the end of each line to raise the administrative distance. You’ll see why soon when we get to dynamic routing. Many times this is also referred to as a floating static route because the static route has a higher administrative distance than any routing protocol and will only be used if the routes found with the routing protocols go down. Here’s the output:
Corp#config t
Corp(config)#ip route 150
Corp(config)#ip route s0/1 150
Corp(config)#do show run | begin ip route
ip route 150
ip route Serial0/1 150

I needed to use different paths for networks and, so I used a next-hop address for the SF router and an exit interface for the LA router. After the router has been configured, you can just type show ip route to see the static routes:
Corp(config)#do show ip route
S  [150/0] via is subnetted, 2 subnets
C          is directly connected, Serial0/1
L           is directly connected, Serial0/1 C is directly connected, Serial0/0
L           is directly connected, Serial0/0 S is directly connected, Serial0/1 is subnetted, 1 subnets
C          is directly connected, FastEthernet0/0
L           is directly connected, FastEthernet0/0

The Corp router is configured to route and know all routes to all networks. But can you see a difference in the routing table for the routes to SF and LA? That’s right! The next-hop configuration showed up as via, and the route configured with an exit interface configuration shows up as static but also as directly connected! This demonstrates how they are functionally the same but will display differently in the routing table. Understand that if the routes don’t appear in the routing table, it’s because the router can’t communicate with the next-hop address you’ve configured. But you can still use the permanent parameter to keep the route in the routing table even if the next-hop device can’t be contacted.
The S in the first routing table entry means that the route is a static entry. The [150/0] stands for the administrative distance and metric to the remote network, respectively.
Okay—we’re good. The Corp router now has all the information it needs to communicate with the other remote networks. Still, keep in mind that if the SF and LA routers aren’t configured with all the same information, the packets will be discarded. We can fix this by configuring static routes.

The SF router is directly connected to networks and, which means I’ve got to configure the following static routes on the SF router:

The configuration for the SF router is revealed in the following output. Remember that we’ll never create a static route to any network we’re directly connected to as well as the fact that we must use the next hop of since that’s our only router connection. Let’s check out the commands:
SF(config)#ip route 150
SF(config)#ip route 150
SF(config)#ip route 150
SF(config)#do show run | begin ip route
ip route 150
ip route 150
ip route 150

By looking at the routing table, you can see that the SF router now understands how to find each network:
SF(config)#do show ip route
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 is subnetted, 3 subnets S [150/0] via
C          is directly connected, Serial0/0/0 L                  is directly connected, Serial0/0 S [150/0] via is subnetted, 1 subnets  S [150/0] via

And we now can rest assured that the SF router has a complete routing table as well. As soon as the LA router has all the networks in its routing table, SF will be able to communicate with all remote networks!


The LA router is directly connected to and, so these are the routes that must be added:

And here’s the LA router’s configuration:
LA#config t
LA(config)#ip route 150
LA(config)#ip route 150
LA(config)#ip route 150
LA(config)#do show run | begin ip route
ip route 150
ip route 150
ip route 150

This output displays the routing table on the LA router:
LA(config)#do sho ip route
S  [150/0] via is subnetted, 3 subnets
C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1 S     [150/0] via
C  is directly connected, FastEthernet0/0

L  is directly connected, FastEthernet0/0 is subnetted, 1 subnets
S           [150/0] via

LA now shows all five networks in the internetwork, so it too can now communicate with all routers and networks. But before we test our little network, as well as our DHCP server, let’s cover one more topic.

Default Routing

The SF and LA routers that I’ve connected to the Corp router are considered stub routers. A stub indicates that the networks in this design have only one way out to reach all other networks, which means that instead of creating multiple static routes, we can just use a single default route. This default route is used by IP to forward any packet with a destination not found in the routing table, which is why it is also called a gateway of last resort. Here’s the configuration I could have done on the LA router instead of typing in the static routes due to its stub status:
LA#config t
LA(config)#no ip route 150
LA(config)#no ip route 150
LA(config)#no ip route 150
LA(config)#ip route
LA(config)#do sho ip route
[output cut]
Gateway of last resort is to network is subnetted, 1 subnets
C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1  
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 S* [1/0] via
Okay—I’ve removed all the initial static routes I had configured, and adding a default route is a lot easier than typing a bunch of static routes! Can you see the default route listed last in the routing table? The S* shows that as a candidate for the default route. And I really want you to notice that the gateway of last resort is now set too. Everything the router receives with a destination not found in the routing table will be forwarded to You need to be careful where you place default routes because you can easily create a network loop!
So we’re there—we’ve configured all our routing tables! All the routers have the correct routing table, so all routers and hosts should be able to communicate without a hitch—for now. But if you add even one more network or another router to the internetwork, you’ll have to update each and every router’s routing tables by hand—ugh! Not really a problem at all if you’ve got a small network like we do, but it would be a time- consuming monster if you’re dealing with a large internetwork!

Verifying Your Configuration

But we’re not done yet—once all the routers’ routing tables are configured, they must be verified. The best way to do this, besides using the show ip route command, is via Ping. I’ll start by pinging from the Corp router to the SF router.
Here’s the output I got:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms

Here you can see that I pinged from the Corp router to the remote interface of the SF router. Now let’s ping the remote network on the LA router, and after that, we’ll test our DHCP server and see if that is working too!
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

And why not test my configuration of the DHCP server on the Corp router while we’re at it? I’m going to go to each host on the SF and LA routers and make them DHCP clients. By the way, I’m using an old router to represent “hosts,” which just happens to work great for studying purposes. Here’s how I did that:

SF_PC(config)#int e0 SF_PC(config-if)#ip address dhcp SF_PC(config-if)#no shut
Interface Ethernet0 assigned DHCP address, mask
LA_PC(config)#int e0 LA_PC(config-if)#ip addr dhcp LA_PC(config-if)#no shut
Interface Ethernet0 assigned DHCP address, mask

Nice! Don’t you love it when things just work the first time? Sadly, this just isn’t exactly a realistic expectation in the networking world, so we must be able to troubleshoot and verify our networks. Let’s verify our DHCP server with a few of the commands you learned back in Chapter 7:
Corp#sh ip dhcp binding
Bindings from all pools not associated with VRF:
IP address                                  Client-ID/                                                       Lease expiration Type
Hardware address/ User name                               0063.6973.636f.2d30.                       Sept 16 2013 10:34 AM
30                               0063.6973.636f.2d30.                       Sept 16 2013 10:46 AM

We can see from earlier that our little DHCP server is working! Let’s try another couple of commands:
Corp#sh ip dhcp pool SF_LAN
Pool SF_LAN :
Utilization mark (high/low)                                                                   : 100 / 0 Subnet size (first/next)                                                                   : 0 / 0 Total addresses                                                                   : 254
Leased addresses                                                   : 3
Pending event                                                          : none
1 subnet is currently in the pool :
Current index                               IP address range                                                             Leased addresses                                                -                                                          3

Corp#sh ip dhcp conflict
IP address                             Detection method                  Detection time                                                  VRF

The last command would tell us if we had two hosts with the same IP address, so it’s good news because there are no conflicts reported! Two detection methods are used to confirm this:
A ping from the DHCP server to make sure no other host responds before handing out an address A gratuitous ARP from a host that receives a DHCP address from the server The DHCP client will send an ARP request with its new IP address looking to see if anyone responds, and if so, it will report the conflict to the server. Okay, since we can communicate from end to end and to each host without a problem while receiving DHCP addresses from our server, I’d say our static and default route configurations have been a success— cheers!

Dynamic Routing
Dynamic routing is when protocols are used to find networks and update routing tables on routers. This is whole lot easier than using static or default routing, but it will cost you in terms of router CPU processing and bandwidth on network links. A routing protocol defines the set of rules used by a router when it communicates routing information between neighboring routers.
The routing protocol I’m going to talk about in this chapter is Routing Information Protocol (RIP) versions 1 and 2. Two types of routing protocols are used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). IGPs are used to exchange routing information with routers in the same autonomous system (AS). An AS is either a single network or a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing-table information are in the same AS. EGPs are used to communicate between ASs. An example of an EGP is Border Gateway Protocol (BGP), which we’re not going to bother with because it’s beyond the scope of this book. Since routing protocols are so essential to dynamic routing, I’m going to give you the basic information you need to know about them next. Later on in this chapter, we’ll focus on configuration.

Routing Protocol Basics

There are some important things you should know about routing protocols before we get deeper into RIP routing. Being familiar with administrative distances and the three different kinds of routing protocols, for example. Let’s take a look.

Administrative Distances

The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.
If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be chosen and placed in the routing table.
If both advertised routes to the same network have the same AD, then routing protocol metrics like hop count and/or the bandwidth of the lines will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table, but if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network, meaning the protocol will send data down each link.
Table 9.1 shows the default administrative distances that a Cisco router uses to decide which route to take to a remote network.
 Default administrative distances

Route Source
Default AD
Connected interface
Static route
External BGP

External EIGRP
Internal BGP
255 (This route will never be used.)

If a network is directly connected, the router will always use the interface connected to the network. If you configure a static route, the router will then believe that route over any other ones it learns about. You can change the administrative distance of static routes, but by default, they have an AD of 1. In our previous static route configuration, the AD of each route is set at 150. This AD allows us to configure routing protocols without having to remove the static routes because it’s nice to have them there for backup in case the routing protocol experiences some kind of failure.
If you have a static route, an RIP-advertised route, and an EIGRP- advertised route listing the same network, which route will the router go with? That’s right—by default, the router will always use the static route unless you change its AD—which we did!

Routing Protocols

There are three classes of routing protocols:
Distance vector The distance-vector protocols in use today find the best path to a remote network by judging distance. In RIP routing, each instance where a packet goes through a router is called a hop, and the route with the least number of hops to the network will be chosen as the best one. The vector indicates the direction to the remote network. RIP is a distance-vector routing protocol and periodically sends out the entire routing table to directly connected neighbors.
Link state In link-state protocols, also called shortest-path-first (SPF) protocols, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Link-state routers know more about the internetwork than any distance- vector routing protocol ever could. OSPF is an IP routing protocol that’s completely link-state. Link-state routing tables are not exchanged periodically. Instead, triggered updates containing only specific link-state information are sent. Periodic keepalives that are small and efficient, in the form of hello messages, are exchanged between directly connected neighbors to establish and maintain neighbor relationships. Advanced distance vector Advanced distance-vector protocols use aspects of both distance-vector and link-state protocols, and EIGRP is a great example. EIGRP may act like a link-state routing protocol because it uses a Hello protocol to discover neighbors and form neighbor relationships and because only partial updates are sent when a change occurs. However, EIGRP is still based on the key distance-vector routing protocol principle that information about the rest of the network is learned from directly connected neighbors.
There’s no set of rules to follow that dictate exactly how to broadly configure routing protocols for every situation. It’s a task that really must be undertaken on a case-by-case basis, with an eye on specific requirements of each one. If you understand how the different routing protocols work, you can make good, solid decisions that will solidly meet the individual needs of any business!

Routing Information Protocol (RIP)

Routing Information Protocol (RIP) is a true distance-vector routing protocol. RIP sends the complete routing table out of all active interfaces every 30 seconds. It relies on hop count to determine the best way to a remote network, but it has a maximum allowable hop count of 15 by default, so a destination of 16 would be considered unreachable. RIP works okay in very small networks, but it’s super inefficient on large networks with slow WAN links or on networks with a large number of routers installed and completely useless on networks that have links with variable bandwidths!
RIP version 1 uses only classful routing, which means that all devices in the network must use the same subnet mask. This is because RIP version 1 doesn’t send updates with subnet mask information in tow. RIP version 2 provides something called prefix routing and does send subnet mask information with its route updates. This is called classless routing.
So, with that let’s configure our current network with RIPv2, before we move onto the next chapter.

Configuring RIP Routing

To configure RIP routing, just turn on the protocol with the router rip command and tell the RIP routing protocol the networks to advertise. Remember that with static routing, we always configured remote networks and never typed a route to our directly connected networks? Well, dynamic routing is carried out the complete opposite way. You would never type a remote network under your routing protocol—only enter your directly connected networks! Let’s configure our three-router internetwork, revisited in Figure 9.9, with RIP routing.


RIP has an administrative distance of 120. Static routes have an administrative distance of 1 by default, and since we currently have static routes configured, the routing tables won’t be populated with RIP information by default. We’re still good though because I added the 150 to the end of each static route!
You can add the RIP routing protocol by using the router rip command and the network command. The network command tells the routing protocol which classful network to advertise. By doing this, you’re activating the RIP routing process on the interfaces whose addressing falls within the specified classful networks configured with the network command under the RIP routing process.
Look at the Corp router configuration to see how easy this is. Oh wait— first, I want to verify my directly connected networks so I know what to configure RIP with:
Corp#sh ip int brief
Interface                             IP-Address                                               OK? Method Status Protocol
FastEthernet0/0                                                   YES manual up up
Serial0/0                                                                 YES manual up up
FastEthernet0/1               unassigned                     YES unset                                               administratively down down
Serial0/1                                                                  YES manual up up
Corp#config t
Corp(config)#router rip
Corp(config-router)#version 2
Corp(config-router)#no auto-summary

That’s it—really! Typically just two or three commands and you’re done, which sure makes your job a lot easier than dealing with static routes, doesn’t it? Be sure to keep in mind the extra router CPU process and bandwidth that you’re consuming.
Anyway, so what exactly did I do here? I enabled the RIP routing protocol, added my directly connected networks, made sure I was only running RIPv2, which is a classless routing protocol, and then I disabled auto-summary. We typically don’t want our routing protocols summarizing for us because it’s better to do that manually and both RIP and EIGRP (before 15.x code) auto-summarize by default. So a general rule of thumb is to disable auto-summary, which allows them to advertise subnets.
Notice I didn’t type in subnets, only the classful network address, which is betrayed by the fact that all subnet bits and host bits are off! That’s because with dynamic routing, it’s not my job and it’s up to the routing protocol to find the subnets and populate the routing tables. And since we have no router buddies running RIP, we won’t see any RIP routes in the routing table yet.
Let’s configure our SF router now, which is connected to two networks. We need to configure both directly connected classful networks, not subnets:

SF#sh ip int brief
Interface                                IP-Address                                                  OK? Method Status Protocol
FastEthernet0/0                                                        YES manual up up
FastEthernet0/1                 unassigned                     YES unset                                                  administratively down down
Serial0/0/0                                                                  YES manual up up
Serial0/0/1                           unassigned                     YES unset                                                  administratively down down
SF#config SF(config)#router rip
SF(config-router)#version 2 SF(config-router)#no auto-summary SF(config-router)#do show ip route
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 is subnetted, 3 subnets
R          [120/1] via, 00:00:08, Serial0/0/0
C          is directly connected, Serial0/0/0 L                  is directly connected, Serial0/0 S [150/0] via is subnetted, 1 subnets
R          [120/1] via, 00:00:08, Serial0/0/0
That was pretty straightforward. Let’s talk about this routing table. Since we have one RIP buddy out there with whom we are exchanging routing tables, we can see the RIP networks coming from the Corp router. All the other routes still show up as static and local. RIP also found both connections through the Corp router to networks and But we’re not done yet!


Let’s configure our LA router with RIP, only I’m going to remove the default route first, even though I don’t have to. You’ll see why soon:
LA#config t
LA(config)#no ip route
LA(config)#router rip
LA(config-router)#no auto LA(config-router)#vers 2 LA(config-router)#do show ip route

R  [120/2] via, 00:00:10, Serial0/0/1 is subnetted, 3 subnets
C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1
R          [120/1] via, 00:00:10, Serial0/0/1
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 is subnetted, 1 subnets
R          [120/1] via, 00:00:10, Serial0/0/1

The routing table is sprouting new R’s as we add RIP buddies! We can still see that all routes are in the routing table.
This output shows us basically the same routing table and the same entries that it had when we were using static routes—except for those R’s. An R indicates that the networks were added dynamically using the RIP routing protocol. The [120/1] is the administrative distance of the route
(120) along with the metric, which for RIP is the number of hops to that remote network (1). From the Corp router, all networks are one hop away.
So, while yes, it’s true that RIP has worked in our little internetwork, it’s just not a great solution for most enterprises. Its maximum hop count of only 15 is a highly limiting factor. And it performs full routing-table updates every 30 seconds, which would bring a larger internetwork to a painful crawl in no time!
There’s still one more thing I want to show you about RIP routing tables and the parameters used to advertise remote networks. Using a different router on a different network as an example for a second, look into the following output. Can you spot where the following routing table shows [120/15] in the network metric? This means that the administrative distance is 120, the default for RIP, but the hop count is
15. Remember that each time a router sends out an update to a neighbor router, the hop count goes up by one incrementally for each route! Here’s that output now:
Router#sh ip route is subnetted, 12 subnets
C          is directly connected, FastEthernet0/1
L           is directly connected, FastEthernet0/1 C is directly connected, FastEthernet0/0
L           is directly connected, FastEthernet/0/0 R [120/2] via, 00:00:15, Serial0/0/1
R          [120/2] via, 00:00:15, Serial0/0/1
R          [120/1] via, 00:00:00, FastEthernet0/1

R       [120/15] via, 00:00:15, Serial0/0/1
R          [120/1] via, 00:00:15, Serial0/0/1
R          [120/1] via, 00:00:15, Serial0/0/1
R          [120/2] via, 00:00:15, Serial0/0/1
R          [120/2] via, 00:00:15, Serial0/0/1
C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1
R          [120/1] via, 00:00:15, Serial0/0/1

So this [120/15] is really bad. We’re basically doomed because the next router that receives the table from this router will just discard the route to network since the hop count would rise to 16, which is invalid!

Holding Down RIP Propagations

You probably don’t want your RIP network advertised everywhere on your LAN and WAN. There’s enough stress in networking already and not a whole lot to be gained by advertising your RIP network to the Internet!
There are a few different ways to stop unwanted RIP updates from propagating across your LANs and WANs, and the easiest one is through the passive-interface command. This command prevents RIP update broadcasts from being sent out of a specified interface but still allows that same interface to receive RIP updates.
Here’s an example of how to configure a passive-interface on the Corp router’s Fa0/1 interface, which we will pretend is connected to a LAN that we don’t want RIP on (and the interface isn’t shown in the figure):
Corp#config t
Corp(config)#router rip
Corp(config-router)#passive-interface FastEthernet 0/1

This command will stop RIP updates from being propagated out of FastEthernet interface 0/1, but it can still receive RIP updates.

Advertising a Default Route Using RIP
Now I’m going to guide you through how to advertise a way out of your autonomous system to other routers, and you’ll see this is completed the same way with OSPF. Imagine that our Corp router’s Fa0/0 interface is connected to some type of Metro-Ethernet as a connection to the Internet. This is a pretty common configuration today that uses a LAN interface to connect to the ISP instead of a serial interface.
If we do add an Internet connection to Corp, all routers in our AS (SF and LA) must know where to send packets destined for networks on the Internet or they’ll just drop the packets when they get a remote request.
One solution to this little hitch would be to place a default route on every router and funnel the information to Corp, which in turn would have a default route to the ISP. Most people do this type of configuration in small- to medium-size networks because it actually works pretty well!
But since I’m running RIPv2 on all routers, I’ll just add a default route on the Corp router to our ISP, as I would normally. I’ll then add another command to advertise my network to the other routers in the AS as the default route to show them where to send packets destined for the Internet.

Here’s my new Corp configuration:
Corp(config)#ip route fa0/0
Corp(config)#router rip
Corp(config-router)#default-information originate

Now, let’s take a look at the last entry found in the Corp routing table:
S* is directly connected, FastEthernet0/0

Let’s see if the LA router can see this same entry:
LA#sh ip route
Gateway of last resort is to network

R  [120/2] via, 00:00:04, Serial0/0/1 is subnetted, 2 subnets
C          is directly connected, Serial0/0/1
L           is directly connected, Serial0/0/1
R          [120/1] via, 00:00:04, Serial0/0/1
C  is directly connected, FastEthernet0/0 L is directly connected, FastEthernet0/0 is subnetted, 1 subnets
R          [120/1] via, 00:00:04, Serial0/0/1
R  [120/3] via, 00:00:04,
R  [120/2] via, 00:00:05,
R* [120/1] via, 00:00:05, Serial0/0/1

Can you see that last entry? It screams that it’s an RIP injected route, but it’s also a default route, so our default-information originate command is working! Last, notice that the gateway of last resort is now set as well.
If all of what you’ve learned is clear and understood, congratulations— you’re ready to move on to the next chapter right after you go through the written and hands-on labs, and while you’re at it, don’t forget the review questions!


This chapter covered IP routing in detail. Again, it’s extremely important to fully understand the basics we covered in this chapter because everything that’s done on a Cisco router will typically have some kind of IP routing configured and running.

You learned how IP routing uses frames to transport packets between routers and to the destination host. From there, we configured static routing on our routers and discussed the administrative distance used by IP to determine the best route to a destination network. You found out that if you have a stub network, you can configure default routing, which sets the gateway of last resort on a router.
We then discussed dynamic routing, specifically RIPv2 and how it works on an internetwork, which is not very well!

Exam Essentials
Describe the basic IP routing process. You need to remember that the frame changes at each hop but that the packet is never changed or manipulated in any way until it reaches the destination device (the TTL field in the IP header is decremented for each hop, but that’s it!).
List the information required by a router to successfully route packets. To be able to route packets, a router must know, at a minimum, the destination address, the location of neighboring routers through which it can reach remote networks, possible routes to all remote networks, the best route to each remote network, and how to maintain and verify routing information.
Describe how MAC addresses are used during the routing process. A MAC (hardware) address will only be used on a local LAN. It will never pass a router’s interface. A frame uses MAC (hardware) addresses to send a packet on a LAN. The frame will take the packet to either a host on the LAN or a router’s interface (if the packet is destined for a remote network). As packets move from one router to another, the MAC addresses used will change, but normally the original source and destination IP addresses within the packet will not. View and interpret the routing table of a router. Use the show ip route command to view the routing table. Each route will be listed along with the source of the routing information. A C to the left of the route will indicate directly connected routes, and other letters next to the route can also indicate a particular routing protocol that provided the information, such as, for example, R for RIP. Differentiate the three types of routing. The three types of routing are static (in which routes are manually configured at the CLI), dynamic (in which the routers share routing information via a routing protocol), and default routing (in which a special route is configured for all traffic without a more specific destination network found in the table).
Compare and contrast static and dynamic routing. Static routing creates no routing update traffic and creates less overhead on the router and network links, but it must be configured manually and does not have the ability to react to link outages. Dynamic routing creates routing update traffic and uses more overhead on the router and network links.
Configure static routes at the CLI. The command syntax to add a route is ip route [destination_network] [mask] [next-hop_address or exitinterface] [administrative_distance] [permanent].
Create a default route. To add a default route, use the command  syntax ip route ip-address or exit interface type and number.
Understand administrative distance and its role in the selection of the best route. Administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. Administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. All routing protocols are assigned a default AD, but it can be changed at the CLI.
Differentiate distance-vector, link-state, and hybrid routing protocols. Distance-vector routing protocols make routing decisions based on hop count (think RIP), while link-state routing protocols are able to consider multiple factors such as bandwidth available and building a topology table. Hybrid routing protocols exhibit characteristics of both types.
Configure RIPv2 routing. To configure RIP routing, first you must be in global configuration mode and then you type the command router rip. Then you add all directly connected networks, making sure to use the classful address and the version 2 command and to disable auto- summarization with the no auto-summary command.

Written Lab 9

In this section, you’ll complete the following lab to make sure you’ve got the information and concepts contained within them fully dialed in:
Lab 9.1: IP Routing
You can find the answers to this lab in Appendix A, “Answers to Written Labs.”
Write the answers to the following questions:
1.            At the appropriate command prompt, create a static route to network with a next-hop gateway of and an administrative distance of 150.
2.            When a PC sends a packet to another PC in a remote network, what destination addresses will be in the frame that it sends to its default gateway?
3.            At the appropriate command prompt, create a default route to
4.            On which type of network is a default route most beneficial?
5.            At the appropriate command prompt, display the routing table on your router.
6.            When creating a static or default route, you don’t have to use the next- hop IP address; you can use the                                                           .
7.           True/False: To reach a remote host, you must know the MAC address of the remote host.
8.            True/False: To reach a remote host, you must know the IP address of the remote host.
9.            At the appropriate command prompt(s), prevent a router from propagating RIP information out serial 1.
0.         True/False: RIPv2 is considered classless.

Hands-on Labs

In the following hands-on labs, you will configure a network with three routers. These exercises assume all the same setup requirements as the

labs found in earlier chapters. You can use real routers, the LammleSim IOS version found at, or the Cisco Packet Tracer program to run these labs.
This chapter includes the following labs: Lab 9.1: Creating Static Routes
Lab 9.2: Configuring RIP Routing
The internetwork shown in the following graphic will be used to configure all routers.

Table 9.2 shows our IP addresses for each router (each interface uses a
/24 mask).
 Our IP addresses

IP Address
These labs were written without using the LAN interface on the Lab_B router. You can choose to add that LAN into the labs if necessary. Also, if you have enough LAN interfaces, then you don’t need to add the serial interfaces into this lab. Using all LAN interfaces is fine.

Hands-on Lab 9.1: Creating Static Routes

In this lab, you will create a static route in all three routers so that the routers see all networks. Verify with the Ping program when complete.
1.         The Lab_A router is connected to two networks, and You need to add routes to networks and Use the following commands to add the static routes:
Lab_A#config t
Lab_A(config)#ip route
Lab_A(config)#ip route

2.       Save the current configuration for the Lab_A router by going to privileged mode, typing copy run start, and pressing Enter.
3.       On the Lab_B router, you have direct connections to networks and You need to add routes to networks and Use the following commands to add the static routes:
Lab_B#config t
Lab_B(config)#ip route
Lab_B(config)#ip route

4.         Save the current configuration for router Lab_B by going to the enabled mode, typing copy run start, and pressing Enter.
5.         On router Lab_C, create a static route to networks and, which are not directly connected. Create static routes so that router Lab_C can see all networks, using the commands shown here:
Lab_C#config t
Lab_C(config)#ip route
Lab_C(config)#ip route

6.         Save the current configuration for router Lab_C by going to the enable mode, typing copy run start, and pressing Enter.

7.         Check your routing tables to make sure all four networks show up by executing the show ip route command.
8.         Now ping from each router to your hosts and from each router to each router. If it is set up correctly, it will work.

Hands-on Lab 9.2: Configuring RIP Routing

In this lab, we will use the dynamic routing protocol RIP instead of static routing.
1.  Remove any static routes or default routes configured on your routers by using the no ip route command. For example, here is how you would remove the static routes on the Lab_A router:
Lab_A#config t
Lab_A(config)#no ip route
Lab_A(config)#no ip route

Do the same thing for routers Lab_B and Lab_C. Verify that only your directly connected networks are in the routing tables.
2.       After your static and default routes are clear, go into configuration mode on router Lab_A by typing config t.
3.       Tell your router to use RIP routing by typing router rip and pressing Enter, as shown here:
config t router rip

4.       Add the network number for the networks you want to advertise. Since router Lab_A has two interfaces that are in two different networks, you must enter a network statement using the network ID of the network in which each interface resides. Alternately, you could use a summarization of these networks and use a single statement, minimizing the size of the routing table. Since the two networks are and, the network summarization would include both subnets. Do this by typing network and pressing Enter.
5.       Press Ctrl+Z to get out of configuration mode.

6.        The interfaces on Lab_B and Lab_C are in the and networks; therefore, the same summarized network statement will work there as well. Type the same commands, as shown here:
Config t Router rip

7.       Verify that RIP is running at each router by typing the following commands at each router:
show ip protocols

(Should indicate to you that RIP is present on the router.)
show ip route

(Should have routes present with an R to the left of them.)
show running-config or show run

(Should indicate that RIP is present and the networks are being advertised.)
8.       Save your configurations by typing copy run start or copy running- config startup-config and pressing Enter at each router.
9.        Verify the network by pinging all remote networks and hosts.


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