Summarizing Routes in OSPF

Problem

You want to minimize the size of a backbone routers link-state database by summarizing the routes sent to the backbone area.

Solution

Summarize the routes that are flooded into the backbone area by the ABR:

[edit protocols ospf] aviva@RouterJ# set area 0.0.0.3 area-range 10.0.0.0/16

Discussion

OSPF route summarization aggregates routes sent by nonbackbone areas to the backbone routers so that the size of their link-state databases is reduced. In this recipe, the networks in Area 0.0.0.3 are 10.0.8.1/28, 10.0.8.2/28, 10.0.13.1/28, and 10.0.13.2/28 (see Figure 12-3), which can be summarized as 10.0.0.0/24.

If we check on one of the backbone routers before configuring route summarization, we see four Type 3 (Summary) LSAs in the link-state database:

aviva@RouterG> show ospf database OSPF link state database, area 0.0.0.0 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.17.1 192.168.17.1 0x80000189 16 0x2 0x926e 60 Router 192.168.18.1 192.168.18.1 0x80000124 2601 0x2 0x97cd 60 Router *192.168.19.1 192.168.19.1 0x8000009c 126 0x2 0xb4c7 48 Network *10.0.0.1 192.168.19.1 0x80000090 726 0x2 0x9aae 32 Network *10.0.1.2 192.168.19.1 0x80000090 426 0x2 0x92b3 32 Network 10.0.2.1 192.168.18.1 0x8000009b 1651 0x2 0x6cd1 32 Summary 10.0.8.0 192.168.17.1 0x80000009 16 0x2 0xa62f 28 Summary 10.0.13.0 192.168.17.1 0x80000002 16 0x2 0xa8c 28 Summary 192.168.15.1 192.168.17.1 0x80000002 16 0x2 0x1510 28 Summary 192.168.16.1 192.168.17.1 0x80000002 16 0x2 0x7de7 28 ASBRSum 192.168.15.1 192.168.17.1 0x80000002 16 0x2 0x71d 28 OSPF AS SCOPE link state database Type ID Adv Rtr Seq Age Opt Cksum Len Extern 10.0.15.0 192.168.15.1 0x80000001 161 0x2 0x3f01 36 Extern 172.19.121.0 192.168.15.1 0x80000001 161 0x2 0x859a 36 Extern 172.19.121.0 192.168.17.1 0x80000003 472 0x2 0x73a8 36 Extern 192.168.15.1 192.168.15.1 0x80000001 161 0x2 0x6d9 36 Extern 192.168.17.1 192.168.17.1 0x80000003 240 0x2 0xddfb 36

After the ABR for Area 0.0.0.3 begins aggregating the routes, the number of Type 3 LSAs sent to the backbone area decreases to three, and two LSAs from that area, 10.0.8.0 and 10.0.13.0, are combined into one, 10.0.0.0:

aviva@RouterG> show ospf database OSPF link state database, area 0.0.0.0 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.17.1 192.168.17.1 0x8000018c 20 0x2 0x8c71 60 Router 192.168.18.1 192.168.18.1 0x80000124 2842 0x2 0x97cd 60 Router *192.168.19.1 192.168.19.1 0x8000009c 367 0x2 0xb4c7 48 Network *10.0.0.1 192.168.19.1 0x80000091 67 0x2 0x98af 32 Network *10.0.1.2 192.168.19.1 0x80000090 667 0x2 0x92b3 32 Network 10.0.2.1 192.168.18.1 0x8000009b 1892 0x2 0x6cd1 32 Summary 10.0.0.0 192.168.17.1 0x80000001 20 0x2 0xf59f 28 Summary 192.168.15.1 192.168.17.1 0x80000005 20 0x2 0xf13 28 Summary 192.168.16.1 192.168.17.1 0x80000005 20 0x2 0x77ea 28 ASBRSum 192.168.15.1 192.168.17.1 0x80000005 20 0x2 0x120 28 OSPF AS SCOPE link state database Type ID Adv Rtr Seq Age Opt Cksum Len Extern 10.0.15.0 192.168.15.1 0x80000002 10 0x2 0x3d02 36 Extern 172.19.121.0 192.168.15.1 0x80000001 402 0x2 0x859a 36 Extern 172.19.121.0 192.168.17.1 0x80000004 117 0x2 0x71a9 36 Extern 192.168.15.1 192.168.15.1 0x80000001 402 0x2 0x6d9 36 Extern 192.168.17.1 192.168.17.1 0x80000003 481 0x2 0xddfb 36