route summarization question

Aman_saini

I have a doubt on the route summarization
Let me explain it with a example if we summarize a route 10.1.0.0/22 which probably have 10.1.1.0, 10.1.2.0, 10.1.3.0. . And some how a network goes down (eg. 10.1.2.0) then how a router tell it to the other routers because it was sending updates of 10.1.0.0/22 .

Forsaken_GA

It wouldn't.

You generally only summarize at points where the routers doing the summarization are the only points of ingress. Any routers further upstream don't care if the routes are up or down, as they have to go through those routers regardless.

For example, let's say you have R1, and it's connected to an upstream provider called BB1.

Let's say R2, R3, and R4 are all connected to R1.

R1 = 10.1.0.0
R2 = 10.1.1.0
R3 = 10.1.2.0
R4 = 10.1.3.0

R1 sends 10.1.0.0/22 to BB1 as a summary.

No matter what network BB1 is trying to get to in that range, it has to go through R1, so it doesn't need to know about the more specific routes. Instead of having 4 routes in BB1's routing table, it only has 1. This saves on resources, which in the global internet routing table, is important.

So if R4 goes down, and BB1 had traffic to forward to 10.1.3.0, it would send the traffic to R1. Since R4 is down, R1 would no longer have a route to 10.1.3.0, and it would just drop it. If BB1 did have the route to 10.1.3.0 via R1, and R4 went down, the R1 would withdraw the route, and BB1 would drop the traffic itself, so the net result of not summarizing is that the traffic would be dropped earlier.

If the route is down, then the route is down. It's all a question of do you want to save some bandwidth on the links, or do you want to save some router resources. It's a judgement call.

I personally prefer to summarize at my borders, as internal links may go down, and I don't want to cause reconvergence events outside of my network if at all possible (and some providers will kill your peering session if you do cause convergence issues on their side too often, like say if you've got a router that's overloaded and constantly has links flapping, or have a router with some bad hardware acting funky). If you've got a suitably large network, then causing convergence events internally is also a bad thing, so you may be doing some summarization internally.

Aman_saini

Thanks greatly explained