Manual route summarization (EIGRP) problem.

[Reprovoid]

  Hi.I've setup a small network In PT and I've enabled manual route summarization for EIGRP on one of the routers serial Interfaces.When I check the neighbouring connected routers EIGRP routes I see that both the manually summarized route (which has a lower metric) and the specific routes(with no auto-summary) are shown.

Router#sh ip route eigrp

     10.0.0.0/8 is variably subnetted, 10 subnets, 3 masks

D       10.0.0.0/30 [90/21024000] via 10.0.6.1, 00:07:01, Serial0/1/0

D       10.0.1.0/30 [90/21024000] via 10.0.6.1, 00:07:01, Serial0/1/0

                    [90/21024000] via 10.0.8.1, 00:00:13, Serial0/0/0

D       10.0.2.0/23 [90/20514560] via 10.0.8.1, 00:00:13, Serial0/0/0

D       10.0.2.0/30 [90/21536000] via 10.0.6.1, 00:00:15, Serial0/1/0

D       10.0.3.0/24 [90/21026560] via 10.0.6.1, 00:00:14, Serial0/1/0

D       10.0.4.0/24 [90/21026560] via 10.0.6.1, 00:06:58, Serial0/1/0

                    [90/21026560] via 10.0.8.1, 00:00:13, Serial0/0/0

D       10.0.5.0/24 [90/20514560] via 10.0.6.1, 00:07:01, Serial0/1/0

I know the routes to 10.0.2.0 and 10.0.3.0 via 10.0.8.1 have a lower metric and I tried summarizing them with a 255.255.254.0 mask.Now It's showing the routes via 10.0.6.1 which have higher metrics along with the summarized route.I know I've done something wrong but I'm not sure what.

[Peter Paluch · Accepted solution]

Hello Reprovoid,

What you see is normal and natural. Your router simply learns about three different networks - 10.0.2.0/23, 10.0.2.0/24 and 10.0.3.0/24 (remember that two networks are same only if their addresses and masks both match). For each of them individually, it selects the shortest route and places it into the routing table. Your router does not make any assumptions about the relation of these networks - that the 10.0.2.0/23 is a summary of the two remaining networks. That's why it holds both summarized and unsummarized routes in its routing table.

If a part of a network is summarized then it should be summarized on all routers that connect it to the rest of the network. Otherwise, some routers will advertise the summary network while other will advertise the individual subnets (also called components). The rest of network therefore learns about all networks - both the components and the summary network. Not only does this defeat the purpose of summarization (make the routing tables of the remaining routers smaller), but also it causes all the traffic for the summarized networks to actually flow exclusively through the routers that do not perform the summarization - because of the longest-prefix-match lookup logic in routing tables.

Please feel welcome to ask further!

Best regards,

Peter

[Reprovoid]

Thank you! You explained It so well I understood It before finishing the first paragraph! (famous last words)