Auto-summary Essentials + an Example

Today’s post is a ramble on the [no] auto-summary command. Think you know it? The proof of the pudding: give me one sentence that specifically states on what routers you need to consider the auto-summary setting, and characterize what happens differently depending on the setting. Can you do it? Throw out your answers here. Today I’ll answer it in the context of an example, as promised in the previous post; check out that post before reading more if you want to try it out first.

Last post, I asked you to look back at a lab from the CCENT Skills blog. That lab had the requirement that the routers advertise subnets instead of classful networks – but the lab also stated that you cannot configure commands that are unnecessary. In short, that means you have to figure out on which of the three routers you need to configure no auto-summary, and on which of the three you do not need to bother changing from the default of auto-summary.

The (default) setting of auto-summary tells a router’s IGP (RIP, EIGRP, OSPF) to change its behavior in regards to what routes it advertises in a particular case. That case? When that one router has interfaces in more than one classful network, and that one routing protocol is also enabled on those interfaces.

In those cases, the routing protocol does not simply advertise routes for subnets. Instead of advertising routes inside one classful network, the router advertises a route for the entire classful network – a summary route – when advertising out the interface in the other network.

Clear as mud, right? That’s why examples help so much with this topic. So, looking back at the lab, which routers have interfaces in multiple classful networks? R1 does not, but R2 and R3 do. So auto-summary has no effect on R1, no matter whether turned on or off.

Next, consider what happens if R2 and R3 were configured with the default of no auto-summary. R2’s S0/0/1 interface sits in class A network 10.0.0.0. When R2 advertises routes out that interface, towards R1, the advertisement would list a route for the entire class B network 172.16.0.0/16, and it would not list routes for any subnets of class B network 172.16.0.0/16. Similarly, R2 would advertise a route for class A network 10.0.0.0/8 out its S0/0/0 and F0/0 interfaces, which are in network 172.16.0.0, but R2 would not advertise subnets of network 10.0.0.0. Figure 1 shows the big concept.

Figure 1  R2’s Automatically Summarized Routes

By disabling auto-summary on R2 using the no auto-summary RIP-2 subcommand, R2 does not advertise the summary routes shown in the figure, but instead advertises the subnets it has routes for.

Finally, just to emphasize it because it’s easy to miss, the lab solution back in the CCENT Skills blog correctly shows only R2 and R3 with a no auto-summary command. This command, for IGPs, controls the action based on the router’s local interfaces as enabled for the IGP routing protocol. R1 has interfaces in network 10.0.0.0 only, so the no auto-summary command has no effect.