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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…

Open Shortest Path First (OSPF)

The following ICND1 exam topics
are covered in this chapter :

1 IP Routing Technologies
■ Configure and verify OSPF (single area)
■ Benefit of single area
■ Configure OSPF v2
■ Router ID
■ Passive interface

Image result for Open Shortest Path"
Open Shortest Path First (OSPF) is by far the most popular
and important routing protocol in use today—so important,
I’m devoting this entire chapter to it! Sticking with the same
approach we’ve adhered to throughout this book, we’ll begin with the basics by completely
familiarizing you with key OSPF terminology. Once we’ve covered that thoroughly, I’ll guide
you through OSPF’s internal operation and then move on to tell you all about OSPF’s many
advantages over RIP.
This chapter is going to be more than chock full of vitally important information and it’s
also going to be really exciting because together, we’ll explore some seriously critical factors
and issues innate to implementing OSPF! I’ll walk you through exactly how to implement
single-area OSPF in a variety of networking environments and then demonstrate some great
techniques you’ll need to verify that everything is configured correctly and running smoothly.
To find up-to-the-minute updates for this chapter, please see or the book’s web page at
Open Shortest Path First (OSPF) Basics
Open Shortest Path First is an open standard routing protocol that’s been implemented by
a wide variety of network vendors, including Cisco. And it’s that open standard characteristic that’s the key to OSPF’s flexibility and popularity.
Most people opt for OSPF, which works by using the Dijkstra algorithm to initially
construct a shortest path tree and follows that by populating the routing table with the
resulting best paths. EIGRP’s convergence time may be blindingly fast, but OSPF isn’t that
far behind, and its quick convergence is another reason it’s a favorite. Another two great
advantages OSPF offers are that it supports multiple, equal-cost routes to the same destination, and like EIGRP, it also supports both IP and IPv6 routed protocols.
Here’s a list that summarizes some of OSPF’s best features:
uu Allows for the creation of areas and autonomous systems
uu Minimizes routing update traffic

u Is highly flexible, versatile, and scalable
uu Supports VLSM/CIDR
uu Offers an unlimited hop count
uu Is open standard and supports multi-vendor deployment
Because OSPF is the first link-state routing protocol that most people run into, it’s a
good idea to size it up against more traditional distance-vector protocols like RIPv2 and
RIPv1. Table 9.1 presents a nice comparison of all three of these common protocols.
388 Chapter 9 u Open Shortest Path First (OSPF)
I want you know that OSPF has many features beyond the few I’ve listed in Table 9.1 and
all of them combine to produce a fast, scalable, robust protocol that’s also flexible enough to
be actively deployed in a vast array of production networks!
One of OSPF’s most useful traits is that its design is intended to be hierarchical in use,
meaning that it allows us to subdivide the larger internetwork into smaller internetworks
called areas. It’s a really powerful feature that I recommend using, and I promise to show
you how to do that later in the chapter.
Here are three of the biggest reasons to implement OSPF in a way that makes full use of
its intentional, hierarchical design:
uu To decrease routing overhead
uu To speed up convergence
uu To confine network instability to single areas of the network
Because free lunches are invariably hard to come by, all this wonderful functionality
predictably comes at a price and doesn’t exactly make configuring OSPF any easier. But
no worries—we’ll crush it!
Let’s start by checking out Figure 9.1, which shows a very typical, yet simple OSPF design.
I really want to point out the fact that some routers connect to the backbone—called area 0—
the backbone area. OSPF absolutely must have an area 0, and all other areas should connect
to it except for those connected via virtual links, which are beyond the scope of this book. A
router that connects other areas to the backbone area within an AS is called an area border
router (ABR), and even these must have at least one of their interfaces connected to area 0.

OSPF runs great inside an autonomous system, but it can also connect multiple autonomous
systems together. The router that connects these ASs is called an autonomous system boundary router (ASBR). Ideally, your aim is to create other areas of networks to help keep route 

Open Shortest Path First (OSPF) Basics 389
updates to a minimum, especially in larger networks. Doing this also keeps problems from
propagating throughout the network, affectively isolating them to a single area.
But let’s pause here to cover some key OSPF terms that are really essential for you to nail
down before we move on any further.
OSPF Terminology
Imagine being given a map and compass with no prior concept of east, west, north or south—
not even what rivers, mountains, lakes, or deserts are. I’m guessing that without any ability to
orient yourself in a basic way, your cool, new tools wouldn’t help you get anywhere but completely lost, right? This is exactly why we’re going to begin exploring OSPF by getting you solidly acquainted with a fairly long list of terms before setting out from base camp into the great
unknown! Here are those vital terms to commit to memory now:
Link A link is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it’s considered to be a link. This link, or interface, will
have up or down state information associated with it as well as one or more IP addresses.
Router ID The router ID (RID) is an IP address used to identify the router. Cisco chooses
the router ID by using the highest IP address of all configured loopback interfaces. If no
loopback interfaces are configured with addresses, OSPF will choose the highest IP address
out of all active physical interfaces. To OSPF, this is basically the “name” of each router.
Neighbor Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. OSPF neighbors must
have a number of common configuration options to be able to successfully establish a
neighbor relationship, and all of these options must be configured exactly the same way:
uu Area ID
uu Stub area flag
uu Authentication password (if using one)
uu Hello and Dead intervals
Adjacency An adjacency is a relationship between two OSPF routers that permits the
direct exchange of route updates. Unlike EIGRP, which directly shares routes with all of its
neighbors, OSPF is really picky about sharing routing information and will directly share
routes only with neighbors that have also established adjacencies. And not all neighbors will
become adjacent—this depends upon both the type of network and the configuration of the
routers. In multi-access networks, routers form adjacencies with designated and backup des￾ignated routers. In point-to-point and point-to-multipoint networks, routers form adjacencies with the router on the opposite side of the connection.
Designated router A designated router (DR) is elected whenever OSPF routers are connected to the same broadcast network to minimize the number of adjacencies formed and to
publicize received routing information to and from the remaining routers on the broadcast
network or link. Elections are won based upon a router’s priority level, with the one having 390 Chapter 9 u Open Shortest Path First (OSPF)
the highest priority becoming the winner. If there’s a tie, the router ID will be used to break
it. All routers on the shared network will establish adjacencies with the DR and the BDR,
which ensures that all routers’ topology tables are synchronized.
Backup designated router A backup designated router (BDR) is a hot standby for the
DR on broadcast, or multi-access, links. The BDR receives all routing updates from OSPF
adjacent routers but does not disperse LSA updates.
Hello protocol The OSPF Hello protocol provides dynamic neighbor discovery and maintains neighbor relationships. Hello packets and Link State Advertisements (LSAs) build and
maintain the topological database. Hello packets are addressed to multicast address
Neighbor ship database The neighbor ship database is a list of all OSPF routers for which
Hello packets have been seen. A variety of details, including the router ID and state, are
maintained on each router in the neighbor ship database.
Topological database The topological database contains information from all of the Link
State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the
shortest path to every network.
LSA packets are used to update and maintain the topological database.
Link State Advertisement A Link State Advertisement (LSA) is an OSPF data packet con￾taining link-state and routing information that’s shared among OSPF routers. There are different types of LSA packets, and I’ll cover these in the ICND2 book. An OSPF router will
exchange LSA packets only with routers to which it has established adjacencies.
OSPF areas An OSPF area is a grouping of contiguous networks and routers. All routers in
the same area share a common area ID. Because a router can be a member of more than one
area at a time, the area ID is associated with specific interfaces on the router. This would allow
some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of
the routers within the same area have the same topology table. When configuring OSPF with
multiple areas, you’ve got to remember that there must be an area 0 and that this is typically
considered the backbone area. Areas also play a role in establishing a hierarchical network
organization—something that really enhances the scalability of OSPF!
Broadcast (multi-access) Broadcast (multi-access) networks such as Ethernet allow multiple devices to connect to or access the same network, enabling a broadcast ability in which
a single packet is delivered to all nodes on the network. In OSPF, a DR and BDR must be
elected for each broadcast multi-access network.
Nonbroadcast multi-access Nonbroadcast multi-access (NBMA) networks are networks
such as Frame Relay, X.25, and Asynchronous Transfer Mode (ATM). These types of
networks allow for multi-access without broadcast ability like Ethernet. NBMA networks
require special OSPF configuration to function properly.

Open Shortest Path First (OSPF) Basics 391
Point-to-point Point-to-point refers to a type of network topology made up of a direct
connection between two routers that provides a single communication path. The point-to-point connection can be physical—for example, a serial cable that directly connects two
routers—or logical, where two routers thousands of miles apart are connected by a circuit
in a Frame Relay network. Either way, point-to-point configurations eliminate the need for
DRs or BDRs.
Point-to-multipoint Point-to-multi-point refers to a type of network topology made up
of a series of connections between a single interface on one router and multiple destination
routers. All interfaces on all routers share the point-to-multipoint connection and belong
to the same network. Point-to-multipoint networks can be further classified according to
whether they support broadcasts or not. This is important because it defines the kind of
OSPF configurations you can deploy.
All of these terms play a critical role when you’re trying to understand how OSPF actually
works, so again, make sure you’re familiar with each of them. Having these terms down will
enable you to confidently place them in their proper context as we progress on our journey
through the rest of this chapter!
OSPF Operation
Fully equipped with your newly acquired knowledge of the terms and technologies we
just covered, it’s now time to delve into how OSPF discovers, propagates, and ultimately
chooses routes. Once you know how OSPF achieves these tasks, you’ll understand how
OSPF operates internally really well.
OSPF operation is basically divided into these three categories:
uu Neighbor and adjacency initialization
uu LSA flooding
uu SPF tree calculation
The beginning neighbor/adjacency formation stage is a very big part of OSPF operation.
When OSPF is initialized on a router, the router allocates memory for it, as well as for the
maintenance of both neighbor and topology tables. Once the router determines which interfaces have been configured for OSPF, it will then check to see if they’re active and begin
sending Hello packets.
F ig u re 9. 2 The Hello protocol
The Hello protocol is used to discover neighbors, establish adjacencies, and maintain
relationships with other OSPF routers. Hello packets are periodically sent out of each
enabled OSPF interface and in environments that support multicast.

392 Chapter 9 u Open Shortest Path First (OSPF)
The address used for this is, and the frequency with which Hello packets are sent
out depends upon the network type and topology. Broadcast and point-to-point networks send
Hellos every 10 seconds, whereas non-broadcast and point-to-multipoint networks send them
every 30 seconds.
LSA Flooding
LSA flooding is the method OSPF uses to share routing information. Via LSU packets, LSA
information containing link-state data is shared with all OSPF routers within an area. The
network topology is created from the LSA updates, and flooding is used so that all OSPF
routers have the same topology map to make SPF calculations with.
Efficient flooding is achieved through the use of a reserved multicast address:
(AllSPFRouters). LSA updates, which indicate that something in the topology has changed,
are handled a bit differently. The network type determines the multicast address used for
sending updates. Table 9.2 contains the multicast addresses associated with LSA flooding.
Point-to-multipoint networks use the adjacent router’s unicast IP address.
Table 9. 2 LSA update multicast addresses
Network Type Multicast Address Description
Point-to-point AllSPFRouters
Broadcast AllDRouters
Point-to-multipoint NA
Once the LSA updates have been flooded throughout the network, each recipient must
acknowledge that the flooded update has been received. It’s also important for recipients to
validate the LSA update.
SPF Tree Calculation
Within an area, each router calculates the best/shortest path to every network in that same
area. This calculation is based upon the information collected in the topology database and
an algorithm called shortest path first (SPF). Picture each router in an area constructing
a tree—much like a family tree—where the router is the root and all other networks are
arranged along the branches and leaves. This is the shortest path tree used by the router to
insert OSPF routes into the routing table.
It’s important to understand that this tree contains only networks that exist in the same
area as the router itself does. If a router has interfaces in multiple areas, then separate trees
will be constructed for each area. One of the key criteria considered during the route selection process of the SPF algorithm is the metric or cost of each potential path to a network.
But this SPF calculation doesn’t apply to routes from other areas.

Configuring OSPF 393
OSPF Metrics
OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface
included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing
interfaces along the path. Because cost is an arbitrary value as defined in RFC 2338, Cisco
had to implement its own method of calculating the cost for each OSPF-enabled interface.
Cisco uses a simple equation of 108/bandwidth, where bandwidth is the configured bandwidth for the interface. Using this rule, a 100 Mbps Fast Ethernet interface would have a
default OSPF cost of 1 and a 1,000 Mbps Ethernet interface would have a cost of 1.
Important to note is that this value can be overridden with the ip ospf cost command.
The cost is manipulated by changing the value to a number within the range of 1 to 65,535.
Because the cost is assigned to each link, the value must be changed on the specific interface
you want to change the cost on.
Cisco bases link cost on bandwidth. Other vendors may use other metrics
to calculate a given link’s cost. When connecting links between routers
from different vendors, you’ll probably have to adjust the cost to match
another vendor’s router because both routers must assign the same cost
to the link for OSPF to work properly.
Configuring OSPF
Configuring basic OSPF isn’t as simple as configuring RIP and EIGRP, and it can get really
complex once the many options that are allowed within OSPF are factored in. But that’s
okay because you really only need to focus on basic, single-area OSPF configuration at this
point. Coming up, I’ll show you how to configure single-area OSPF.
The two factors that are foundational to OSPF configuration are enabling OSPF and
configuring OSPF areas.
Enabling OSPF
The easiest and also least scalable way to configure OSPF is to just use a single area. Doing
this requires a minimum of two commands.
The first command used to activate the OSPF routing process is as follows:
Router(config)#router ospf ?
<1-65535> Process ID
A value in the range from 1 to 65,535 identifies the OSPF process ID. It’s a unique number
on this router that groups a series of OSPF configuration commands under a specific running
process. Different OSPF routers don’t have to use the same process ID to communicate. It’s a
394 Chapter 9 u Open Shortest Path First (OSPF)
purely local value that doesn’t mean a lot, but you still need to remember that it cannot start
at 0; it has to start at a minimum of 1.
You can have more than one OSPF process running simultaneously on the same router
if you want, but this isn’t the same as running multi-area OSPF. The second process will
maintain an entirely separate copy of its topology table and manage its communications
independently of the first one and you use it when you want OSPF to connect multiple ASs
together. Also, because the Cisco exam objectives only cover single-area OSPF with each
router running a single OSPF process, that’s what we’ll focus on in this book.
The OSPF process ID is needed to identify a unique instance of an OSPF
database and is locally significant.
Configuring OSPF Areas
After identifying the OSPF process, you need to identify the interfaces that you want to
activate OSPF communications on as well as the area in which each resides. This will
also configure the networks you’re going to advertise to others. OSPF uses wildcards in
the configuration, which are also used in the access list configurations that we’ll cover
in Chapter 12, “Security.”
Here’s an example of a basic OSPF configuration for you, showing our second minimum command needed, the network command:
Router#config t
Router(config)#router ospf 1
Router(config-router)#network area ?
 <0-4294967295> OSPF area ID as a decimal value
 A.B.C.D OSPF area ID in IP address format
Router(config-router)#network area 0
The areas can be any number from 0 to 4.2 billion. Don’t get these numbers
confused with the process ID, which ranges from 1 to 65,535.
Remember, the OSPF process ID number is irrelevant. It can be the same on every router
on the network, or it can be different—doesn’t matter. It’s locally significant and just enables
the OSPF routing on the router.
The arguments of the network command are the network number ( and the wild￾card mask ( The combination of these two numbers identifies the interfaces
that OSPF will operate on and will also be included in its OSPF LSA advertisements. Based
on my sample configuration, OSPF will use this command to find any interface on the router
configured in the network and will place any interface it finds into area 0. Notice Configuring OSPF 395
that you can create about 4.2 billion areas! In reality, a router wouldn’t let you create that
many, but you can certainly name them using the numbers up to 4.2 billion. You can also
label an area using an IP address format.
Let me stop here a minute to give you a quick explanation of wildcards: A 0 octet in the
wildcard mask indicates that the corresponding octet in the network must match exactly.
On the other hand, a 255 indicates that you don’t care what the corresponding octet is in
the network number. A network and wildcard mask combination of would
match an interface configured exactly with only, and nothing else. This is really useful if you want to activate OSPF on a specific interface in a very clear and simple way. If
you insist on matching a range of networks, the network and wildcard mask combination
of would match any interface in the range of to
Because of this, it’s simpler and safer to stick to using wildcard masks of and identify each OSPF interface individually. Once configured, they’ll function exactly the same—
one way is really isn’t better than the other.
The final argument is the area number. It indicates the area to which the interfaces
identified in the network and wildcard mask portion belong. Remember that OSPF routers
will become neighbors only if their interfaces share a network that’s configured to belong
to the same area number. The format of the area number is either a decimal value from the
range 1 to 4,294,967,295 or a value represented in standard dotted-decimal notation. For
example, area is a legitimate area and is identical to area 0.
Wildcard Example
Before getting down to configuring our network, let’s take a quick peek at a more complex
OSPF network configuration to find out what our OSPF network statements would be if we
were using subnets and wildcards.
In this scenario, you have a router with these four subnets connected to four different
All interfaces need to be in area 0, so it seems to me the easiest configuration would look
like this:
Test#config t
Test(config)#router ospf 1
Test(config-router)#network area 0
Okay—I’ll admit that preceding example is actually pretty simple, but easy isn’t always
best—especially when dealing with OSPF! So even though this is an easy-button way to configure OSPF, it doesn’t make good use of its capabilities and what fun is that? Worse yet, the
objectives aren’t very likely to present something this simple for you! So let’s create a separate
396 Chapter 9 u Open Shortest Path First (OSPF)
network statement for each interface using the subnet numbers and wildcards. Doing that
would look something like this:
Test#config t
Test(config)#router ospf 1
Test(config-router)#network area 0
Test(config-router)#network area 0
Test(config-router)#network area 0
Test(config-router)#network area 0
Wow, now that’s a different looking config! Truthfully, OSPF would work exactly the
same way as it would with the easy configuration I showed you first—but unlike the easy
configuration, this one covers the objectives!
And although this looks a bit complicated, trust me, it really isn’t. All you need for clarity is
to fully understand your block sizes! Just remember that when configuring wildcards, they’re
always one less than the block size. A /28 is a block size of 16, so we would add our network
statement using the subnet number and then add a wildcard of 15 in the interesting octet. For
the /30, which is a block size of 4, we would go with a wildcard of 3. Once you practice this a
few times, it gets really easy. And do practice because we’ll deal with them again when we get
to access lists later on!
Let’s use Figure 9.3 as an example and configure that network with OSPF using wildcards to make sure you have a solid grip on this. The figure shows a three-router network
with the IP addresses of each interface.
F ig u re 9. 3 Sample OSPF wildcard configuration
Fa0/0 Fa0/0 Fa0/0
G0/0 G0/1 G0/0 G0/0
Lab A Lab B Lab C
The very first thing you need to be able to do is to look at each interface and determine the subnet that the addresses are in. Hold on, I know what you’re thinking: “Why
don’t I just use the exact IP addresses of the interface with the wildcard?” Well,
you can, but we’re paying attention to Cisco exam objectives here, not just what’s easiest, remember?

Configuring OSPF 397
The IP addresses for each interface are shown in the figure. The Lab_A router has two
directly connected subnets: and Here’s the OSPF configuration using wildcards:
Lab_A#config t
Lab_A(config)#router ospf 1
Lab_A(config-router)#network area 0
Lab_A(config-router)#network area 0
The Lab_A router is using a /29, or, mask on the Fa0/0 interface. This is
a block size of 8, which is a wildcard of 7. The G0/0 interface is a mask of—
block size of 4, with a wildcard of 3. Notice that I typed in the network number, not the
interface number. You can’t configure OSPF this way if you can’t look at the IP address and
slash notation and then figure out the subnet, mask, and wildcard, can you? So don’t take your
exam until you can do this.
Here are other two configurations to help you practice:
Lab_B#config t
Lab_B(config)#router ospf 1
Lab_B(config-router)#network area 0
Lab_B(config-router)#network area 0
Lab_B(config-router)#network area 0
Lab_C#config t
Lab_C(config)#router ospf 1
Lab_C(config-router)#network area 0
Lab_C(config-router)#network area 0
As I mentioned with the Lab_A configuration, you’ve got to be able to determine the subnet, mask, and wildcard just by looking at the IP address and mask of an interface. If you
can’t do that, you won’t be able to configure OSPF using wildcards as I just demonstrated.
So go over this until you’re really comfortable with it!
Configuring Our Network with OSPF
Okay—now we get to have some fun! Let’s configure our internetwork with OSPF using
just area 0. OSPF has an administrative distance of 110, but let’s remove RIP while we’re
at it because I don’t want you to get in the habit of having RIP running on your network.
There’s a bunch of different ways to configure OSPF, and as I said, the simplest
and easiest is to use the wildcard mask But I want to demonstrate that we can 398 Chapter 9 u Open Shortest Path First (OSPF)
configure each router differently with OSPF and still come up with the exact same result.
This is one reason why OSPF is more fun and challenging than other routing protocols—
it gives us all a lot more ways to screw things up, which automatically provides a troubleshooting opportunity! We’ll use our network as shown in Figure 9.4 to configure OSPF
and by the way, notice I added a new router!
F ig u re 9. 4 Our new network layout
Fa0/0 Fa0/0 S0/0/0
S0/0/1 S0/0 S0/1
Here’s the Corp router’s configuration:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 YES manual up
Serial0/0 YES manual up
FastEthernet0/1 unassigned YES unset administratively down
Serial0/1 YES manual up
Corp#config t
Corp(config)#no router rip
Corp(config)#router ospf 132
Corp(config-router)#network area 0
Corp(config-router)#network area 0
Corp(config-router)#network area 0

Configuring OSPF 399
Alright—it looks like we have a few things to talk about here. First, I removed RIP and
then added OSPF. Why did I use OSPF 132? It really doesn’t matter—the number is irrelevant. I guess it just felt good to use 132. But notice that I started with the show ip int brief
command, just like when I was configuring RIP. I did this because it’s always important to
verify exactly what you are directly connected to. Doing this really helps prevent typos!
The network commands are pretty straightforward. I typed in the IP address of each inter￾face and used the wildcard mask of, which means that the IP address must precisely
match each octet. This is actually one of those times where easier is better, so just do this:
Corp(config)#router ospf 132
Corp(config-router)#network area 0
Nice—there’s only one line instead of two for the network! I really want
you to understand that OSPF will work the same here no matter which way you configure
the network statement. Now, let’s move on to SF. To simplify things, we’re going to use our
same sample configuration.
The SF router has two directly connected networks. I’ll use the IP addresses on each interface to configure this router.
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
Serial0/0/0 YES manual up up
Serial0/0/1 unassigned YES unset administratively down
SF#config t
SF(config)#no router rip
SF(config)#router ospf 300
SF(config-router)#network area 0
SF(config-router)#network area 0
*Apr 30 00:25:43.810: %OSPF-5-ADJCHG: Process 300, Nbr on Serial0/0/0
from LOADING to FULL, Loading Done
Here, all I did was to first disable RIP, turn on OSPF routing process 300, and then I
added my two directly connected networks. Now let’s move on to LA!
We’re going to give some attention to the LA router that’s directly connected to two networks:
LA#sh ip int brief
Interface IP-Address OK? Method Status Protocol

400 Chapter 9 u Open Shortest Path First (OSPF)
FastEthernet0/0 YES manual up
FastEthernet0/1 unassigned YES unset administratively down
Serial0/0/0 unassigned YES unset administratively down
Serial0/0/1 YES manual up
LA#config t
LA(config)#router ospf 100
LA(config-router)#network area 0
LA(config-router)#network area 0
*Apr 30 00:56:37.090: %OSPF-5-ADJCHG: Process 100, Nbr on Serial0/0/1
from LOADING to FULL, Loading Done
Remember that when you’re configuring dynamic routing, using the show ip int brief
command first will make it all so much easier!
And don’t forget, I can use any process ID I want, as long as it’s a value from 1 to 65,535,
because it doesn’t matter if all routers use the same process ID. Also, notice that I used different wildcards in this example. Doing this works really well too.
Okay, I want you to think about something for a second before we move onto more
advanced OSPF topics: What if the Fa0/1 interface of the LA router was connected to a
link that we didn’t want, or need to have on in order to have OSPF working, as shown in
Figure 9.5?
F ig u re 9.5 Adding a non-OSPF network to LA router
OSPF network
Non-OSPF network
You’ve seen this before because I demonstrated this already back in Chapter 8, in the
RIP section. We can use the same command that we did under that routing process here as
well! Take a look:
LA(config)#router ospf 100
LA(config-router)#passive-interface fastEthernet 0/1
Even though this is pretty simple, you’ve really got to be careful before you configure
this command on your router! I added this command as an example on interface Fa0/1,
which happens to be an interface we’re not using in this network because I want OSPF to
work on my other router’s interfaces.
Configuring OSPF 401
Now it’s time to configure our Corp router to advertise a default route to the SF and LA
routers because doing so will make our lives a lot easier. Instead of having to configure all
our routers with a default route, we’ll only configure one router and then advertise that this
router is the one that holds the default route—elegant!
In Figure 9.4, keep in mind that, for now, the corporate router is connected to the Internet
off of Fa0/0. We’ll create a default route toward this imaginary Internet and then tell the
other routers that this is the route they’ll use to get to the Internet. Here is the configuration:
Corp#config t
Corp(config)#ip route Fa0/0
Corp(config)#router ospf 1
Corp(config-router)#default-information originate
Now, let’s check and see if our other routers have received this default route from the
Corp router:
SF#show ip route
[output cut]
E1 - OSPF external type 1, E2 - OSPF external type 2
[output cut]
O*E2 [110/1] via, 00:01:54, Serial0/0/0
Sure enough—the last line in the SF router shows that it received the advertisement from
the Corp router regarding the fact that the corporate router is the one holding the default
route out of the AS.
But hold on a second! I need to configure our new router into my lab to create the example
network we’ll use from here on. Here’s the configuration of the new router that I connected to
the same network that the Corp router is connected to via the Fa0/0 interface:
Router#config t
Router(config)#hostname Boulder
Boulder(config)#int f0/0
Boulder(config-if)#ip address
Boulder(config-if)#no shut
*Apr 6 18:01:38.007: %LINEPROTO-5-UPDOWN: Line protocol on Interface
FastEthernet0/0, changed state to up
Boulder(config-if)#router ospf 2
Boulder(config-router)#network area 0
*Apr 6 18:03:27.267: %OSPF-5-ADJCHG: Process 2, Nbr on
FastEthernet0/0 from LOADING to FULL, Loading Done

402 Chapter 9 u Open Shortest Path First (OSPF)
This is all good, but I need to make sure that you don’t follow my example to a tee
because here, I just quickly brought a router up without setting my passwords first. I can
get away with this only because I am in a nonproduction network, so don’t do this in the
real world where security is key!
Anyway, now that I have my new router nicely connected with a basic configuration,
we’re going to move on to cover loopback interfaces, how to set the router ID (RID) used
with OSPF, and finally, how to verify OSPF.
OSPF and Loopback Interfaces
It’s really vital to configure loopback interfaces when using OSPF. In fact, Cisco suggests
using them whenever you configure OSPF on a router for stability purposes.
Loopback interfaces are logical interfaces, which means they’re virtual, software-only
interfaces, not actual, physical router interfaces. A big reason we use loopback interfaces
with OSPF configurations is because they ensure that an interface is always active and
available for OSPF processes.
Loopback interfaces also come in very handy for diagnostic purposes as well as for OSPF
configuration. Understand that if you don’t configure a loopback interface on a router, the
highest active IP address on a router will become that router’s RID during bootup! Figure 9.6
illustrates how routers know each other by their router ID.
F ig u re 9.6 OSPF router ID (RID)
I need your RID so I can
place you properly in my
neighbor table!
The RID is not only used to advertise routes, it’s also used to elect the designated router
(DR) and the backup designated router (BDR). These designated routers create adjacencies
when a new router comes up and exchanges LSAs to build topological databases.
By default, OSPF uses the highest IP address on any active interface at the
moment OSPF starts up to determine the RID of the router. But this behavior can be overridden via a logical interface. Remember—the highest IP
address of any logical interface will always become a router’s RID!

OSPF and Loopback Interfaces 403
Now it’s time to show you how to configure these logical loopback interfaces and how to
verify them, as well as verify RIDs.
Configuring Loopback Interfaces
Configuring loopback interfaces rocks mostly because it’s the easiest part of OSPF configuration, and we all need a break about now—right? So hang on—we’re in the home stretch!
First, let’s see what the RID is on the Corp router with the show ip ospf command:
Corp#sh ip ospf
 Routing Process "ospf 1" with ID
[output cut]
Okay—we can see that the RID is—the Serial0/0 interface of the router. So
let’s configure a loopback interface using a completely different IP addressing scheme:
Corp(config)#int loopback 0
*Mar 22 01:23:14.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface
 Loopback0, changed state to up
Corp(config-if)#ip address
The IP scheme really doesn’t matter here, but each one being in a separate subnet does!
By using the /32 mask, we can use any IP address we want as long as the addresses are
never the same on any two routers.
Let’s configure the other routers now:
SF#config t
SF(config)#int loopback 0
*Mar 22 01:25:11.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface
 Loopback0, changed state to up
SF(config-if)#ip address
Here’s the configuration of the loopback interface on LA:
LA#config t
LA(config)#int loopback 0
*Mar 22 02:21:59.686: %LINEPROTO-5-UPDOWN: Line protocol on Interface
 Loopback0, changed state to up
LA(config-if)#ip address
I’m pretty sure you’re wondering what the IP address mask of (/32)
means and why we don’t just use instead. While it’s true that either mask
works, the /32 mask is called a host mask and works fine for loopback interfaces. It also 404 Chapter 9 u Open Shortest Path First (OSPF)
allows us to save subnets. Notice how I was able to use, .2, .3, and .4? If I didn’t
use the /32, I’d have to use a separate subnet for each and every router—not good!
One important question to answer before we move on is did we actually change the
RIDs of our router by setting the loopback interfaces? Let’s find out by taking a look at
the Corp’s RID:
Corp#sh ip ospf
 Routing Process "ospf 1" with ID
Okay—what happened here? You would think that because we set logical interfaces, the
IP addresses under them would automatically become the RID of the router, right? Well, sort
of, but only if you do one of two things: either reboot the router or delete OSPF and re-create
the database on your router. Neither is all that great an option, so try to remember to create
your logical interfaces before you start OSPF routing. That way, the loopback interface would
always become your RID straight away!
With all this in mind, I’m going with rebooting the Corp router because it’s the easier of
the two options I have right now.
Now let’s look and see what our RID is:
Corp#sh ip ospf
 Routing Process "ospf 1" with ID
Okay, that did the trick! The Corp router now has a new RID, so I guess I’ll just go
ahead and reboot all my routers to get their RIDs reset to our logical addresses. But should
I really do that?
Maybe not because there is one other way. What do you think about adding a new RID
for the router right under the router ospf process-id command instead? Sounds good, so
I’d say let’s give that a shot! Here’s an example of doing that on the Corp router:
Corp#config t
Corp(config)#router ospf 1
Reload or use "clear ip ospf process" command, for this to take effect
Corp(config-router)#do clear ip ospf process
Reset ALL OSPF processes? [no]: yes
*Jan 16 14:20:36.906: %OSPF-5-ADJCHG: Process 1, Nbr
on Serial0/1 from FULL to DOWN, Neighbor Down: Interface down
or detached
*Jan 16 14:20:36.906: %OSPF-5-ADJCHG: Process 1, Nbr
on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down
or detached
*Jan 16 14:20:36.982: %OSPF-5-ADJCHG: Process 1, Nbr 

Verifying OSPF Configuration 405
on Serial0/1 from LOADING to FULL, Loading Done
*Jan 16 14:20:36.982: %OSPF-5-ADJCHG: Process 1, Nbr
on Serial0/0 from LOADING to FULL, Loading Done
Corp(config-router)#do sh ip ospf
 Routing Process "ospf 1" with ID
Now look at that—it worked! We changed the RID without reloading the router! But
wait—remember, we set a logical loopback interface earlier. Does that mean the loopback
interface will win over the router-id command? Well, we can see our answer… A loop￾back interface will not override the router-id command, and we don’t have to reboot the
router to make it take effect as the RID!
So this process follows this hierarchy:
1. Highest active interface by default.
2. Highest logical interface overrides a physical interface.
3. The router-id overrides the interface and loopback interface.
The only thing left now is to decide whether you want to advertise the loopback interfaces under OSPF. There are pros and cons to using an address that won’t be advertised
versus using an address that will be. Using an unadvertised address saves on real IP address
space, but the address won’t appear in the OSPF table, which means you can’t ping it.
So basically, what you’re faced with here is a choice that equals a trade-off between the
ease of debugging the network and conservation of address space—what to do? A really
tight strategy is to use a private IP address scheme as I did. Do this and all will be well!
Now that we’ve configured all the routers with OSPF, what’s next? Miller time? Nope—
not yet. It’s that verification thing again. We still have to make sure that OSPF is really working, and that’s exactly what we’re going to do next.
Verifying OSPF Configuration
There are several ways to verify proper OSPF configuration and operation, so next, I’m going
to demonstrate the various OSPF show commands you need to know in order to achieve this.
We’re going to start by taking a quick look at the routing table of the Corp router.
First, let’s issue a show ip route command on the Corp router:
O [110/65] via, 1d17h, Serial0/0 is subnetted, 1 subnets is subnetted, 1 subnets
C is directly connected, Loopback0 is subnetted, 4 subnets
C is directly connected, Serial0/1
L is directly connected, Serial0/1
406 Chapter 9 u Open Shortest Path First (OSPF)
C is directly connected, Serial0/0
L is directly connected, Serial0/0
O [110/65] via, 1d17h, Serial0/1 is subnetted, 2 subnets
C is directly connected, FastEthernet0/0
L is directly connected, FastEthernet0/0
The Corp router shows only two dynamic routes for the internetwork, with the O representing OSPF internal routes. The Cs are clearly our directly connected networks, and our
two remote networks are showing up too—nice! Notice the 110/65, which is our adminis￾trative distance/metric.
Now that’s a really sweet-looking OSPF routing table! It’s important to make it easier to
troubleshoot and fix an OSPF network, which is why I always use the show ip int brief
command when configuring my routing protocols. It’s very easy to make little mistakes
with OSPF, so keep your eyes on the details!
It’s time to show you all the OSPF verification commands that you need in your toolbox
for now.
The show ip ospf Command
The show ip ospf command is what you’ll need to display OSPF information for one or all
OSPF processes running on the router. Information contained therein includes the router
ID, area information, SPF statistics, and LSA timer information. Let’s check out the output
from the Corp router:
Corp#sh ip ospf
 Routing Process "ospf 1" with ID
 Start time: 00:08:41.724, Time elapsed: 2d16h
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Incremental-SPF disabled
 Minimum LSA interval 5 secs
 Minimum LSA arrival 1000 msecs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
 Number of external LSA 0. Checksum Sum 0x000000
Verifying OSPF Configuration 407
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
 IETF NSF helper support enabled
 Cisco NSF helper support enabled
 Number of interfaces in this area is 3
 Area has no authentication
 SPF algorithm last executed 00:11:08.760 ago
 SPF algorithm executed 5 times
 Area ranges are
 Number of LSA 6. Checksum Sum 0x03B054
 Number of opaque link LSA 0. Checksum Sum 0x000000
 Number of DCbitless LSA 0
 Number of indication LSA 0
 Number of DoNotAge LSA 0
 Flood list length 0
Notice the router ID (RID) of, which is the highest IP address configured on the router. Hopefully, you also noticed that I set the RID of the corporate router to
the highest available IP address available with IPv4.
The show ip ospf database Command
Using the show ip ospf database command will give you information about the number
of routers in the internetwork (AS) plus the neighboring router’s ID—the topology database
I mentioned earlier. Unlike the show ip eigrp topology command, this command reveals
the OSPF routers, but not each and every link in the AS like EIGRP does.
The output is broken down by area. Here’s a sample output, again from Corp:
Corp#sh ip ospf database
 OSPF Router with ID ( (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count 966 0x80000001 0x007162 1
408 Chapter 9 u Open Shortest Path First (OSPF) 885 0x80000002 0x00D27E 1 886 0x8000007A 0x00BC95 3 1133 0x8000007A 0x00E348 3 925 0x8000004D 0x000B90 5
 Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum 884 0x80000002 0x008CFE
You can see all the routers and the RID of each router—the highest IP address on each
of them. For example, the Link ID and ADV Router of my new Boulder router shows up
twice: once with the directly connected IP address ( and as the RID that I set
under the OSPF process (
The router output shows the link ID—remember that an interface is also a link—and the
RID of the router on that link under the ADV router, or advertising router.
The show ip ospf interface Command
The show ip ospf interface command reveals all interface-related OSPF information.
Data is displayed about OSPF information for all OSPF-enabled interfaces or for specified
interfaces. I’ll highlight some of the more important factors for you. Check it out:
Corp#sh ip ospf int f0/0
FastEthernet0/0 is up, line protocol is up
 Internet Address, Area 0
 Process ID 1, Router ID, Network Type BROADCAST, Cost: 1
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID), Interface address
 Backup Designated router (ID), Interface address
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
 oob-resync timeout 40
 Hello due in 00:00:08
 Supports Link-local Signaling (LLS)
 Cisco NSF helper support enabled
 IETF NSF helper support enabled
 Index 3/3, flood queue length 0
 Next 0x0(0)/0x0(0)
 Last flood scan length is 1, maximum is 1
Verifying OSPF Configuration 409
 Last flood scan time is 0 msec, maximum is 0 msec
 Neighbor Count is 1, Adjacent neighbor count is 1
 Adjacent with neighbor 172.31.1. Suppress hello for 0 neighbor(s)
Okay—so this command has given us the following information:
uu Interface IP address
uu Area assignment
uu Process ID
uu Router ID
uu Network type
uu Cost
uu Priority
uu DR/BDR election information (if applicable)
uu Hello and Dead timer intervals
uu Adjacent neighbor information
The reason I used the show ip ospf interface f0/0 command is because I knew that
there would be a designated router elected on the FastEthernet broadcast multi-access network between our Corp and Boulder routers. The information that I highlighted is all very
important, so make sure you’ve noted it! A good question to ask you here is what are the
Hello and Dead timers set to by default?
Type in the show ip ospf interface command and receive this response:
Corp#sh ip ospf int f0/0
%OSPF: OSPF not enabled on FastEthernet0/0
This error occurs when OSPF is enabled on the router, but not the interface. When this
happens, you need to check your network statements because it means that the interface
you’re trying to verify is not in your OSPF process!
The show ip ospf neighbor Command
The show ip ospf neighbor command is super-useful because it summarizes the pertinent
OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that
information will also be displayed. Here’s a sample:
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface 1 FULL/BDR 00:00:34 FastEthernet0/0 0 FULL/ - 00:00:31 Serial0/1 0 FULL/ - 00:00:32 Serial0/0

410 Chapter 9 u Open Shortest Path First (OSPF)
An Admin Connects Two Disparate Routers Together with OSPF and the
Link Between them Never Comes Up
Quite a few years ago, an admin called me in a panic because he couldn’t get OSPF working between two routers, one of which was an older router that they needed to use while
they were waiting for their new router to be shipped to them.
OSPF can be used in a multi-vendor network so he was confused as to why this wasn’t
working. He turned on RIP and it worked so he was super confused with why OSPF was
not creating adjacencies. I had him use the show ip ospf interface command to look at
the link between the two routers and sure enough, the hello and dead timers didn’t match.
I had him configure the mismatched parameters so they would match, but it still wouldn’t
create an adjacency. Looking more closely at the show ip ospf interface command, I
noticed the cost did not match! Cisco calculated the bandwidth differently than the other
vendor. Once I had him configure both as the same value, the link came up! Always remember, just because OSPF can be used in a multi-vendor network, does not mean it will work
out of the box!
This is a critical command to understand because it’s extremely useful in production
networks. Let’s take a look at the Boulder router output:
Boulder>sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface 1 FULL/DR 00:00:31 FastEthernet0/0
Okay—here we can see that since there’s an Ethernet link (broadcast multi-access) on
the link between the Boulder and the Corp router, there’s going to be an election to determine who will be the designated router (DR) and who will be the backup designated router
(BDR). We can see that the Corp became the designated router, and it won because it had
the highest IP address on the network—the highest RID.
Now the reason that the Corp connections to SF and LA don’t have a DR or BDR listed
in the output is that by default, elections don’t happen on point-to-point links and they
show FULL/ - . But we can still determine that the Corp router is fully adjacent to all three
routers from its output.

The show ip protocols Command
The show ip protocols command is also highly useful, whether you’re running OSPF,
EIGRP, RIP, BGP, IS-IS, or any other routing protocol that can be configured on your router.
It provides an excellent overview of the actual operation of all currently running protocols!
Check out the output from the Corp router:
Corp#sh ip protocols
Routing Protocol is "ospf 1"
 Outgoing update filter list for all interfaces is not set
 Incoming update filter list for all interfaces is not set
 Router ID
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Maximum path: 4
 Routing for Networks: area 0 area 0 area 0
 Reference bandwidth unit is 100 mbps
 Routing Information Sources:
 Gateway Distance Last Update 110 00:21:53 110 00:21:53
 Distance: (default is 110) Distance: (default is 110)
From looking at this output, you can determine the OSPF process ID, OSPF router ID,
type of OSPF area, networks and areas configured for OSPF, and the OSPF router IDs of
neighbors—that’s a lot. It’s super-efficient!
This chapter gave you a great deal of information about OSPF. It’s really difficult to include
everything about OSPF because so much of it falls outside the scope of this chapter and book,
but I’ve given you a few tips here and there, so you’re good to go—as long as you make sure
you’ve got what I presented to you dialed in, that is!
I talked about a lot of OSPF topics, including terminology, operations, and configuration
as well as verification and monitoring.

Each of these topics encompasses quite a bit of information—the terminology section
just scratched the surface of OSPF. But you’ve got the goods you really need for your
studies. Finally, I gave you a tight survey of commands highly useful for observing the
operation of OSPF so you can verify that things are moving along as they should. So eat
it all up, and you’re set!
Exam Essentials
Compare OSPF and RIPv1. OSPF is a link-state protocol that supports VLSM and classless
routing; RIPv1 is a distance-vector protocol that does not support VLSM and supports only
classful routing.
Know how OSPF routers become neighbors and/or adjacent. OSPF routers become
neighbors when each router sees the other’s Hello packets.
Be able to configure single-area OSPF. A minimal single-area configuration involves only
two commands: router ospf process-id and network x.x.x.x y.y.y.y area Z.
Be able to verify the operation of OSPF. There are many show commands that provide useful details on OSPF, and it is useful to be completely familiar with the output of each: show
ip ospf, show ip ospf database, show ip ospf interface, show ip ospf neighbor, and
show ip protocols.

Written Lab 9
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. Write the command that will enable the OSPF process 101 on a router.
2. Write the command that will display details of all OSPF routing processes enabled on
a router.
3. Write the command that will display interface-specific OSPF information.
4. Write the command that will display all OSPF neighbors.
5. Write the command that will display all different OSPF route types that are currently
known by the router.
Hands-on Labs
In this section, you will use the following network and add OSPF routing.
Fa0/0 Fa0/0
S0/0 DCE
Lab A Lab B Lab C
The first lab (Lab 9.1) requires you to configure three routers for OSPF and then view
the configuration. Note that the labs in this chapter were written to be used with real
equipment—but they can be used with any router simulator. You can replace the WAN
links with Ethernet links if you want to.
The labs in this chapter are as follows:
Lab 9.1: Enabling the OSPF Process
Lab 9.2: Configuring OSPF Interfaces
Lab 9.3: Verifying OSPF Operation
Table 9.5 shows our IP addresses for each router (each interface uses a /24 mask).

414 Chapter 9 u Open Shortest Path First (OSPF)
Table 9.5 Our IP addresses
Router Interface IP address
Lab_A Fa0/0
Lab_A S0/0
Lab_B S0/2
Lab_B S0/1
Lab_C S0/0
Lab_C Fa0/0
Hands-on Lab 9.1: Enabling the OSPF Process
This is the first mandatory step in OSPF configuration.
1. Enable OSPF process 100 on Lab_A:
Lab_A#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_A (config)#router ospf 100
Lab_A (config-router)#^Z
2. Enable OSPF process 101 on Lab_B:
Lab_B#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_B (config)#router ospf 101
Lab_B (config-router)#^Z
3. Enable OSPF process 102 on Lab_C:
Lab_C#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_C (config)#router ospf 102
Lab_C (config-router)#^Z
Hands-on Lab 9.2: Configuring OSPF Interfaces
The second mandatory step in OSPF is adding your network statements.
1. Configure the LAN and the network between Lab_A and Lab_B. Assign it to area 0.
Lab_A#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_A (config)#router ospf 100
Lab_A (config-router)#network area 0
Lab_A (config-router)#network area 0
Lab_A (config-router)#^Z
Lab_A #
2. Configure the networks on the Lab_B router. Assign them to area 0.
Lab_B#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_B(config)#router ospf 101
Lab_B(config-router)#network area 0
Lab_B(config-router)#network area 0
Lab_B #
3. Configure the networks on the Lab_C router. Assign them to area 0.
Lab_C#conf t
Enter configuration commands, one per line.
 End with CNTL/Z.
Lab_C(config)#router ospf 102
Lab_C(config-router)#network area 0
Lab_C(config-router)#network area 0

Hands-on Lab 9.3: Verifying OSPF Operation
You need to be able to verify what you configure.
1. Execute a show ip ospf neighbors command from the Lab_A router and view
the results.
Lab_A#sho ip ospf neig
2. Execute a show ip route command to verify that all other routers are learning
all routes.
Lab_A#sho ip route
3. Execute a show ip protocols command to verify OSPF information.
Lab_A#sho ip protocols
4. Execute a show ip OSPF command to verify your RID.
Lab_A#sho ip ospf
5. Execute a show ip ospf interface f0/0 command to verify your timers.
Lab_A#sho ip ospf int f0/0
Review Questions 417
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques￾tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. There are three possible routes for a router to reach a destination network. The first
route is from OSPF with a metric of 782. The second route is from RIPv2 with a metric
of 4. The third is from EIGRP with a composite metric of 20514560. Which route will
be installed by the router in its routing table?
A. RIPv2
D. All three
2. In the accompanying diagram, which of the routers must be ABRs? (Choose all
that apply.)
Area 1
Area 2
Area 3
Backbone Area
A. C
B. D
C. E
D. F
E. G
F. H
3. Which of the following describe the process identifier that is used to run OSPF on a
router? (Choose two.)
A. It is locally significant.
B. It is globally significant.
C. It is needed to identify a unique instance of an OSPF database.
D. It is an optional parameter required only if multiple OSPF processes are running
on the router.
E. All routes in the same OSPF area must have the same process ID if they are to
exchange routing information.
4. All of the following must match for two OSPF routers to become neighbors except which?
A. Area ID
B. Router ID
C. Stub area flag
D. Authentication password if using one
5. In the diagram, by default what will be the router ID of Lab_B?
Fa0/0 Fa0/0 Fa0/0
G0/0 G0/1 G0/1 G0/0
Lab A Lab B Lab C
6. You get a call from a network administrator who tells you that he typed the following
into his router:
Router(config)#router ospf 1
Router(config-router)#network area 0
He tells you he still can’t see any routes in the routing table. What configuration error
did the administrator make?
A. The wildcard mask is incorrect.
B. The OSPF area is wrong.
C. The OSPF process ID is incorrect.
D. The AS configuration is wrong.
7. Which of the following statements is true with regard to the output shown?
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface 1 FULL/BDR 00:00:34 FastEthernet0/0 0 FULL/ - 00:00:31 Serial0/1 0 FULL/ - 00:00:32 Serial0/0
A. There is no DR on the link to
B. The Corp router is the BDR on the link to
C. The Corp router is the DR on the link to
D. The link to is Active.
8. What is the administrative distance of OSPF?
A. 90
B. 100
C. 120
D. 110
9. In OSPF, Hellos are sent to what IP address?
10. What command generated the following output? 1 FULL/BDR 00:00:34 FastEthernet0/0 0 FULL/ - 00:00:31 Serial0/1 0 FULL/ - 00:00:32 Serial0/0
A. show ip ospf neighbor
B. show ip ospf database
C. show ip route
D. show ip ospf interface
11. Updates addressed to are destined for which type of OSPF router?
D. All OSPF routers
12. For some reason, you cannot establish an adjacency relationship on a common Ethernet
link between two routers. Looking at this output, what is the cause of the problem?
Ethernet0/0 is up, line protocol is up
 Internet Address, Area 0
 Process ID 2, Router ID, Network Type BROADCAST, Cost: 10
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID), interface address
 No backup designated router on this network
 Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5
Ethernet0/0 is up, line protocol is up
 Internet Address, Area 0
 Process ID 2, Router ID, Network Type BROADCAST, Cost: 10
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID), interface address
 No backup designated router on this network
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
A. The OSPF area is not configured properly.
B. The priority on RouterA should be set higher.
C. The cost on RouterA should be set higher.
D. The Hello and Dead timers are not configured properly.
E. A backup designated router needs to be added to the network.
F. The OSPF process ID numbers must match.
13. In the work area match each OSPF term (by line) to its definition.
Designated router contains only the best routes
Topological database elected on broadcast networks
Hello protocol contains all routes learned
Routing table provides dynamic neighbor discovery
14. Type the command that will disable OSPF on the Fa0/1 interface under the routing
process. Write only the command and not the prompt.
15. Which two of the following commands will place network into area 0?
(Choose two.)
A. router eigrp 10
B. router ospf 10
C. router rip
D. network
E. network area 0
F. network area0
G. network area 0
16. Given the following output, which statement or statements can be determined to be
true? (Choose all that apply.)
RouterA2# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface 1 FULL/BDR 00:00:29 FastEthernet1/0 2 FULL/BDR 00:00:24 FastEthernet0/0 1 FULL/- 00:00:33 Serial0/1 1 FULL/DR 00:00:32 FastEthernet0/1 1 FULL/DR 00:00:20 FastEthernet0/2 1 FULL/BDR 00:00:23 FastEthernet0/1
<<output omitted>>
A. The DR for the network connected to Fa0/0 has an interface priority higher than 2.
B. This router (A2) is the BDR for subnet
C. The DR for the network connected to Fa0/1 has a router ID of
D. The DR for the serial subnet is
17. What are three reasons for creating OSPF in a hierarchical design? (Choose three.)
A. To decrease routing overhead
B. To speed up convergence
C. To confine network instability to single areas of the network
D. To make configuring OSPF easier
18. Type the command that produced the following output. Write only the command and
not the prompt.
FastEthernet0/0 is up, line protocol is up
 Internet Address, Area 0
 Process ID 1, Router ID, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID), Interface address
Backup Designated router (ID), Interface address
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
 oob-resync timeout 40
 Hello due in 00:00:08
 Supports Link-local Signaling (LLS)
 Cisco NSF helper support enabled
 IETF NSF helper support enabled
 Index 3/3, flood queue length 0
 Next 0x0(0)/0x0(0)
 Last flood scan length is 1, maximum is 1
 Last flood scan time is 0 msec, maximum is 0 msec
 Neighbor Count is 1, Adjacent neighbor count is 1
 Adjacent with neighbor 172.31.1. Suppress hello for 0 neighbor(s)
19. A(n) ____________is an OSPF data packet containing link-state and routing informa￾tion that are shared among OSPF routers.
C. Hello
20. If routers in a single area are configured with the same priority value, what value does
a router use for the OSPF router ID in the absence of a loopback interface?
A. The lowest IP address of any physical interface
B. The highest IP address of any physical interface
C. The lowest IP address of any logical interface
D. The highest IP address of any logical interface


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