CCNP Routing Study Guide- P8 ppt

The difference is that RouterA is internal to Area 0, thereby classifying it as a backbone router: RouterCconfig#router ospf 70 RouterCconfig-router#network 1.1.3.0 0.0.0.255 area 1 Rout

Basic Multi-area Configuration 173

RouterA (config)#router ospf 70

where 70 is the Process ID

Next, we need to identify each of the networks connected to the router that we want to participate in the OSPF process In this example, we have two networks connected to RouterA (1.1.1.0/24 and 1.1.2.0/24):

RouterA(config-router)#network 1.1.1.0 0.0.0.255 area 0

where 1.1.1.0 0.0.0.255 is the network and wildcard mask of a network connected to RouterA and where 0 is the area that network 1.1.1.0/24 is a member of

RouterA(config-router)#network 1.1.2.0 0.0.0.255 area 0

The syntax for RouterB is similar to that used for RouterA The primary difference is that RouterB is connected to two areas:

RouterB(config)#router ospf 70 RouterB(config-router)#network 1.1.2.0 0.0.0.255 area 0 RouterB(config-router)#network 1.1.3.0 0.0.0.255 area 1

The syntax for RouterC is very similar to that of RouterA The difference

is that RouterA is internal to Area 0, thereby classifying it as a backbone router:

RouterC(config)#router ospf 70 RouterC(config-router)#network 1.1.3.0 0.0.0.255 area 1 RouterC(config-router)#network 1.1.4.0 0.0.0.255 area 1

Stub Area Configuration

Since the main purpose of having stub areas (and totally stubby areas)

is to keep such areas from carrying external routes, we need to review some design guidelines before configuring a stub area or a totally stubby area:

 Do not make the backbone area (Area 0) a stub area

 Since external routes are injected by autonomous system boundary routers, do not make any area containing an ASBR a stub area

 Since routers within a stub area use a default route to get out of the stub area, typically there is only one route out of the stub area There-fore, a stub area should usually only contain a single area border router Keep in mind that since a default route is being used, if a stub area contains more than one ABR, a non-optimal path may be used

 If you decide to make a particular area a stub area, be sure to configure

all the routers in the area as stubby If a router within a stub area has

not been configured as stubby, it will not be able to correctly form adjacencies and exchange OSPF routes

With these guidelines in mind, let’s examine a sample configuration for a stub area Consider the network shown in Figure 5.4 We’re going to make Area 25 a stub area In this example, we won’t be concerned with the con-figuration of RouterA, since it does not participate in Area 25 We will then examine the syntax for RouterB, RouterC, and RouterD

Stub Area Configuration 175

F I G U R E 5 4 OPSF configuration example continued—stub area configuration

RouterB

interface Ethernet0

ip address 1.1.1.1 255.255.255.0

!interface Ethernet1

ip address 10.1.1.1 255.255.255.0

!interface Ethernet2

ip address 10.1.2.1 255.255.255.0

!router ospf 10 network 1.0.0.0 0.255.255.255 area 0 network 10.0.0.0 0.255.255.255 area 25 area 25 stub

RouterC

RouterA

RouterD

Area 0 Area 25

Summary Route Information 10.1.1.2/24

e0

e0

e0 10.1.1.1/24

RouterB

External Route Information

Default Route Information

Stub Area

10.1.2.2/24

interface Ethernet0

ip address 10.1.2.2 255.255.255.0

!router ospf 10 network 10.0.0.0.0 255.255.255 area 25 area 25 stub

RouterD

interface Ethernet0

ip adress 10.1.1.2 255.255.255.0

!router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub

First, we’ll configure RouterB Notice that RouterB is an ABR and that it

is the only ABR in Area 25, as recommended in our stub area design lines When configuring an ABR that is a member of a stub area, be cautious

guide-to only configure the stub area as stubby:

RouterB(config-router)#area 25 stub

where 25 is the area that we have designated as stubby

Notice that instead of using two network statements to represent works 10.1.1.0/24 and 10.1.2.0/24, we used a single network statement specifying network 10.0.0.0/8, which includes, or summarizes, these two

net-Totally Stubby Area Configuration 177

networks By using these summary routes where possible, we can reduce the size of a router’s routing tables, thus lowering memory and processor overhead

We will also use the 10.0.0.0/8 summary when we configure RouterC and RouterD Remember that it is critical that all routers that are members of a stub area be configured as stubby for that area Therefore, RouterC and RouterD will have identical OSPF configurations:

RouterC(config)#router ospf 10 RouterC(config-router)#network 10.0.0.0 0.255.255.255 area

25 RouterC(config-router)#area 25 stub

RouterD(config)#router ospf 10 RouterD(config-router)#network 10.0.0.0 0.255.255.255 area

25 RouterD(config-router)#area 25 stub

Let’s review some key elements of our stub area configuration example:

 The syntax to make a router stubby is area area-id stub

 All routers that are part of Area 25 are configured as stubby

 Area 25 has only one ABR (i.e., only one path out of the area)

 The ABR used the area area-id stub command only for Area 25, not for Area 0, which is not stubby

Totally Stubby Area Configuration

Using the same network topology as we had for the stub area uration, let’s examine how to make Area 25 a totally stubby area Remem-bering that the difference between a stub area and a totally stubby area is that a totally stubby area doesn’t have summary routes injected into it, we only need to change the configuration of RouterB Since RouterB is the ABR,

config-it is the router that will have the responsibilconfig-ity for blocking summary routes from entering the stub area So, again consider our network, as illustrated in Figure 5.5

F I G U R E 5 5 OPSF configuration example continued—totally stubby area configuration

RouterB

interface Ethernet0

ip address 1.1.1.1 255.255.255.0

!interface Ethernet1

ip address 10.1.1.1 255.255.255.0

!interface Ethernet2

ip address 10.1.2.1 255.255.255.0

!router ospf 10 network 1.0.0.0 0.255.255.255 area 0 network 10.0.0.0 0.255.255.255 area 25

area 25 stub no-summary

RouterC

RouterA

RouterD

Area 0 Area 25

Default Route Information 10.1.1.2/24

e0

e0

e0 10.1.1.1/24

e1

e2

1.1.1.1/24

Summary Route Information

RouterB

External Route Information

Default Route Information

Totally Stubby Area

10.1.2.1/24 10.1.2.2/24

Totally Stubby Area Configuration 179

RouterC

interface Ethernet0

ip address 10.1.2.2 255.255.255.0

!router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub

RouterD

interface Ethernet0

ip address 10.1.1.2 255.255.255.0

!router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub

Notice that we only have to change, from the previous example, the figuration of RouterB We simply add the no-summary argument to the area area-id stub command:

where 10.0.0.0 0.255.255.255 is a summary network and wildcard mask

of networks connected to RouterB and where 25 is the area that networks 10.1.1.0/24 and 10.1.2.0/24 are members of

RouterB(config-router)#area 25 stub no-summary

where the no-summary argument makes Area 25 totally stubby

Not-So-Stubby Area Configuration

Recall that a not-so-stubby area (NSSA) is useful when we have an area that requires the injection of external routes, although we still want to eliminate the injection of Type 5 LSAs Figure 5.6 presents such a scenario

In Area 1, we want to prevent Area 0 from injecting Type 5 LSAs, yet we still need external routes from the RIP routing process to be injected into Area 1 The solution to these requirements is to make Area 1 an NSSA

F I G U R E 5 6 OPSF configuration example continued—not-so-stubby area configuration

RouterA

interface Ethernet0

ip address 10.1.1.1 255.255.255.0

!interface Ethernet1

ip address 10.1.2.1 255.255.255.0

!router ospf 24 network 10.0.0.0 0.255.255.255 area 0

Not-So-Stubby Area Configuration 181

interface Ethernet1

ip address 1.1.1.1 255.255.255.0

!router ospf 24 network 10.0.0.0 0.255.255.255 area 0 network 1.0.0.0 0.255.255.255 area 1 area 0 range 10.0.0.0 255.255.0.0 area 1 nssa

RouterC

interface Ethernet0

ip address 1.1.1.2 255.255.255.0

!interface Ethernet1

ip address 172.16.1.1 255.255.255.0

!router ospf 24 redistribute rip network 1.0.0.0 0.255.255.255 area 1 default-metric 128

area 1 nssa

!router rip redistribute ospf 24 network 172.16.0.0 default-metric 3

RouterD

interface Ethernet0

ip address 172.16.1.2 255.255.255.0

!interface Ethernet1

ip address 172.16.2.1 255.255.255.0

!router rip network 172.16.0.0

Let’s examine the configuration of each of these routers, beginning with RouterA RouterA is a backbone router (and an internal router), which does not participate in our NSSA (Area 1) Therefore, RouterA doesn’t need any special NSSA configuration However, by way of review, we will still exam-ine its syntax:

summa-RouterB does participate in the NSSA Therefore, it will require a special configuration:

RouterB(config)#router ospf 24 RouterB(config-router)#network 10.0.0.0 0.255.255.255

area 0 RouterB(config-router)#network 1.0.0.0 0.255.255.255

area 1 RouterB(config-router)#area 0 range 10.0.0.0 255.0.0.0

where 10.0.0.0 255.0.0.0 is the network number and subnet mask of a network that summarizes the individual networks within Area 0, thus reduc-ing the number of a router’s routing table entries

RouterB(config-router)#area 1 nssa

where 1 is the area that is being designated as a not-so-stubby area.Notice that the configuration for RouterB included the command area area-id range network_address network_mask, which can be used on area border routers to summarize the IP address space being used by routers within a given area Also notice the area area-id nssa command This command tells the router that the specified area the router is connected to is

a not-so-stubby area As we saw when configuring stub areas, all routers within a not-so-stubby area must agree that they are connected to a NSSA (i.e., be configured with the area area-id nssa command)

Not-So-Stubby Area Configuration 183

To expand upon the idea of advertising summarized routes, the area

area-id range network_address network_mask command is used to summarize intra-area routes on an ABR Similarly, we can summarize external routes on

an autonomous system boundary router (ASBR) with the command address network_address network_mask Proper use of these summarization tools can greatly reduce the number of routes that have to be maintained by

summary-a router, thus reducing memory summary-and processor overhesummary-ad.

RouterC will be an even more complex configuration Not only is RouterC part of an NSSA, it also participates in a RIP routing process In order to exchange its OSPF and RIP routes, RouterC must perform route redistribution (route redistribution is the focus of Chapter 10):

where 128 is the OSPF metric value to be assigned to routes being uted into the OSPF routing process

OSPF Virtual Links

When designing a multi-area OSPF network, all areas should be nected to the backbone area However, there may be instances when an area has to cross another area to reach the backbone area, as shown in Figure 5.7 Since, in this example, Area 20 does not have a direct link to Area 0, we need

con-to create a virtual link

F I G U R E 5 7 OSPF virtual link

The syntax for creating a virtual link across an area isarea area-id virtual-link router-id

where area-id is the number of the transit area, in this example, Area 10, and router-id is the IP address of the highest loopback interface configured

on a router If a loopback interface has not been configured on the router,

then the router-id is the highest IP address configured on the router Note

that a virtual link has area border routers as the endpoints of the link

As shown in Figure 5.8, we are going to create a virtual link from Area 20

to Area 0, with Area 10 acting as the transit area Let’s examine the uration of RouterB and RouterC, since RouterA does not have any virtual-link-specific configuration

OSPF Virtual Links 185

F I G U R E 5 8 OSPF virtual link

Here is the configuration of RouterB and RouterC:

RouterB(config)#router ospf 10 RouterB(config-router)#network 3.0.0.0 0.255.255.255

area 0 RouterB(config-router)#network 4.0.0.0 0.255.255.255

area 10 RouterB(config-router)#area 10 virtual-link 6.6.6.1

where 10 is the Area ID of the transit area and where 6.6.6.1 is the highest loopback address of the ABR joining the transit area to Area 20

RouterC(config)#router ospf 10 RouterC(config-router)#network 4.0.0.0 0.255.255.255

area 10 RouterC(config-router)#network 7.0.0.0 0.255.255.255

area 20 RouterC(config-router)#area 10 virtual-link 5.5.5.1

where 10 is the Area ID of the transit area and where 5.5.5.1 is the highest loopback address of the ABR joining the transit area to the backbone area

RouterA

interface Loopback0

ip address 2.2.2.1 255.255.255.0

!internet Ethernet0

ip address 1.1.1.1 255.255.255.0

!

Area 10 Area 0

e0

e1 e0

7.7.7.1/24

Lo0:6.6.6.1/24 Lo0:5.5.5.1/24

Lo0:2.2.2.1/24

Virtual Link

interface Ethernet1

ip address 3.3.3.1 255.255.255.0

!router ospf 10 network 1.0.0.0 0.255.255.255 area 0 network 3.0.0.0 0.255.255.255 area 0

RouterB

interface Loopback0

ip address 5.5.5.1 255.255.255.0

!interface Ethernet0

ip address 3.3.3.2 255.255.255.0

!interface Ethernet1

ip address 4.4.4.1 255.255.255.0

!router ospf 10 network 3.0.0.0 0.255.255.255 area 0 network 4.0.0.0 0.255.255.255 area 10

ip address 4.4.4.2 255.255.255.0

!interface Ethernet1

ip address 7.7.7.1 255.255.255.0

!router ospf 10 network 4.0.0.0 0.255.255.255 area 10 network 7.0.0.0 0.255.255.255 area 20

area 10 virtual-link 5.5.5.1

Summary 187

Monitoring and Troubleshooting Multi-area OSPF Networks

Cisco’s IOS has several debug and show commands that can be useful

in monitoring and troubleshooting OSPF networks Following is a sampling

of these commands, which can be used to gain information about various OSPF characteristics:

debug ip ospf events Shows information concerning OSPF events, such as the selection of a designated router and the formation of router adjacencies

debug ip ospf packet Shows information contained in each OSPF packet, such as Router ID and Area ID

show ip ospf border-routers Shows an ABR’s internal routing table

show ip ospf virtual-links Shows the status of a router’s virtual link

show ip ospf neighbor Shows neighbor router information, such as Neighbor ID and the state of adjacency with the neighboring router

show ip ospf process-id Shows area information, such as ing the area’s area border router or autonomous system boundary router

identify-show ip ospf database Shows information about a router’s OSPF database, such as a router’s router link states and network link states

Summary

In this chapter, we illustrated the scalability constraints of an OSPF work with a single area We introduced the concept of multi-area OSPF as a solution to these scalability limitations

net-We identified the different categories of routers used in multi-area urations These router categories include backbone router, internal router, area border router, and autonomous system boundary router We explored how these routers can use summarization and default routes to reduce the