Welcome, to Routing Protocol Design.
I know this title feels a little misleading; as we just already covered design considerations for the major routing protocols out there, what could this possibly cover? Well, first layer 3 redistribution is an exam topic and I wanted to make sure we covered that, but also summarization and filtering are concepts that apply to all of the routing protocols and are something that should be considered during a routing domain design or expansion. We’ve gone over the importance of summarization in just about each of the routing protocol sections here but I just want to bring it up one last time to really drive home the overall purpose of summarization and its importance. Filtering and redistribution really go hand in hand as often you’ll use both at the same place and time, as you always should if you’re bringing in or expanding to a separate routing domain not under your direct control.
I really find when it comes to route summarization, there’s a pretty big gap between the theory of what we all learn and the actuality of what’s out there in practice. Most of the time you’ll go into an environment and do a little ‘show ip route’ and get back hundreds of routes! Now, I’m not saying people generally try to roll out a suboptimal network from the getgo, but we might not plan well for growth! Once you start getting requests from the company to add a guest network, or unexpectedly you need to support BYOD, or you start having way more than 100 sites in one region, then your plan might fall apart, right?
I wanted to make the point here because a lot of times it doesn’t quite come out right, that we’re not summarizing because our routers can’t handle it. Of course they can, especially modern routers, a few hundred routes is nothing! What you’re really doing is setting up a shield, a barrier where routing changes in that portion of the network end. We remember that summarization points act as a barrier for query messages from EIGRP, and for LSAs from OSPF. So we have protection in the event a route is flapping in some more remote area of your network that it won’t have any effect on the core or other parts of the network. This is really what summarization is for.
As far as sending summary routes, default routing is the ultimate in summaries and is a no brainer if you only have a single point to exit a network. Why does that branch need to know about all the routes in your campus? It doesn’t since everything needs to go out that MPLS anyway, just send a default route. A good point is that you should ‘send’ a default route, so that if you have changes to me made later, it’ll end up being more automatic and you’ll encounter fewer problems than you having to manually go around and adjust static routes at your branch offices.