Policy Routing and/or Redistribution: gaining a greater understanding

siggnation

So, if I am to understand redistribution with policy and policy routing...

Distribution lists are merely lists that "call upon" ACLs or prefix lists, the lists that actually perform the filtering. Prefix lists are less cpu and memory intensive and filter based on subnet mask, perhaps best used when redistributing routes and wanting to be particular about which subnets are redistributed; whereas ACLs/distribute lists are used to permit a particular host to somewhere? I can't see the advantage of ACLs over prefix lists when redistributing routes.

Route maps (very cool), can be used with redistribution, but mainly for policy routing, and they can call upon ACLs and prefix lists, but don't require them. They use match/set (if/then) statements. For example, I can use a route map to call upon a named prefix list while redistributing routes and setting a metric if those particular prefixes are matched, etc. Can also use route maps for a scenario where I have multiple gateways and want to filter one client's traffic to go to one gateway, and another client's traffic to travel to the other gateway.

Sorry if I lost anyone with my rambling, but this is my way of thinking out loud to try and clearly distinguish the differences between these concepts. Please, if anyone can add anything to clarify, I would greatly appreciate it.

Cheers.

networker050184

I'd say play around with it a bit to get a feel for it. Different tools for different jobs. I'll always use a route-map when I can to call upon ACLs or prefix-lists. And as is usual in networking there is more than one way to accomplish the same thing.

siggnation

networker050184 wrote: »

I'd say play around with it a bit to get a feel for it. Different tools for different jobs. I'll always use a route-map when I can to call upon ACLs or prefix-lists. And as is usual in networking there is more than one way to accomplish the same thing.

Thanks networker. Your response leads me to believe i'm heading in the right direction. The labbing has helped tremendously. There's no way to gain a full understanding without it, in my opinion.

I tried a few Route Maps (with ACL) to control route redistribution from RIP to OSPF with great success. Going for Route Map/prefix list next and maybe a match/set route map for some policy routing if i'm feeling confident enough.

networker050184

BGP is where you really get into the fun with route maps. Lots of things to do there. Most other protocols you'll only use it on redistribution with simple filtering.

OfWolfAndMan

The only time I've used a route map in the real world is for a routing protocol migration, using route tags and filtering only a default route into OSPF

jdballinger

I use route maps all the time. Especially if you're in a BGP rich environment, their flexibility and power become crucial to pulling things off sometimes. For instance I just finished implementing a new extranet, and one of the things that I looked into using was conditional BGP advertisements. These use separate route maps to advertise specific routes depending on the existence (or non-existence) of other routes or conditions. The power behind things like that is amazing. Or using a filter-list to do actual route filtering in OSPF, which is a difficult thing to do as I'm sure you've come across.

siggnation

Great examples guys...

Thanks Wolf.

@jd-- cool man!