Moving on in this chapter of miscellaneous nifty things you can do with routing protocols, we come to route summarization. The idea is that you can reduce the size of your routing table by summarizing a bunch of smaller routes into a single larger route. For instance, let’s say a router was connected to 2 different networks, 10.0.0.0/24 and 10.0.1.0/24. Normally, 2 networks, 2 advertisements, 2 routes in the routing tables right? Well, with summarization, you could condense that down to a single 10.0.0.0/23 summary route that would cover both routes. That’s just an example to give you an idea of what we’re talking about.
Some things to understand about route summarization:
- The summary route will be assigned the same metric by the advertising router as the lowest metric of any routes covered by the summary.
- As mentioned in the opening paragraph, individual routes that make up the summary aren’t advertised by the summarizing router. (Or we wouldn’t be summarizing, now would we?)
- The summary itself won’t be advertised if the router doesn’t have any individual routes that would fall inside that summary route.
- Locally the summarizing router will point the summary route to Null0 (the bit bucket, dead zone, no man’s land, packet cemetery) to avoid routing loops.
- Summary routes make routing tables and topology tables smaller, theoretically improving the performance of convergence. The idea is that there’s less for routing algorithms to think about, and few state changes remote routers would see that would cause a convergence event anyway.
- Summary routes can cause inefficient routing, since there’s fewer individual routes for the router to make a forwarding decision on.
So, how does one perform route summarization with different protocols?
- EIGRP – “ip summary-address eigrp 1 172.16.0.0 255.240.0.0” placed within an interface paragraph would cause EIGRP to advertise 172.16.0.0/12 as a summary route to any neighbors connected on that particular interface. If you want, you could also tag an administrative distance on the tail-end of that statement…although it’s not used for what you might think. You might think that the AD would be advertised with the summary to neighbors – not so. Rather the AD is used by the summarizing EIGRP router to determine whether or not it should place the summary route pointing to Null0 in his table or not. BTW, the EIGRP AD for summary routes defaults to 5.
- OSPF – with OSPF you can only summary at area border routers or autonomous system boundary routers. You can’t summarize within an OSPF area, because then there would be differences within the OSPF link state database. That makes OSPF sad – more technically, it blows the SPF computation right out of the water. Ergo, OSPF won’t allow you to summarize within an area – only ABRs or ASBRs.
- In “router ospf”, “summary-address 10.0.0.0 255.0.0.0” would inject a summarized external route of 10.0.0.0/8 at an ASBR.
- In “router ospf”, “area 54 range 192.168.0.0 255.255.0.0” would inject a summarized route of 192.168.0.0/16 into an area at an ABR. Which is REALLY cool for making your routing tables smaller. Think about this for a second. Let’s say you had 3 areas, 1, 2 and 3, plus backbone area 0. Now let’s say you had an awesome design where you had maybe hundreds of networks in area 1, but they were all in the 10.1.0.0/16 range. And the same story in area 2, but all your routes were in 10.2.0.0/16. Area 3, 10.3.0.0/16. Instead of advertising every single route out of those areas into the other areas, you could summarize at each area’s ABR(s). The only route you’d see in non-area 1 routers for area 1 destinations would be the single summary route – 10.1.0.0/16. How cool is that? Using this technique, you could get your routing table down to something small, tidy and efficient. Easy to manage, easy to troubleshoot, scalable.
- RIP – works like EIGRP, but with notable exceptions. “ip summary-address 10.0.0.0 255.0.0.0” applied to a RIP interface would summarize RIP advertisements for 10-space route to 10.0.0.0/8. But…
- You can only summarize with RIPv2, as RIPv1 doesn’t support VLSM.
- You can’t create a supernet with RIP summarization. Trying to summarize 172.16.0.0 255.254.0.0 wouldn’t work.
- You can only use one summarization statement per classful network. So, you couldn’t advertise multiple /16 summarizations falling under the 10.0.0.8 classful “A” network, for instance. (Side note – does anyone really use RIP as a serious enterprise routing protocol? Someone at Cisco must think so, or we wouldn’t be having this discussion.)