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Chapter 5
Interconnecting OSPF Areas
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:
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 summa-
rization 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 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).
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