Configuring OSPF Summarization Between Areas

OSPF also supports manual summarization of routes on area border routers (ABRs). Consider the following topology (Figure 3-15).

FigU re 3-15 Summarizing OSPF Routes Betw een A reas

FigU re 3-15 Summarizing OSPF Routes Betw een A reas

In Figure 3-15, Router G is an ABR betw een area 0 and area 100 and is responsible for advertising the 16 routes in area 100 to area 0 (the backbone area). Normally, it does not summarize but simply injects 16 routes into area 0. This is another opportunity to summarize because the 16 routes arc contiguous. Again, good address planning makes this possible.

To summarize the routes in area 100 with a single summary route, configure Router G with the OSPF area range command, like this:

router ospf 25

network 172.20.16.0 0.0.15.255 area 100 network 172.20.0.0 0.0.3.255 area 0 area 100 range 172.20.16.0 255.255.240.0

where area 100 range 172.20.16.0 255.255.240.0 is the key command that summarizes the routes in area 100 with a single route 172.20.16.0/20. For an explanation of why 172.20.16.0/ 20 summarizes the 16 routes inside area 100, see the preceding section "Configuring EIGRP Summarization," which has the same summary route.

The other commands configure OSPF routing (see Chapter 2).

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