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NETWORK BASICS

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…

Multi-Area OSPF


Image result for Multi-Area OSPF"1 IP Routing Technologies

■ Configure and verify OSPF (single area)
■ Neighbor adjacencies
■ OSPF states
■ Discuss Multi-area
■ Configure OSPF v2
Configure OSPF v3
■ Router ID
■ LSA types

1 Troubleshooting

■ Troubleshoot and resolve routing issues
Routing is enabled
■ Routing table is correct
■ Correct path selection
■ Troubleshoot and resolve OSPF problems
■ Neighbor adjacencies
■ Hello and Dead timers
■ OSPF area
■ Interface MTU
Network types
■ Neighbor states
■ OSPF topology database

We’ll begin this chapter by focusing on the scalability constraints 
of an Open Shortest Path First (OSPF) network with a single 
area and move on from there to explore the concept of multi-area OSPF as a solution to these scalability limitations.

I’ll also identify and introduce you to the various categories of routers used in multi-area 
configurations, including backbone routers, internal routers, area border routers (ABRs), 
and autonomous system boundary routers (ASBRs).
The functions of different OSPF Link-State Advertisements (LSAs) are absolutely crucial 
for you to understand for success in taking the Cisco exam, so I’ll go into detail about the 
types of LSAs used by OSPF, as well as, the Hello protocol and different neighbor states 
when an adjacency is taking place.

And because troubleshooting is always a vital skill to have, I’ll guide you through the process with a collection of show commands that can be effectively used to monitor and troubleshoot a multi-area OSPF implementation. Finally, I’ll end the chapter with the easiest part: 
configuring and verifying OSPFv3.
To find up-to-the-minute updates for this chapter, please see 
www.lammle.com/forum or the book’s web page at www.sybex.com.
OSPF Scalability
At this point, and before you read this chapter, be sure that you have the foundation of single area OSPF down pat. I’m sure you remember OSPF’s significant advantage over distance vector protocols like RIP, due to OSPF’s ability to represent an entire network within its 
link-state database, which dramatically reduces the time required for convergence!
But what does a router actually go through to give us this great performance? Each router 
recalculates its database every time there’s a topology change. If you have numerous routers 
in an area, they’ll clearly have lots of links. Every time a link goes up or down, an LSA Type 1 
packet is advertised, forcing all of the routers in the same area to recalculate their shortest path 
first (SPF) tree. Predictably, this kind of heavy lifting requires a ton of CPU overhead. On top of that, each router must hold the entire link-state database that represents the topology of the entire network, which results in considerable memory overhead. As if all that weren’t enough, each router also holds a complete copy of the routing table, adding more to the already heavy overhead burden on memory. And keep in mind that the number of entries in the routing table OSPF Scalability can be much greater than the number of networks in the routing table because there are typically multiple routes to to the same remote networks!
Considering these OSPF factors, it’s easy to imagine that in a really large network, single area OSPF presents some serious scalability challenges,  We’ll move on in a bit to compare the single-area OSPF network in that illustration to our multi-area networks.

OSPF single-area network: All routers flood the network with link-state information to all other routers within the same area.
Area 0
Single-area OSPF design places all routers into a single OSPF area, which results in many 
LSAs being processed on every router.
Fortunately, OSPF allows us to take a large OSPF topology and break it down into 
multiple, more manageable areas, as illustrated in OSPF multi-area network: All routers flood the network only within 
their area.
Area 1 Area 2
Area 0

Multi-Area OSPF

Just take a minute to think about the advantages of this hierarchical approach. First, routers that are internal to a defined area don’t need to worry about having a link-state database 
for the entire network because they need one for only their own areas. This factor seriously 
reduces memory overhead! Second, routers that are internal to a defined area now have to 
recalculate their link-state database only when there’s a topology change within their given 
area. Topology changes in one area won’t cause global OSPF recalculations, further reducing processor overhead. Finally, because routes can be summarized at area boundaries, the routing tables on each router just don’t need to be nearly as huge as they would be in a single-area environment! 
But of course there’s a catch: As you start subdividing your OSPF topology into multiple 
areas, the configuration gets more complex, so we’ll explore some strategic ways to finesse 
the configuration plus look at some cool tricks for effectively troubleshooting multi-area 

OSPF networks.

Categories of Multi-area Components In the following sections, I’m going to cover the various roles that routers play in a multi-area OSPF network. You’ll find routers serving as backbone routers, internal routers, area 
border routers, and autonomous system boundary routers. I’ll also introduce you to the 
different types of advertisements used in an OSPF network.
Link-State Advertisements (LSAs) describe a router and the networks that are connected 
to it by sending the LSAs to neighbor routers. Routers exchange LSAs and learn the complete topology of the network until all routers have the exact same topology database. After the topology database is built, OSPF uses the Dijkstra algorithm to find the best path to each remote network and places only the best routes into the routing table. 
Adjacency Requirements
Once neighbors have been identified, adjacencies must be established so that routing (LSA) 
information can be exchanged. There are two steps required to change a neighboring OSPF 
router into an adjacent OSPF router:
1. Two-way communication (achieved via the Hello protocol)
2. Database synchronization, which consists of three packet types being exchanged 
between routers:
 Database Description (DD) packets
 Link-State Request (LSR) packets
 Link-State Update (LSU) packets
Once database synchronization is complete, the two routers are considered adjacent. 
This is how adjacency is achieved, but you need to know when an adjacency will occur.
Categories of Multi-area Components 
It’s important to remember that neighbors will not form an adjacency if the following do 
not match:
Area ID
Subnet
Hello and dead timers
Authentication (if configured)
When adjacencies form, depends on the network type. If the link is point-to-point, the 
two neighbors will become adjacent if the Hello packet information for both routers is configured properly. On broadcast multi-access networks, adjacencies are formed only between the OSPF routers on the network and the DR and BDR.
OSPF Router Roles Routers within a multi-area OSPF network fall into different categories. 
to see the various roles that routers can play.
Router roles: Routers within an area are called internal routers.
Area 2
Boston Norfolk
To a different AS
ABR ABR
San Jose
Area 1
SF
Area 0
ASBR Corp
NY
Oakland
Notice that there are four routers that are part of area 0: the Corp router, SF and NY, 
and the autonomous system border router (ASBR). When configuring multi-area OSPF, one 
area must be called area 0, referred to as the backbone area. All other areas must connect 
to area 0. The four routers are referred to as the backbone routers, which are any routers 
that exist either partially or completely in OSPF area 0. 
Another key distinction about the SF and NY routers connecting to other areas is that 
they have interfaces in more than one area. This makes them area border routers (ABRs)
because in addition to having an interface in area 0, SF has an interface in area 1 and NY 
has an interface in area 2.

Multi-Area OSPF

An ABR is a router that belongs to more than one OSPF area. It maintains information 
from all directly connected areas in its topology table but doesn’t share the topological 
details from one area with the other. But it will forward routing information from one area 
to the other. The key concept here is that an ABR separates the LSA flooding zone, is a primary point for area address summarization, and typically has the source default route, all 
while maintaining the link-state database (LSDB) for each area it’s connected to. 
Remember that a router can play more than one role. In Figure 20.3, SF and 
NY are both backbone routers and area border routers.
Let’s turn our focus to the San Jose and Oakland routers. You can see that all interfaces 
on both of these routers reside only in area 
1. Because all of San Jose’s and Oakland’s interfaces are internal to a single area, they’re called internal routers. 
An internal router is any router with all of its interfaces included as members of the same area. This also applies to the Boston and Norfolk routers and their relationship to area 2. The Corp router is internal to area 0.
Finally, the ASBR is unique among all routers in our example because of its connection to 
an external autonomous system (AS). When an OSPF network is connected to an EIGRP network, a Border Gateway Protocol (BGP) network, or a network running any other external routing process, it’s referred to as an AS.
An autonomous system boundary router (ASBR) is an OSPF router with at least one 
interface connected to an external network or different AS. A network is considered external if it’s either running a routing protocol other than OSPF. An ASBR is responsible for 
injecting route information learned via the external network into OSPF.
I want to point out that an ASBR doesn’t automatically exchange routing information 
between its OSPF routing process and the external routing process that it’s connected to. 
These routes are exchanged through a method called route redistribution, which is beyond 
the scope of this book.
Link-state Advertisements
You know that a router’s link-state database is made up of Link-State Advertisements (LSAs). 
But just as there are several OSPF router categories to remember, there are also various types of LSAs to keep in mind—five of them, to be exact. These LSA classifications may not seem important at first, but you’ll see why they are when we cover how the various types of OSPF areas operate. Let’s start by exploring the different types of LSAs that Cisco uses: Type 1 LSA Referred to as a router link advertisement (RLA), or just router LSA, a Type 1 LSA is sent by every router to other routers in its area. This advertisement contains the status of a router’s link in the area to which it is connected. If a router is connected to multiple areas, then it will send separate Type 1 LSAs for each of the areas it’s connected to. Type 1 LSAs contain the router ID (RID), interfaces, IP information, and current interface state. For example, 

Categories of Multi-area Components 

in the network  router SF will send an LSA Type 1 advertisement for its interface into area 0 and a separate LSA Type 1 advertisement for its interfaces into area 1 describing the state of its links. The same will happen with the other routers 
Type 1 Link-state Advertisements
Area 2
Boston Norfolk
To a different AS
ABR ABR
San Jose
Area 1
SF
Area 0
ASBR Corp
NY
Oakland
Type 1
Type 1
Type 1
Type 1
Type 1
Type 1
Type 1: Here is the status of my links!
Type 2 LSA Referred to as a network link advertisement (NLA), a Type 2 LSA is generated by designated routers (DRs). Remember that a designated router is elected to represent 
other routers in its network, and it establishes adjacencies with them. The DR uses a Type 2 
LSA to send out information about the state of other routers that are part of the same network. Note that the Type 2 LSA is flooded to all routers that are in the same area as the one containing the specific network but not to any outside of that area. These updates contain the DR and BDR IP information.
Type 3 LSA Referred to as a summary link advertisement (SLA), a Type 3 LSA is generated 
by area border routers. These ABRs send Type 3 LSAs toward the area external to the one 
where they were generated. The Type 3 LSA advertises networks, and these LSAs advertise 
inter-area routes to the backbone area (area 0). Advertisements contain the IP information and RID of the ABR that is advertising an LSA Type 3.
The word summary often invokes images of a summarized network address 
that hides the details of many small subnets within the advertisement of a 
single large one. But in OSPF, Summary Link Advertisements don’t necessarily contain network summaries. Unless the administrator manually creates a 
summary, the full list of individual networks available within an area will be 
advertised by the SLAs.
854 Chapter 20 u Multi-Area OSPF
Type 4 LSA Type 4 LSAs are generated by area border routers. These ABRs send a Type 4 
LSA toward the area external to the one in which they were generated. These are also summary LSAs like Type 3, but Type 4 are specifically used to inform the rest of the OSPF areas how to get to the ASBR. 
Type 5 LSA Referred to as AS external link advertisements, a Type 5 LSA is sent by autonomous system boundary routers to advertise routes that are external to the OSPF autonomous system and are flooded everywhere. A Type 5 LSA is generated for each individual external 
network advertised by the ASBR.
Figure 20.5 shows how each LSA type would be used in a multi-area OSPF network.
F ig u re 20.5 Basic LSA types
LSA Type Description
1 Router LSA
2 Network LSA
3 Summary LSA
4 ASBR Summary LSA
5 Autonomous system LSA
Area 1 Area 0
ASBR ABR
Corp
Type 4
Type 1 or 2 Type 1 or 2
Type 3
Type 5
It’s important to understand the different LSA types and how they work. Looking at 
Figure 20.5, you can see that Type 1 and 2 are flooded between routers in their same area. 
Type 3 LSAs from the Corp router (which is an ABR and maintains the LSDB for each area 
it is connected to) will summarize information learned from area 1 into area 0 and vice 
versa. The ASBR will flood Type 5 LSAs into area 1, and the Corp router will then flood 
Type 4 LSAs into area 0, telling all routers how to get to the ASBR, basically becoming a 
proxy ASBR. 
OSPF Hello Protocol
The Hello protocol provides a lot of information to neighbors. The following is communicated 
between neighbors, by default, every 10 seconds:
Router ID (RID) This is the highest active IP address on the router. The highest loopback 
IP addresses are used first. If no loopback interfaces are configured, OSPF will choose from 
physical interfaces instead.

Categories of Multi-area Components 

Hello/Dead interval The period between Hello packets is the Hello time, which is 10 seconds by default. The dead time is the length of time allotted for a Hello packet to be received before a neighbor is considered down—four times the Hello interval, unless otherwise configured.
Neighbors The information includes a list of the router IDs for all the originating router’s 
neighbors; neighbors being defined as routers that are attached to a common IP subnet and 
use identical subnet masks.
Area ID This represents the area that the originating router interface belongs to.
Router priority The priority is an 8-bit value used to aid in the election of the DR and 
BDR. This isn’t set on point-to-point links!
DR IP address This is the router ID of the current DR.
BDR IP address This is the router ID of the current BDR.
Authentication data This is the authentication type and corresponding information 
(if configured).
The mandatory information within the Hello update that must match exactly are the 
hello and dead timer values intervals, area ID, OSPF area type, subnet, and authentication 
data if used. If any of those don’t match perfectly, no adjacency will occur!
Neighbor States Before we move on to configuration, verification, and troubleshooting OSPF, it’s important for you to grasp how OSPF routers traverse different states when adjacencies are being established.
When OSPF routers are initialized, they first start exchanging information using the 
Hello protocol via the multicast address 224.0.0.5. After the neighbor relationship is 
established between routers, the routers synchronize their link-state database (LSDB) by 
reliably exchanging LSAs. They actually exchange quite a bit of vital information when 
they start up.
The relationship that one router has with another consists of eight possible states. All 
OSPF routers begin in the DOWN state, and if all is well, they’ll progress to either the 2WAY or FULL state with their neighbors. Figure 20.6 shows this neighbor state progression.
F ig u re 20.6 OSPF neighbor states, part 1
Corp Branch
DOWN state
INIT state
2WAY state
224.0.0.5
Hello?
 Multi-Area OSPF
The process starts by sending out Hello packets. Every listening router will then add 
the originating router to the neighbor database. The responding routers will reply with all 
of their Hello information so that the originating router can add them to its own neigh￾bor table. At this point, we will have reached the 2WAY state—only certain routers will 
advance beyond this to establish adjacencies.
Here’s a definition of the eight possible relationship states:
DOWN In the DOWN state, no Hello packets have been received on the interface. Bear 
in mind that this does not imply that the interface itself is physically down.
ATTEMPT In the ATTEMPT state, neighbors must be configured manually. It applies 
only to non-broadcast multi-access (NBMA) network connections.
INIT In the INIT state, Hello packets have been received from another router. Still, 
the absence of the Router ID for the receiving router in the Neighbor field indicates that 
bidirectional communication hasn’t been established yet.
2WAY In the 2WAY state, Hello packets that include their own router ID in the Neighbor 
field have been received. Bidirectional communication has been established. In broadcast 
multi-access networks, an election can occur after this point.
After the DR and BDR have been selected, the routers will enter into the EXSTART state 
and the routers are ready to discover the link-state information about the internetwork and 
create their LSDB. This process is illustrated in Figure 20.7.
F ig u re 20.7 OSPF router neighbor states, part 2
Corp Branch
LSDB summary
EXSTART state
EXCHANGE state
LOADING state
FULL state
LSAck
I need info on a network!
Here is that info!
LSAck
LSU
LSU
EXSTART In the EXSTART state, the DR and BDR establish adjacencies with each router 
in the network. A master-slave relationship is created between each router and its adjacent 
DR and DBR. The router with the highest RID becomes the master, and the master-slave 
election dictates which router will start the exchange. Once routers exchange DBD packets, 
the routers will move into the EXCHANGE state.
Basic Multi-area Configuration 857
EXCHANGE In the EXCHANGE state, routing information is exchanged using 
Database Description (DBD or DD) packets and Link-State Request (LSR) and Link-State 
Update packets may also be sent. When routers start sending LSRs, they’re considered to 
be in the LOADING state.
LOADING In the LOADING state, Link-State Request (LSR) packets are sent to 
neighbors to request any Link-State Advertisements (LSAs) that may have been missed 
or corrupted while the routers were in the EXCHANGE state. Neighbors respond with 
Link-State Update (LSU) packets, which are in turn acknowledged with Link-State 
Acknowledgement (LSAck) packets. When all LSRs have been satisfied for a given 
router, the adjacent routers are considered synchronized and enter the FULL state.
FULL In the FULL state, all LSA information is synchronized among neighbors and 
adjacency has been established. OSPF routing can begin only after the FULL state has 
been reached!
It’s important to understand that routers should be in the 2WAY and FULL states and the 
others are considered transitory. Routers shouldn’t remain in any other state for extended 
period of times. Let’s configure OSPF now to see what we’ve covered so far in action.
Basic Multi-area Configuration
Basic multi-area configuration isn’t all that hard. Understanding your design, layout, types of 
LSAs, DRs and configuring the elections, troubleshooting, and fully comprehending what’s 
happening in the background are really the most complicated aspects of OSPF.
As I was saying, configuring OSPF is pretty simple, and you’ll see toward the end of this 
chapter that configuring OSPFv3 is even easier! After I show you the basic OSPF multi-area 
configuration in this section, we’ll work on the verification of OSPF and then go through a 
detailed troubleshooting scenario just as we did with EIGRP. Let’s get the ball rolling with 
the multi-area configuration shown in Figure 20.8.
We’ll use the same routers we’ve been working with throughout all the chapters, but 
we’re going to create three areas. The routers are still configured with the IPv6 addresses 
from my last EIGRPv6 section in the previous chapter, and I’ve also verified that the IPv4 
addresses are on the interfaces and working as well since then, so we’re all set to rock the 
configs for this chapter! Here’s the Corp configuration:
Corp#config t
Corp(config)#router ospf 1
Corp(config-router)#router-id 1.1.1.1
Corp(config-router)#Reload or use "clear ip ospf process" command, for this to
take effect
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Corp(config-router)#network 10.10.0.0 0.0.255.255 area 0
Corp(config-router)#network 172.16.10.0 0.0.0.3 area 1
Corp(config-router)#network 172.16.10.4 0.0.0.3 area 2
F ig u re 20. 8 Our internetwork
s0/1
s0/0
Corp
SF
NY
10.10.10.0/24
g0/0
.2
10.10.11.0/24
g0/1
.6
Area 0
Area 1
Area 2
172.16.10.0/30
172.16.10.4/30
10.10.20.0/24
10.10.30.0/24
10.10.40.0/24
10.10.50.0/24
Pretty straightforward, but let’s talk about it anyway. First I started the OSPF process 
with the router ospf process-id command, using any number from 1–65,535 because 
they’re only locally significant so they don’t need to match my neighbor routers. I set the 
RID of the router only to remind you that this can be configured under the router process, 
but with our small network it wouldn’t really be necessary to mess with RIDs if this was 
an actual production network. The one thing that you need to keep in mind here is that 
in OSPF, the RID must be different on each router. With EIGRP, they can all be the same 
because they are not as important in that process. Still, as I showed you in the EIGRPv6 
section, we still need them!
Anyway, at this point in the configurations I needed to choose my network statements 
for the OSPF process to use, which allowed me to place my four interfaces on the Corp 
router into three different areas. In the first network statement, 10.10.0.0 0.0.255.255, 
placed the gi0/0 and gi0/1 interfaces into area 0. The second and third statements needed to 
be more exact since there are /30 networks. 172.16.10.0 0.0.0.3 tells OSPF process 1 to go 
find an active interface that’s configured with 172.16.10.1 or .2 and to place that interface 
into area 
1. The last line tells the OSPF process to go find any active interface configured 
with 172.16.10.5 or .6 and place that interface into area 2. The wildcard of 0.0.0.3 means 
the first three octets can match any value, but the last octet is a block size of 4. 
The only thing different about these configurations from those in the single-area OSPF is 
the different areas at the end of the command—that’s it!
Verifying and Troubleshooting Multi-area OSPF Networks 859
Here is the configuration for the SF and NY routers: 
SF(config)#router ospf 1
SF(config-router)#network 10.10.0.0 0.0.255.255 area 1
SF(config-router)#network 172.16.0.0 0.0.255.255 area 1
NY(config)#router ospf 1
NY(config-router)#network 0.0.0.0 255.255.255.255 area 2
00:01:07: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0/0/0 from LOADING to 
FULL, 
Loading Done
I configured each one slightly different from the Corp router, but since they didn’t have 
an interface in more than area 1, I had more leeway in configuring them. For the NY router 
I just configured a network statement (0.0.0.0 255.255.255.255) that says “go find any 
active interface and place it into area 2!” I’m not recommending that you configure your 
routers in such a broad manner; I just wanted to show you your options.
Now that our three routers are configured, let’s verify our internetwork.
Verifying and Troubleshooting 
Multi-area OSPF Networks
Cisco’s IOS has several show and debug commands that can help you monitor and troubleshoot OSPF networks. A sampling of these commands, which can be used to gain information about various OSPF characteristics, is included in Table 20.1.
Table 20.1 OSPF verification commands
Command Provides the following
show ip ospf neighbor Verifies your OSPF-enabled interfaces
show ip ospf interface Displays OSPF-related information on an OSPF-enabled 
interface
show ip protocols Verifies the OSPF process ID and that OSPF is enabled on 
the router
show ip route Verifies the routing table, and displays any OSPF injected routes
show ip ospf database Lists a summary of the LSAs in the database, with one line of 
output per LSA, organized by type
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 Multi-Area OSPF
Let’s go through some verification commands—the same commands we used to verify 
our single-area OSPF network—then we’ll move onto the OSPF troubleshooting scenario 
section.
Okay, once you’ve checked the link between your neighbors and can use the Ping program, 
the best command when verifying a routing protocol is to always check the status of your 
neighbor’s connection first. The show ip ospf neighbor command is super useful because it 
summarizes the pertinent OSPF information regarding neighbors and their 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
172.16.10.2 0 FULL/ - 00:00:34 172.16.10.2 Serial0/0/0
172.16.10.6 0 FULL/ - 00:00:31 172.16.10.6 Serial0/0/1
SF#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:39 172.16.10.1 Serial0/0/0
NY#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:34 172.16.10.5 Serial0/0/0
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 see that the Corp router is fully adjacent to all three routers from its output.
The output of this command shows the neighbor ID, which is the RID of the router. 
Notice in the output of the Corp router that the RIDs for the SF and NY routers were chosen 
based on highest IP address of any active interface when I started the OSPF process on those 
routers. Both the SF and NY routers see the Corp router RID as 1.1.1.1 because I set that 
manually under the router ospf process command.
Next we see the Pri field, which is the priority field that’s set to 1 by default. Don’t 
forget that on point-to-point links, elections don’t happen, so the interfaces are all set to 0 
in this example because none of these routers will have elections on these interfaces with 
each other over this serial WAN network. The state field shows Full/-, which means all 
routers are synchronized with their LSDB, and the /- means there is no election on this 
type of interface. The dead time is counting down, and if the router does not hear from 
this neighbor before this expires, the link will be considered down. The address is the 
actual address of the neighbor’s interface connecting to the router, and the interface is the 
neighbor’s interface—not your interface!
Verifying and Troubleshooting Multi-area OSPF Networks 861
The show ip ospf Command
We use the show ip ospf command 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 1.1.1.1
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 It is an area border router
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 Number of external LSA 0. Checksum Sum 0x000000
 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 3. 3 normal 0 stub 0 nssa
 External flood list length 0
 Area BACKBONE(0)
 Number of interfaces in this area is 2
 Area has no authentication
 SPF algorithm executed 19 times
 Area ranges are
 Number of LSA 7. Checksum Sum 0x0384d5
 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
 Area 1
 Number of interfaces in this area is 1
 Area has no authentication
 SPF algorithm executed 43 times
 Area ranges are
 Number of LSA 7. Checksum Sum 0x0435f8
 Number of opaque link LSA 0. Checksum Sum 0x000000
 Number of DCbitless LSA 0
862 Chapter 20 u Multi-Area OSPF
 Number of indication LSA 0
 Number of DoNotAge LSA 0
 Flood list length 0
 Area 2
 Number of interfaces in this area is 1
 Area has no authentication
 SPF algorithm executed 38 times
 Area ranges are
 Number of LSA 7. Checksum Sum 0x0319ed
 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
You’ll notice that most of the preceding information wasn’t displayed with this command 
output in single-area OSPF. We have more displayed here because it’s providing information about each area we’ve configured on this router.
The show ip ospf interface Command
The show ip ospf interface command displays all interface-related OSPF information. 
Data is displayed for all OSPF-enabled interfaces or for specified interfaces. I’ll highlight 
some important portions I want you to pay special attention to.
Corp#sh ip ospf interface gi0/0
GigabitEthernet0/0 is up, line protocol is up
 Internet address is 10.10.10.1/24, Area 0
 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID) 1.1.1.1, Interface address 10.10.10.1
 No backup designated router on this network
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Re-transmit 5
 Hello due in 00:00:05
 Index 1/1, 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 0, Adjacent neighbor count is 0
 Suppress hello for 0 neighbor(s)
Verifying and Troubleshooting Multi-area OSPF Networks 863
Let’s take a look at a serial interface so we can compare it to the Gigabit Ethernet inter￾face just shown. The Ethernet network is a broadcast multi-access network by default, and 
the serial interface is a point-to-point non-broadcast multi-access network, so they will act 
differently with OSPF:
Corp#sh ip ospf interface s0/0/0
Serial0/0/0 is up, line protocol is up
 Internet address is 172.16.10.1/30, Area 1
 Process ID 1, Router ID 1.1.1.1, Network Type POINT-TO-POINT, Cost: 64
 Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
 No designated router on this network
 No backup designated router on this network
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Re-transmit 5
 Hello due in 00:00:02
 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 172.16.10.2
 Suppress hello for 0 neighbor(s)
The following information is displayed via this command:
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
I used the show ip ospf interface gi0/0 command first because I knew that there 
would be a designated router elected on the Ethernet broadcast multi-access network on 
the Corp router, even though it has no one to run against, which means the Corp router 
automatically wins. The information that I bolded is all very important! What are the hello 
and dead timers set to by default? Even though I haven’t talked much about the cost out￾put on an interface, it can also be very important. Two OSPF routers still could create an 
adjacency if the costs don’t match, but it could lead to certain links not being utilized. We’ll 
discuss this more at the end of the verification section. 
864 Chapter 20 u Multi-Area OSPF
Neighbor Routers Don’t Form an Adjacency
I’d like to talk more about the adjacency issue and how the show ip ospf interface
command can help you solve problems, especially in multi-vendor networks. 
Years ago I was consulting with the folks at a large PC/laptop manufacturer and was helping them build out their large internetwork. They were using OSPF because their company 
was a worldwide company and used many types of routers from all manufacturers.
I received a call from a remote branch informing me that they installed a new router but it 
was not seeing the Cisco router off their Ethernet interface. Of course it was an emergency 
because this new router was holding some important WAN links to a new remote location 
that needed to be up yesterday!
After calming down the person on the phone, I simply had the admin use the show ip 
ospf interface fa0/0 command and verify the hello and dead timers and the area 
configured for that interface and then had him verify that the IP addresses were correct 
between routers and that there was no passive interface set.
Then I had him verify that same information on the neighbor, and sure enough the 
neighbors’ hello and dead timers didn’t match. Quick and easy fix on the interface of 
the Cisco router with the ip ospf dead 30 command, and they were up!
Always remember that OSPF can work with multi-vendor routers, but no one ever said it 
works out of the box between various vendors!
The show ip protocols Command
The show ip protocols command is also 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 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 1.1.1.1
 Number of areas in this router is 3. 3 normal 0 stub 0 nssa
Verifying and Troubleshooting Multi-area OSPF Networks 865
 Maximum path: 4
 Routing for Networks:
 10.10.0.0 0.0.255.255 area 0
 172.16.10.0 0.0.0.3 area 1
 172.16.10.4 0.0.0.3 area 2
 Routing Information Sources:
 Gateway Distance Last Update
 1.1.1.1 110 00:17:42
 172.16.10.2 110 00:17:42
 172.16.10.6 110 00:17:42
 Distance: (default is 110)
Here we can determine the OSPF process ID, OSPF router ID, type of OSPF area, 
networks, and the three areas configured for OSPF, as well as, the OSPF router IDs of 
neighbors—that’s a lot. Read efficient!
The show ip route Command
Now would be a great time to issue a show ip route command on the Corp router. The 
Corp router shows only four dynamic routes for our internetwork, with the O representing 
OSPF internal routes. The Cs clearly represent our directly connected networks, but our 
four remote networks are also showing up—nice! Notice the 110/65, which is the administrative distance/metric:
Corp#sh ip route
[output cut]
 10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/0
L 10.10.10.1/32 is directly connected, GigabitEthernet0/0
C 10.10.11.0/24 is directly connected, GigabitEthernet0/1
L 10.10.11.1/32 is directly connected, GigabitEthernet0/1
O 10.10.20.0/24 [110/65] via 172.16.10.2, 02:18:27, Serial0/0/0
O 10.10.30.0/24 [110/65] via 172.16.10.2, 02:18:27, Serial0/0/0
O 10.10.40.0/24 [110/65] via 172.16.10.6, 03:37:24, Serial0/0/1
O 10.10.50.0/24 [110/65] via 172.16.10.6, 03:37:24, Serial0/0/1
 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.16.10.0/30 is directly connected, Serial0/0/0
L 172.16.10.1/32 is directly connected, Serial0/0/0
C 172.16.10.4/30 is directly connected, Serial0/0/1
L 172.16.10.5/32 is directly connected, Serial0/0/1
866 Chapter 20 u Multi-Area OSPF
In addition, you can use the show ip route ospf command to get only OSPF-injected 
routes in your routing table. I can’t stress enough how useful this is when dealing with 
large networks!
Corp#sh ip route ospf
 10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
O 10.10.20.0 [110/65] via 172.16.10.2, 02:18:33, Serial0/0/0
O 10.10.30.0 [110/65] via 172.16.10.2, 02:18:33, Serial0/0/0
O 10.10.40.0 [110/65] via 172.16.10.6, 03:37:30, Serial0/0/1
O 10.10.50.0 [110/65] via 172.16.10.6, 03:37:30, Serial0/0/1
Now that’s a really nice-looking OSPF routing table! Troubleshooting and fixing an OSPF 
network is as vital a skill to have as it is in any other networking environment, 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 pay very close attention to the details—especially 
when troubleshooting!
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. This is the topology 
database I referred to earlier.
The output is broken down by area. Here’s a sample, again from Corp:
Corp#sh ip ospf database
 OSPF Router with ID (1.1.1.1) (Process ID 1)
 Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 196 0x8000001a 0x006d76 2
 Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.16.10.0 1.1.1.1 182 0x80000095 0x00be04
172.16.10.4 1.1.1.1 177 0x80000096 0x009429
10.10.40.0 1.1.1.1 1166 0x80000091 0x00222b
10.10.50.0 1.1.1.1 1166 0x80000092 0x00b190
10.10.20.0 1.1.1.1 1114 0x80000093 0x00fa64
10.10.30.0 1.1.1.1 1114 0x80000094 0x008ac9
Verifying and Troubleshooting Multi-area OSPF Networks 867
 Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1118 0x8000002a 0x00a59a 2
172.16.10.2 172.16.10.2 1119 0x80000031 0x00af47 4
 Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.10.10.0 1.1.1.1 178 0x80000076 0x0021a5
10.10.11.0 1.1.1.1 178 0x80000077 0x0014b0
172.16.10.4 1.1.1.1 173 0x80000078 0x00d00b
10.10.40.0 1.1.1.1 1164 0x80000074 0x005c0e
10.10.50.0 1.1.1.1 1164 0x80000075 0x00eb73
 Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1119 0x8000002b 0x005cd6 2
172.16.10.6 172.16.10.6 1119 0x8000002d 0x0020a3 4
 Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
10.10.10.0 1.1.1.1 179 0x8000007a 0x0019a9
10.10.11.0 1.1.1.1 179 0x8000007b 0x000cb4
172.16.10.0 1.1.1.1 179 0x8000007c 0x00f0ea
10.10.20.0 1.1.1.1 1104 0x80000078 0x003149
10.10.30.0 1.1.1.1 1104 0x80000079 0x00c0ae
Corp#
Considering we only have eight networks configured in our internetwork, there’s a huge 
amount of information in this database! You can see all the routers and the RID of each—
the highest IP address related to individual routers. And each output under each area repre￾sents LSA Type 1, indicating the area they’re connected to.
The router output also shows the link ID. Remember that an interface is also a link, as is 
the RID of the router on that link under the ADV router—the advertising router.
So far, this has been a great chapter, brimming with detailed OSPF information, a whole 
lot more than what was needed to meet past Cisco objectives, for sure! Next, we’ll use the 
same sample network that I built in Chapter 5 on EIGRP and run through a troubleshooting 
scenario using multi-area OSPF. 
868 Chapter 20 u Multi-Area OSPF
Troubleshooting OSPF Scenario
When you notice problems with your OSPF network, it’s wise to first test your layer 3 connectivity with Ping and the traceroute command to see if your issue is a local one. If all 
looks good locally, then follow these Cisco-provided guidelines:
1. Verify your adjacency with your neighbor routers using the show ip ospf neighbors
command. If you are not seeing your neighbor adjacencies, then you need to verify that 
the interfaces are operational and enabled for OSPF. If all is well with the interfaces, 
verify the hello and dead timers next, and establish that the interfaces are in the same 
area and that you don’t have a passive interface configured.
2. Once you’ve determined that your adjacencies to all neighbors are working, use the 
show ip route to verify your layer 3 routes to all remote networks. If you see no OSPF 
routes in the routing table, you need to verify that you don’t have another routing protocol running with a lower administrative distance. You can use show ip protocols
to see all routing protocols running on your router. If no other protocols are running, 
then verify your network statements under the OSPF process. In a multi-area network, 
make sure all non-backbone area routers are directly connected to area 0 through an 
ABR or they won’t be able to send and receive updates.
3. If you can see all the remote networks in the routing table, move on to verify the path 
for each network and that each path for specific networks is correct. If not, you need 
to verify the cost on your interfaces with the show ip ospf interface command. You 
may need to adjust the cost on an interface either higher or lower, depending on which 
path you want OSPF to use for sending packets to a remote network. Remember—the 
path with the lowest cost is the preferred path!
Okay, with our marching orders for troubleshooting OSPF in hand, let’s take a look at 
Figure 20.9, which we’ll use to verify our network now.
F ig u re 20. 9 Our internetwork
Area 0
10.1.1.0/24
192.168.1.1
s0/0
192.168.1.2
s0/0/0
Area 1
10.2.2.0/24
Internal Corp
Area 1
Branch
Here’s the OSPF configuration on the three routers:
Corp(config-if)#router ospf 1
Corp(config-router)#network 10.1.1.0 0.0.0.255 area 0
Corp(config-router)#network 192.168.1.0 0.0.0.3 area 1
Troubleshooting OSPF Scenario 869
Internal(config)#router ospf 3
Internal(config-router)#network 10.1.1.2 0.0.0.0 area 0
Branch(config-if)#router ospf 2
Branch(config-router)#network 192.168.1.2 0.0.0.0 area 0
Branch(config-router)#network 10.2.2.1 0.0.0.0 area 0
Let’s check out our network now, beginning by checking the layer 1 and layer 2 status 
between routers:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial0/0 192.168.1.1 YES manual up up
The IP addresses look correct and the layer 1 and 2 status is up/up, so next we’ll use the 
Ping program to check connectivity like this:
Corp#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Corp#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Nice—I can ping both directly connected neighbors, so this means layers 1, 2, and 3 are 
working between neighbor routers. This is a great start, but it still doesn’t mean OSPF is 
actually working yet. If any of the preceding commands had failed, I first would’ve verified 
layers 1 and 2 to make sure my data link was working between neighbors and then moved 
on to verify my layer 3 IP configuration. 
Since our data link appears to be working between each neighbor, our next move is to 
check the OSPF configuration and status of the routing protocol. I’ll start with the interfaces:
Corp#sh ip ospf interface s0/0
Serial0/0 is up, line protocol is up
 Internet Address 192.168.1.1/30, Area 1
 Process ID 1, Router ID 192.168.1.1, Network Type POINT_TO_POINT, Cost: 100
 Transmit Delay is 1 sec, State POINT_TO_POINT
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
 oob-resync timeout 40
 Hello due in 00:00:03
870 Chapter 20 u Multi-Area OSPF
 Supports Link-local Signaling (LLS)
 Cisco NSF helper support enabled
 IETF NSF helper support enabled
 Index 1/2, 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 192.168.1.2
 Suppress hello for 0 neighbor(s)
I’ve highlighted the important statistics that you should always check first on an OSPF 
interface. You need to verify that the interface is configured in the same area as the neigh￾bor and that the hello and dead timers match. A cost mismatch won’t stop an adjacency 
from forming, but it could cause ugly routing issues. We’ll explore that more in a minute.
For now let’s take a look at the LAN interface that’s connecting to the Internal router:
Corp#sh ip ospf int f0/0
FastEthernet0/0 is up, line protocol is up
 Internet Address 10.1.1.1/24, Area 0
 Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID) 192.168.1.1, Interface address 10.1.1.1
 Backup Designated router (ID) 10.1.1.2, Interface address 10.1.1.2
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
 oob-resync timeout 40
 Hello due in 00:00:00
 Supports Link-local Signaling (LLS)
 Cisco NSF helper support enabled
 IETF NSF helper support enabled
 Index 1/1, 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 10.1.1.2 (Backup Designated Router)
 Suppress hello for 0 neighbor(s)
We’ll focus on the same key factors on a LAN interface that we did on our serial interface: 
the area ID and hello and dead timers. Notice that the cost is 1. According to Cisco’s method 
of calculating cost, anything 100 Mbps or higher will always be a cost of 1 and serial links 
with the default bandwidth are always 64. This can cause problems in a large network with 
Troubleshooting OSPF Scenario 871
lots of high-bandwidth links. One thing to take special note of is that there’s a designated and 
backup designated router on a broadcast multi-access network. DRs and BDRs won’t cause 
a routing problem between neighbors, but it’s still a consideration when designing and con￾figuring in a really large internetwork environment. But we won’t be focusing on that for our 
purposes here. It’s just something to keep in mind.
Staying with the troubleshooting step of checking our interfaces, look at the error I received 
when I tried to verify OSPF on the fa0/1 interface of the Corp router (which we’re not using): 
Corp#sh ip ospf int fa0/1
%OSPF: OSPF not enabled on FastEthernet0/1
Corp#
I got this error because the network statements under the OSPF process are not enabled 
for the network on the fa0/1 interface. If you receive this error, immediately check your 
network statements! 
Next, let’s check out the networks we’re routing for with the show ip protocols
command:
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 192.168.1.1
 It is an area border router
 Number of areas in this router is 2. 2 normal 0 stub 0 nssa
 Maximum path: 4
 Routing for Networks:
 10.1.1.0 0.0.0.255 area 0
 192.168.1.0 0.0.0.3 area 1
 Reference bandwidth unit is 100 mbps
 Routing Information Sources:
 Gateway Distance Last Update
 192.168.1.2 110 00:28:40
 Distance: (default is 110)
From this output we can check our process ID, as well as, reveal if we have an ACL set on 
our routing protocol, just as we found when troubleshooting EIGRP in the last chapter. But 
this time, we’ll first examine the network statements and the area they’re configured for—
most important, the specific areas that each interface is configured for. This is key because 
if your neighbor’s interface isn’t in the same area, you won’t be able to form an adjacency! 
This command’s output provides a great view of what exactly we typed in for the network 
statements under the OSPF process. Also, notice that the default reference bandwidth is set 
to 100 Mbps. I’ll talk about this factor at the end of this section.
872 Chapter 20 u Multi-Area OSPF
I want to point out that the neighbor IP address and administrative distance is listed. 
OSPF uses 110 by default, so remember that if EIGRP were running here, we wouldn’t 
see OSPF routes in the routing table because EIGRP has an AD of 90!
Next, we’ll look at our neighbor table on the Corp router to find out if OSPF has formed 
an adjacency with the Branch router: 
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.1.1.2 1 FULL/BDR 00:00:39 10.1.1.2 FastEthernet0/0
Okay, we’ve finally zeroed in on our problem—the Corp router can see the Internal 
router in area 0 but not the Branch router in area 1! What now?
First, let’s review what we know so far about the Corp and Branch router. The data link 
is good, and we can use Ping successfully between the routers. This shouts out that we have 
a routing protocol issue, so we’ll look further into the details of the OSPF configuration on 
each router. Let’s run a show ip protocols on the Branch router:
Branch#sh ip protocols
Routing Protocol is "eigrp 20"
 Outgoing update filter list for all interfaces is not set
 Incoming update filter list for all interfaces is not set
 Default networks flagged in outgoing updates
 Default networks accepted from incoming updates
 EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
 EIGRP maximum hopcount 100
 EIGRP maximum metric variance 1
 Redistributing: eigrp 20
 EIGRP NSF-aware route hold timer is 240s
 Automatic network summarization is not in effect
 Maximum path: 4
 Routing for Networks:
 10.0.0.0
 192.168.1.0
 Routing Information Sources:
 Gateway Distance Last Update
 (this router) 90 3d22h
 192.168.1.1 90 00:00:07
 Distance: internal 90 external 170
Routing Protocol is "ospf 2"
 Outgoing update filter list for all interfaces is not set
Troubleshooting OSPF Scenario 873
 Incoming update filter list for all interfaces is not set
 Router ID 192.168.1.2
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Maximum path: 4
 Routing for Networks:
 10.2.2.1 0.0.0.0 area 1
 192.168.1.2 0.0.0.0 area 1
 Reference bandwidth unit is 100 mbps
 Passive Interface(s):
 Serial0/0/0
 Routing Information Sources:
 Gateway Distance Last Update
 192.168.1.1 110 03:29:07
 Distance: (default is 110)
Do you see two routing protocols running on the Branch router? Both EIGRP and 
OSPF are running, but that’s not necessarily our problem. The Corp router would need 
to be running EIGRP, and if so, we would have only EIGRP routes in our routing table 
because EIGRPs have the lower AD of 90 versus OSPF’s AD of 110. 
Let’s check the routing table of the Branch router and see if the Corp router is also running 
EIGRP. This will be easy to determine if we discover EIGRP-injected routes in the table:
Branch#sh ip route
[output cut]
 10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
D 10.1.1.0 [90/2172416] via 192.168.1.1, 00:02:35, Serial0/0/0
 192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
Okay—so yes, the Corp router is clearly running EIGRP. This is a leftover configuration 
from Chapter 5. All I need to do to fix this issue is to disable EIGRP on the Branch router. 
After that, we should see OSPF in the routing table:
Branch#config t
Branch(config)#no router eigrp 10
Branch(config)#do sh ip route
[output cut]
 10.0.0.0/24 is subnetted, 1 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
 192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
874 Chapter 20 u Multi-Area OSPF
That’s not so good—I disabled the EIGRP protocol on the Branch router, but we still 
didn’t receive OSPF updates! Let investigate further using the show ip protocols command 
on the Branch router:
Branch#sh ip protocols
Routing Protocol is "ospf 2"
 Outgoing update filter list for all interfaces is not set
 Incoming update filter list for all interfaces is not set
 Router ID 192.168.1.2
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Maximum path: 4
 Routing for Networks:
 10.2.2.1 0.0.0.0 area 1
 192.168.1.2 0.0.0.0 area 1
 Reference bandwidth unit is 100 mbps
 Passive Interface(s):
 Serial0/0/0
 Routing Information Sources:
 Gateway Distance Last Update
 192.168.1.1 110 03:34:19
 Distance: (default is 110)
Do you see the problem? There’s no ACL, the networks are configured correctly, but 
see the passive interface for Serial0/0/0? That will definitely prevent an adjacency from 
happening between the Corp and Branch routers! Let’s fix that:
Branach#show run
[output cut]
!
router ospf 2
 log-adjacency-changes
 passive-interface Serial0/0/0
 network 10.2.2.1 0.0.0.0 area 1
 network 192.168.1.2 0.0.0.0 area 1
!
[output cut]
Branch#config t
Branch(config)#router ospf 2
Branch(config-router)#no passive-interface serial 0/0/0
Let’s see what our neighbor table and routing table look like now:
Branch#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.1 0 FULL/ - 00:00:32 192.168.1.1 Serial0/0/0
Troubleshooting OSPF Scenario 875
Branch#sh ip route
 10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
O IA 10.1.1.0 [110/65] via 192.168.1.1, 00:01:21, Serial0/0/0
 192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
Awesome—our little internetwork is finally happy! That was actually pretty fun and 
really not all that hard once you know what to look for.
But there’s one more thing we need to cover before moving onto OSPFv3—load balancing 
with OSPF. To explore that, we’ll use Figure 20.10, wherein I added another link between 
the Corp and Branch routers.
F ig u re 20.10 Our internetwork with dual links
Area 0
10.1.1.0/24
192.168.1.1/30
s0/0
Area 1
10.2.2.0/24
Internal Corp
Area 1
Branch
192.168.1.1/30
s0/0/0
g0/0 g0/0
First, it’s clear that having a Gigabit Ethernet interface between our two routers is way 
better than any serial link we could possibly have, which means we want the routers to use 
the LAN link. We can either disconnect the serial link or use it as a backup link.
Let’s start by looking at the routing table and seeing what OSPF found:
Corp#sh ip route ospf
 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O 10.2.2.0 [110/2] via 192.168.1.6, 00:00:13, GigabitEthernet0/1
Look at that! OSPF wisely went with the Gigabit Ethernet link because it has the lowest 
cost. Although it’s possible you’ll have to mess with the links to help OSPF choose the best 
paths, it’s likely best to just leave it alone at this point. 
But that wouldn’t be very much fun, now would it? Instead, let’s configure OSPF to 
fool it into thinking the links are equal so it will use both of them by setting the cost on 
the interfaces to the same value:
Corp#config t
Corp(config)#int g0/1
Corp(config-if)#ip ospf cost 10
Corp(config-if)#int s0/0/0
Corp(config-if)#ip ospf cost 10
876 Chapter 20 u Multi-Area OSPF
Obviously you need to deploy this configuration on both sides of the link, and I’ve already 
configured the Branch router as well. Now that both sides are configured with the same cost, 
let’s check out our routing table now:
Corp#sh ip route ospf
 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O 10.2.2.0 [110/11] via 192.168.1.2, 00:01:23, Serial0/0/0
 [110/11] via 192.168.1.6, 00:01:23, GigabitEthernet0/1
I’m not saying you should configure a serial link and Gigabit Ethernet link as equal costs 
as I just demonstrated, but there are times when you need to adjust the cost for OSPF. If 
you don’t have multiple links to any remote networks, you really don’t need to worry about 
this, but with regard to the objectives, you absolutely must understand the cost, how it 
works, and how to set it so OSPF can choose a preferred path. And there’s still one more 
thing about cost I want to cover with you.
It’s possible to change the reference bandwidth of the router, but you need to make sure 
all the routers within the OSPF AS have the same reference bandwidth. The default reference 
bandwidth is 108, which is 100,000,000, or the equivalent of the bandwidth of Fast Ethernet, 
which is 100 Mbps, as demonstrated via show ip ospf and the show ip protocols command:
Routing for Networks:
 10.2.2.1 0.0.0.0 area 1
 192.168.1.2 0.0.0.0 area 1
Reference bandwidth unit is 100 mbps
This will basically make any interface running 100 Mbps or higher have a cost of 1. 
The default is 100, and if you change it to 1,000, it will increase the cost by a factor of 10. 
Again, if you do want to change this, you must make sure to configure the change on all 
routers in your AS! Here is how you would do that:
Corp(route)#router ospf 1
Corp(config-router)#auto-cost reference-bandwidth ?
 <1-4294967> The reference bandwidth in terms of Mbits per second
Now, finally, let’s get to the easy section of the chapter!
OSPFv3
The new version of OSPF continues the trend of routing protocols having a lot in common 
with their IPv4 versions. The foundation of OSPF remains the same—it’s still a link-state 
routing protocol that divides an entire internetwork or autonomous system into areas, 
establishing a hierarchy.
In OSPF version 2, the router ID (RID) is determined by the highest IP addresses assigned 
to the router. And as you now know, the RID can be assigned. In version 3, nothing has 
OSPFv3 877
really changed because you can still assign the RID, area ID, and link-state ID, which remain 
32-bit values. 
Adjacencies and next-hop attributes now use link-local addresses, but OSPFv3 still uses 
multi-cast traffic to send its updates and acknowledgements. It uses the addresses FF02::5 
for OSPF routers and FF02::6 for OSPF-designated routers. These new addresses are the 
replacements for 224.0.0.5 and 224.0.0.6, respectively.
Other, less flexible IPv4 protocols don’t give us the ability that OSPFv2 does to assign 
specific networks and interfaces into the OSPF process, but this is still configured under 
the router configuration process. And with OSPFv3, just as with the EIGRPv6 routing pro￾tocols we’ve talked about, the interfaces and therefore the networks attached to them are 
configured directly on the interface in interface configuration mode.
The configuration of OSPFv3 is going to look like this: First, optionally start by assigning 
the RID, but if you have IPv4 addresses assigned to your interface, you can let OSPF pick the 
RID just as we did with OSPFv2:
Router(config)#ipv6 router osfp 10
Router(config-rtr)#router-id 1.1.1.1
You get to perform some other configurations from router configuration mode, like 
summarization and redistribution, but again, we don’t even need to configure OSPFv3 
from this prompt if we configure it from the interface!
A simple interface configuration looks like this:
Router(config-if)#ipv6 ospf 10 area 0.0.0.0
So, if we just go to each interface and assign a process ID and area—poof, we’re done! 
See? Easy! As the configuration shows, I configured the area as 0.0.0.0, which is the same 
thing as just typing area 0. We’ll use Figure 20.11, which is the same network and IPv6-
addressing we used in the EIGRPv6 section in Chapter 5.
F ig u re 20.11 Configuring OSPFv3
2001:db8:3c4d:13::/64
2001:db8:3c4d:11::/64
2001:db8:3c4d:12::/64
2001:db8:3c4d:14::/64
Corp
g0/0
g0/1
g0/0
g0/1
g0/0
g0/1
SF
NY
2001:db8:3c4d:17::/64
2001:db8:3c4d:18::/64
2001:db8:3c4d:15::/64
2001:db8:3c4d:16::/64
878 Chapter 20 u Multi-Area OSPF
Okay, so all we have to do to enable OSPF on the internetwork is go to each interface 
that we want to run it on. Here’s the Corp configuration:
Corp#config t
Corp(config)#int g0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int g0/1
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/0/1
Corp(config-if)#ipv6 ospf 1 area 0
That wasn’t so bad—much easier than it was with IPv4! To configure OSPFv3, you 
just need to establish the specific interfaces you’ll be using! Let’s configure the other two 
routers now: 
SF#config t
SF(config)#int g0/0
SF(config-if)#ipv6 ospf 1 area 1
SF(config-if)#int g0/1
SF(config-if)#ipv6 ospf 1 area 1
SF(config-if)#int s0/0/0
SF(config-if)#ipv6 ospf 1 area 0
01:03:55: %OSPFv3-5-ADJCHG: Process 1, Nbr 192.168.1.5 on Serial0/0/0 from 
LOADING to 
FULL, Loading Done
Sweet—the SF has become adjacent to the Corp router! One interesting output line I want 
to point out is that the IPv4 RID is being used in the OSPFv3 adjacent change. I didn’t set 
the RIDs manually because I knew I had interfaces with IPv4 addresses already on them, 
which the OSPF process would use for a RID. 
Now let’s configure the NY router:
NY(config)#int g0/0
NY(config-if)#ipv6 ospf 1 area 2
%OSPFv3-4-NORTRID:OSPFv3 process 1 could not pick a router-id,please configure 
manually
NY(config-if)#ipv6 router ospf 1
NY(config-rtr)#router-id 1.1.1.1
NY(config-if)#int g0/0
NY(config-if)#ipv6 ospf 1 area 2
NY(config-if)#int g0/1
NY(config-if)#ipv6 ospf 1 area 2
OSPFv3 879
NY(config-if)#int s0/0/0
NY(config-if)#ipv6 ospf 1 area 0
00:09:00: %OSPFv3-5-ADJCHG: Process 1, Nbr 192.168.1.5 on Serial0/0/0 from 
LOADING to 
FULL, Loading Done
Our adjacency popped up—this is great. But did you notice that I had to set the RID? 
That’s because there wasn’t an IPv4 32-bit address already on an interface for the router to 
use as the RID, so it was mandatory to set the RID manually!
Without even verifying our network, it appears it’s up and running. Even so, it’s always 
important to verify!
Verifying OSPFv3
I’ll start as usual with the show ipv6 route ospf command:
Corp#sh ipv6 route ospf
OI 2001:DB8:3C4D:15::/64 [110/65]
 via FE80::201:C9FF:FED2:5E01, Serial0/0/1
OI 2001:DB8:3C4D:16::/64 [110/65]
 via FE80::201:C9FF:FED2:5E01, Serial0/0/1
O 2001:DB8:C34D:11::/64 [110/128]
 via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
OI 2001:DB8:C34D:17::/64 [110/65]
 via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
OI 2001:DB8:C34D:18::/64 [110/65]
 via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
Corp#
Perfect. I see all six subnets. Notice the O and OI? The O is intra-area and the OI is inter￾area, meaning it’s a route from a different area. You can’t simply distinguish the area by look￾ing at the routing table though. Plus, don’t forget that the routers communicate with their 
neighbor via link-local addresses: via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0, for example.
Let’s take a look at the show ipv6 protocols command:
Corp#sh ipv6 protocols
IPv6 Routing Protocol is "connected"
IPv6 Routing Protocol is "static
IPv6 Routing Protocol is "ospf 1"
 Interfaces (Area 0)
 GigabitEthernet0/0
 GigabitEthernet0/1
 Serial0/0/0
 Serial0/0/1
880 Chapter 20 u Multi-Area OSPF
This just tells us which interfaces are part of OSPF process 1, area 0. To configure OSPFv3, 
you absolutely have to know which interfaces are in use. Sho ip int brief can really help you 
if you’re having a problem finding your active interfaces.
Let’s take a look at the Gigabit Ethernet OSPFv3 active interface on the Corp router:
Corp#sh ipv6 ospf int g0/0
GigabitEthernet0/0 is up, line protocol is up
 Link Local Address FE80::2E0:F7FF:FE0A:3301 , Interface ID 1
 Area 0, Process ID 1, Instance ID 0, Router ID 192.168.1.5
 Network Type BROADCAST, Cost: 1
 Transmit Delay is 1 sec, State DR, Priority 1
 Designated Router (ID) 192.168.1.5, local address FE80::2E0:F7FF:FE0A:3301
 No backup designated router on this network
 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
 Hello due in 00:00:09
 Index 1/1, 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 0, Adjacent neighbor count is 0
 Suppress hello for 0 neighbor(s)
This is basically the same information we saw earlier in the verification and trouble￾shooting section. Let’s take a look at the neighbor table on the Corp router via show ipv6 
ospf neighbor:
Corp#sh ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
2.2.2.2 0 FULL/ - 00:00:36 4 Serial0/0/1
192.168.1.6 0 FULL/ - 00:00:39 4 Serial0/0/0
Okay, we can see our two neighbors, and there’s also a slight difference in this version’s 
command from OSPFv2. We still see the RID on the left and that we’re also fully adjacent 
with both our neighbors—the dash is there because there are no elections on serial point￾to-point links. But we don’t see the neighbor’s IPv6 address listed as we did with OSPFv2’s 
IPv4 addreses, which were listed in the interface ID field.
There’s one other command I want to finish with—the show ipv6 ospf command:
Corp#sh ipv6 ospf
 Routing Process "ospfv3 1" with ID 192.168.1.5
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
Exam Essentials 881
 Number of external LSA 0. Checksum Sum 0x000000
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Reference bandwidth unit is 100 mbps
 Area BACKBONE(0)
 Number of interfaces in this area is 4
 SPF algorithm executed 10 times
 Number of LSA 10. Checksum Sum 0x05aebb
 Number of DCbitless LSA 0
 Number of indication LSA 0
 Number of DoNotAge LSA 0
 Flood list length 0
This shows the process ID and RID, our reference bandwidth for this interface, and how 
many interfaces we have in each area, which in our example is only area 0.
Holy output! Now that’s what I call a fun chapter. The best thing you can do to get a 
solid grasp of OSPF and OSPv3 multi-area networks is to gather up some routers and spend 
some quality time with them, practicing everything we’ve covered!
Summary
In this chapter, you learned about the scalability constraints of a single-area OSPF net￾work, and you were introduced to the concept of multi-area OSPF as a solution to these 
scalability limitations.
You’re now able to identify the different categories of routers used in multi-area configu￾rations, including the backbone router, internal router, area border router, and autonomous 
system boundary router.
I detailed the function of different OSPF Link-State Advertisements (LSAs) and you dis￾covered how these LSAs can be minimized through the effective implementation of specific 
OSPF area types. I discussed the Hello protocols and the different neighbor states experi￾enced when an adjacency is taking place.
Verification and troubleshooting are very large parts of the objectives, and I covered 
everything you need to know in order to verify and troubleshoot OSPFv2 and meet those 
requirements.
Finally, we ended the chapter with the easiest part: configuring and verifying OSPFv3.
Exam Essentials
Know the scalability issues multi-area OSPF addresses. The primary problems in single-area 
OSPF networks are the large size of the topology and routing tables, as well as, the excessive 
computation of the SPF algorithm due to the large number of link-state updates that occur in 
this single area.
882 Chapter 20 u Multi-Area OSPF
Know the different types of OSPF routers. Backbone routers have at least one interface 
in area 0. Area border routers (ABRs) belong to two or more OSPF areas simultaneously. 
Internal routers have all of their interfaces within the same area. Autonomous system 
boundary routers (ASBRs) have at least one interface connected to an external network.
Know the different types of LSA packets. There are seven different types of LSA pack￾ets that Cisco uses, but here are the ones you need to remember: Type 1 LSAs (router link 
advertisements), Type 2 LSAs (network link advertisements), Type 3 and 4 LSAs (summary 
LSAs), and Type 5 LSAs (AS external link advertisements). Know how each functions.
Be able to monitor multi-area OSPF. There are a number of commands that provide 
information useful in a multi-area OSPF environment: show ip route ospf, show ip ospf 
neighbor, show ip ospf, and show ip ospf database. It’s important to understand what 
each provides.
Be able to troubleshoot OSPF networks. It’s important that you can work your way 
through the troubleshooting scenario that I presented in this chapter. Be able to look for 
neighbor adjacencies, and if they are not there, look for ACLs set on the routing protocol, 
passive interfaces, and wrong network statements.
Understand how to configure OSPFv3. OSPFv3 uses the same basic mechanisms that 
OSPFv2 uses, but OSPFv3 is more easily configured by placing the configuring OSPFv3 
on a per-interface basis with the ipv6 ospf process-ID area area.
Hands-on Labs 883
Written Lab 6
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. What type of LSAs are sent by an ASBR?
2. What state would a router adjacency be in after the INIT state has finished?
3. What LSA types are sent by ABR toward the area external to the one in which they 
were generated?
4. When would you see an adjacency show this: FULL/-?
5. True/False: OSPFv3 is configured per area, per interface.
6. Which OSPF state uses DBD packets and LSRs?
7. Which LSA type is referred to as a router link advertisement (RLA)?
8. What is the command to configure OSPFv3 on an interface with process ID 1 into area 0?
9. What must match exactly between two routers to form an adjacency when using OSPFv3?
10. How can you see all the routing protocols configured and running on your router from 
user mode?
Hands-on Labs
In this section, you will use the following network and add OSPF and OSPFv3 routing.
Area 0
10.1.1.0/24
192.168.1.1
s0/0
192.168.1.2
s0/0
Area 1
10.2.2.0/24
RouterA
Area 1
RouterB g0/0 g0/0
The first lab requires you to configure two routers with OSPF and then verify the con￾figuration. In the second, you will be asked to enable OSPFv3 routing on the same network. 
Note that the labs in this chapter were written to be used with real equipment—real cheap 
equipment, that is. As with Chapter 5 on EIGRP, I wrote these labs with the cheapest, oldest 
routers I had lying around so you can see that you don’t need expensive gear to get through 
some of the hardest labs in this book. However, you can use the free LammleSim IOS version 
simulator or Cisco’s Packet Tracer to run through these labs.
884 Chapter 20 u Multi-Area OSPF
The labs in this chapter are as follows:
Lab 6.1: Configuring and Verifying Multi-Area OSPF 
Lab 6.2: Configuring and Verifying OSPFv3
Hands-on Lab 6.1: Configuring and Verifying OSPF 
Multi-Area
In this lab, you’ll configure and verify multi-area OSPF: 
1. Implement OSPFv2 on RouterA based on the information in the diagram.
RouterA#conf t
RouterA(config)#router ospf 10
RouterA(config-router)#network 10.0.0.0 0.255.255.255 area 0
RouterA(config-router)#network 192.168.1.0 0.0.0.255 area 0
2. Implement OSPF on RouterB based on the diagram.
RouterB#conf t
RouterB(config)#router ospf 1
RouterB(config-router)#network 192.168.1.2 0.0.0.0 area 0
RouterB(config-router)#network 10.2.2.0 0.0.0.255 area 1
3. Display all the LSAs received on RouterA.
RouterA#sh ip ospf database
 OSPF Router with ID (192.168.1.1) (Process ID 10)
 Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.1.1.2 10.1.1.2 380 0x80000035 0x0012AB 1
192.168.1.1 192.168.1.1 13 0x8000000A 0x00729F 3
192.168.1.2 192.168.1.2 10 0x80000002 0x0090F9 2
 Net Link States (Area 0)
Hands-on Labs 885
Link ID ADV Router Age Seq# Checksum
10.1.1.2 10.1.1.2 381 0x80000001 0x003371
 Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.2.2.0 192.168.1.2 8 0x80000001 0x00C3FD
RouterA#
4. Display the routing table for RouterA. 
RouterA#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 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
Gateway of last resort is not set
 10.0.0.0/24 is subnetted, 2 subnets
O IA 10.2.2.0 [110/101] via 192.168.1.2, 00:00:29, Serial0/0
C 10.1.1.0 is directly connected, FastEthernet0/0
 192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0
RouterA#
5. Display the neighbor table for RouterA. 
RouterA#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.2 0 FULL/ - 00:00:35 192.168.1.2 Serial0/0
10.1.1.2 1 FULL/DR 00:00:34 10.1.1.2 FastEthernet0/0
RouterA#
886 Chapter 20 u Multi-Area OSPF
6. Use the show ip ospf command on RouterB to see that it is an ABR.
RouterB#sh ip ospf
 Routing Process "ospf 1" with ID 192.168.1.2
 Start time: 1w4d, Time elapsed: 00:07:04.100
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 It is an area border router
 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
 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 2. 2 normal 0 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
 Area BACKBONE(0)
 Number of interfaces in this area is 1
 Area has no authentication
 SPF algorithm last executed 00:06:44.492 ago
 SPF algorithm executed 3 times
 Area ranges are
 Number of LSA 5. Checksum Sum 0x020DB1
 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
 Area 1
 Number of interfaces in this area is 1
Hands-on Labs 887
 Area has no authentication
 SPF algorithm last executed 00:06:45.640 ago
 SPF algorithm executed 2 times
 Area ranges are
 Number of LSA 3. Checksum Sum 0x00F204
 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
Hands-on Lab 6.2: Configuring and Verifying OSPFv3
In this lab, you will configure and verify OSPFv3: 
1. Implement OSPFv3 on RouterA. Since the routers have IPv4 addresses, we don’t need 
to set the RID of the router.
RouterA#config t
RouterA(config)#int g0/0
RouterA(config-if)#ipv6 ospf 1 area 0
RouterA(config-if)#int s0/0
RouterA(config-if)#ipv6 ospf 1 area 0
That’s all there is to it! Nice.
2. Implement OSPFv3 on RouterB.
RouterB#config t
RouterB(config)#int s0/0/0
RouterB(config-if)#ipv6 ospf 1 area 0
RouterB(config-if)#int f0/0
RouterB(config-if)#ipv6 ospf 1 area 1
Again, that’s all there is to it!
3. Display the routing table for RouterA.
RouterA#sh ipv6 route ospf
IPv6 Routing Table - 11 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
 U - Per-user Static route
 I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
 O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
888 Chapter 20 u Multi-Area OSPF
 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
 D - EIGRP, EX - EIGRP external
OI 2001:DB8:3C4D:15::/64 [110/65]
 via FE80::21A:2FFF:FEE7:4398, Serial0/0
RouterA#
Notice that the one route OSPFv3 found is an inter-area route, meaning the network is 
another area.
4. Display the neighbor table for RouterA.
RouterA#sh ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
192.168.1.2 1 FULL/ - 00:00:32 6 Serial0/0
RouterA#
5. Display the show ipv6 ospf command on RouterB.
RouterB#sh ipv6 ospf
 Routing Process "ospfv3 1" with ID 192.168.1.2
 It is an area border router
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 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
 Number of areas in this router is 2. 2 normal 0 stub 0 nssa
 Reference bandwidth unit is 100 mbps
 Area BACKBONE(0)
 Number of interfaces in this area is 1
 SPF algorithm executed 3 times
 Number of LSA 7. Checksum Sum 0x041C1B
 Number of DCbitless LSA 0
 Number of indication LSA 0
 Number of DoNotAge LSA 0
 Flood list length 0
 Area 1
 Number of interfaces in this area is 1
 SPF algorithm executed 2 times
 Number of LSA 5. Checksum Sum 0x02C608
Hands-on Labs 889
 Number of DCbitless LSA 0
 Number of indication LSA 0
 Number of DoNotAge LSA 0
 Flood list length 0
RouterB#
890 Chapter 20 u Multi-Area OSPF
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. Which of the following are scalability issues with single-area OSPF networks? (Choose 
all that apply.)
A. Size of the routing table
B. Size of the OSPF database
C. Maximum hop-count limitation
D. Recalculation of the OSPF database
2. Which of the following describes a router that connects to an external routing process 
(e.g., EIGRP)?
A. ABR
B. ASBR
C. Type 2 LSA
D. Stub router
3. Which of the following must match in order for an adjacency to occur between routers? 
(Choose three.)
A. Process ID
B. Hello and dead timers
C. Link cost
D. Area
E. IP address/subnet mask
4. Which OSPF state do two routers forming an adjacency appear as in the show ip ospf 
neighbor output after adding neighbors into the table and exchanging hello information?
A. ATTEMPT
B. INIT
C. 2WAY
D. EXSTART
E. FULL
Review Questions 891
5. You need to set up a preferred link that OSPF will use to route information to a 
remote network. Which command will allow you to set the interface link as preferred 
over another?
A. ip ospf preferred 10
B. ip ospf priority 10
C. ospf bandwidth 10
D. ip ospf cost 10
6. When would a router’s neighbor table show the FULL/DR state?
A. After the first Hello packets are received by a neighbor
B. When all information is synchronized among adjacent neighbors
C. When the router’s neighbor table is too full of information and is discarding 
neighbor information
D. After the EXSTART state
7. Which is/are true regarding OSPFv3? (Choose all that apply.)
A. You must add network statements under the OSPF process.
B. There are no network statements in OSPFv3 configurations.
C. OSPFv3 uses a 128-bit RID.
D. If you have IPv4 configured on the router, it is not mandatory that you configure 
the RID.
E. If you don’t have IPv4 configured on the router, it is mandatory that you configure 
the RID.
F. OSPFv3 doesn’t use LSAs like OSPFv2 does.
8. What is the default cost of a Fast Ethernet interface configured with OSPF?
A. 1
B. 10
C. 100
D. 1,000
9. Which type of LSA is generated by DRs and referred to as a network link advertise￾ment (NLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5
892 Chapter 20 u Multi-Area OSPF
10. Which type of LSA is generated by ABRs and refers to a summary link advertise￾ment (SLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5
11. Which command will show all the LSAs known by a router?
A. show ip ospf
B. show ip ospf neighbor
C. show ip ospf interface
D. show ip ospf database
12. Using the following illustration, what is the cost from R1’s routing table to reach the 
network with Server 1? Each Gigabit Ethernet link has a cost of 4, and each serial link 
has a cost of 15.
R1 R2
R3 R5
R4
g0/2
g0/0
HostA
Server 1
g0/0
g0/0
g0/1
EoMPLS
A. 100
B. 23
C. 64
D. 19
E. 27
Review Questions 893
13. Using the following illustration, which of the following are true? (Choose all that apply.)
R1 R2
R3 R5
R4
g0/2
g0/0
HostA
Area 2 Area 0 Area 1
Server 1
g0/0
g0/0
g0/1
EoMPLS
A. R1 is an internal router.
B. R3 would see the networks connected to the R1 router as an inter-area route.
C. R2 is an ASBR.
D. R3 and R4 would receive information from R2 about the backbone area, and the 
same LSA information would be in both LSDBs.
E. R4 is an ABR.
14. Which of the following could cause two routers to not form an adjacency? (Choose all 
that apply.)
A. They are configured in different areas.
B. Each router sees the directly connected link as different costs.
C. Two different Process ID’s configured. 
D. ACL is configured on the routing protocol.
E. IP address/mask.
F. Passive interface is configured.
15. Which of the following IOS commands shows the state of an adjacency with directly 
connected routers?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database
894 Chapter 20 u Multi-Area OSPF
16. What command will show you the DR and DBR address of the area you are connected 
to directly with an interface? 
A. show interface s0/0/0
B. show interface fa0/0
C. show ip ospf interface s0/0/0
D. show ip ospf interface fa0/0
17. Which of the following could be causing a problem with the Corp router not forming 
an adjacency with its neighbor router? (Choose all that apply.)
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 10
 Router ID 1.1.1.1
 Number of areas in this router is 3. 3 normal 0 stub 0 nssa
 Maximum path: 4
 Routing for Networks:
 10.10.0.0 0.0.255.255 area 0
 172.16.10.0 0.0.0.3 area 1
 172.16.10.4 0.0.0.3 area 2
 Reference bandwidth unit is 100 mbps
 Passive Interface(s):
 Serial0/0/0
 Routing Information Sources:
 Gateway Distance Last Update
 1.1.1.1 110 00:17:42
 172.16.10.2 110 00:17:42
 172.16.10.6 110 00:17:42
 Distance: (default is 110)
A. The routers are configured with the wrong network statements.
B. They have different maximum paths configured.
C. There is a passive interface configured.
D. There is an ACL set stopping Hellos.
E. The costs of the links between the routers are configured differently.

F. They are in different areas.

Review Questions 895
18. Which of the following is/are true? (Choose all that apply.)
A. The reference bandwidth for OSPF and OSPFv3 is 1.
B. The reference bandwidth for OSPF and OSPFv3 is 100.
C. You change the reference bandwidth from global config with the command auto-cost 
reference bandwidth number.
D. You change the reference bandwidth under the OSPF router process with the 
command auto-cost reference bandwidth number.
E. Only one router needs to set the reference bandwidth if it is changed from its default.
F. All routers in a single area must set the reference bandwidth if it is changed from 
its default.
G. All routers in the AS must set the reference bandwidth if it is changed from its default.
19. Which of the following interfaces would have the same default cost? (Choose two.)
A. Fast Ethernet
B. Ethernet
C. Serial
D. Gigabit Ethernet
20. What is the default cost of a serial interface with OSPF?
A. 1
B. 10
C. 32
D. 64

E. 100

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