You want to reduce the size of your routing tables to improve the stability and efficiency of the routing process.
The ip summary-address eigrp configuration command allows you to configure manual summary addresses on a per-interface basis:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#interface Serial0/0.2 Router1(config-subif)#ip summary-address eigrp 55 172.25.0.0 255.255.0.0 Router1(config-subif)#end Router1#
By default, EIGRP will automatically summarize subnet routes into network-level routes. You can disable this with the no auto-summary configuration command:
Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#no auto-summary
Router1(config-router)#end
Router1#
Summarization is one of the most powerful features of EIGRP, and one of the most frequently overlooked ways to improve network efficiency. Unlike RIP, which summarizes along classful network boundaries, EIGRP uses CIDR, allowing you to summarize at any bit in the address as well as allowing supernets. And, while OSPF also allows this sort of summarization, as we will discuss in Chapter 8, OSPF can only summarize at the ABR. Conversely, EIGRP allows you to summarize at any router in the network. This means that you can have multiple hierarchical levels of address summarization with EIGRP, which can greatly improve the maximum size and efficiency of a large network that is designed properly to allow it.
You can see all of the summarization information, including which interfaces will send out summary addresses, using the show ip protocols command:
Router1#show ip protocols
Routing Protocol is "eigrp 55"
Outgoing update filter list for all interfaces is not set
Redistributed static filtered by 7
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: static, eigrp 55
Automatic network summarization is not in effect
Address Summarization:
172.25.0.0/16 for Serial0/0.2
Summarizing with metric 28160
Maximum path: 4
Routing for Networks:
10.0.0.0
172.22.0.0
172.25.0.0
Routing Information Sources:
Gateway Distance Last Update
10.1.1.1 90 1d23h
172.25.1.7 90 00:00:57
172.25.2.2 90 00:00:57
172.22.1.4 90 00:00:57
Distance: internal 90 external 170
Router1#
Note that when you summarize like this, the router doing the summarization will install a special route pointing to the null interface:
Router1#show ip route 172.25.0.0
<lines deleted for brevity>
D 172.25.0.0/16 is a summary, 00:00:23, Null0
Router1#
In this example, we have only summarized 172.25.0.0/16 on interface Serial0/0.2. However, it is important to remember that you can summarize several networks at the same time on a single interface by simply configuring all of the different summary addresses, as follows:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#interface Serial0/0.2 Router1(config-subif)#ip summary-address eigrp 55 172.25.0.0 255.255.0.0 Router1(config-subif)#ip summary-address eigrp 55 10.0.0.0 255.0.0.0 80 Router1(config-subif)#end Router1#
When it summarizes addresses, EIGRP will automatically suppress all of the routes that are included in the summary. Of course, if there are no routes to summarize, the router won't distribute the summary address.
The metric of this summary route will be equal to the best metric of the routes being summarized. It is important to remember this because, if the route with the best metric goes away for any reason, EIGRP will change the metric of the summary. So, if the route with the best metric is unstable, it will make the summary route unstable. If you want to ensure that this doesn't happen, you can configure a static route within the summarized range and point it to a null interface. Then you must configure the router to redistribute this static route into EIGRP.
The following example shows a CIDR supernet summarization:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#interface Serial0/0.2 Router1(config-subif)#ip summary-address eigrp 55 0.0.0.0 0.0.0.0 Router1(config-subif)#end Router1#
In this case, if there are any routes to distribute at all, EIGRP will distribute only the default route 0.0.0.0/0, and suppress all of the individual routes. This is actually an extremely useful technique on low-speed WAN links, particularly when this link represents the only path to the rest of the network. In such cases, the remote site only needs to know that it can get to everything it needs through this link. Further, because routing is always done by taking the longest match first, if the remote site happens to have more specific routing information for a particular destination, it won't use this summary route.
You could accomplish the same thing by injecting a default route (as shown in Recipe 7.5) and filtering out everything except 0.0.0.0/0 using a distribute list (as in Recipe 7.2). But this summary address technique does both of these actions in a single step. Furthermore, with this technique, the default route appears in the routing table as an internal route:
Router2#show ip route eigrp D* 0.0.0.0/0 [90/2172416] via 172.25.2.1, 00:00:30, Serial0.1 Router2#
Recipe 7.2; Recipe 7.5; Chapter 8
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