Configuring OSPF for a Not-So-Stubby Area

Configuring OSPF for a Not-So-Stubby Area
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 examine its syntax:
RouterA(config)#router ospf 24
where 24 is the Process ID.
RouterA(config-router)#network 10.0.0.0 0.255.255.255 area 0
where 10.0.0.0 0.255.255.255 is a network and wildcard mask summarization of the networks
connected to RouterA and where 0 is the area that networks 10.1.1.0/24 and 10.1.2.0/24
are members of.
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 reducing 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.
192 Chapter 6  Interconnecting OSPF Areas
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 to other areas.
Notice also 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 (in
other words, be configured with the area area-id nssa command).
To expand upon the idea of advertising summarized routes, the area area-id range
network_address network_mask command is used to summarize inter-area routes on an
ABR. Similarly, we can summarize external routes on an autonomous system boundary router
(ASBR) with the command summary-address network_address network_mask. Proper use
of these summarization tools can greatly reduce the number of routes that have to be maintained
by a router, thus reducing memory and processor overhead.
RouterC will be an even more complex configuration. Not only is RouterC part of an NSSA,
it also participates in an 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):
RouterC(config)#router ospf 24
RouterC(config-router)#redistribute rip
where rip is the routing protocol whose routes are being injected into the OSPF routing process.
RouterC(config-router)#network 1.0.0.0 0.255.255.255 area 1
RouterC(config-router)#default-metric 128
where 128 is the OSPF metric value to be assigned to routes being redistributed into the OSPF
routing process.
RouterC(config-router)#area 1 nssa
RouterC(config-router)#router rip
This enables the RIP routing process on the router.
RouterC(config-router)#redistribute ospf 24
where ospf 24 is the routing process whose routes are being injected into the RIP routing process.
RouterC(config-router)#network 172.16.0.0
RouterC(config-router)#default-metric 3
where 3 is the RIP metric value (hop count) to be assigned to OSPF routes being redistributed
into the RIP routing process.
RouterD is internal to the RIP routing process. Therefore, RouterD does not require any
NSSA-specific configuration:
RouterD(config)#router rip
RouterD(config-router)#network 172.16.0.0