When you configure multipoint redistribution it’s a good idea to use route tagging to prevent advertising something from routing protocol “A” into “B” and back into “A”. Let me give you an example:
Above you see routing protocol A and B. At the bottom we find network 18.104.22.168 /24. Let’s say that R1 redistributes this network from routing protocol A into B. R2 will learn about this network from routing protocol B and will redistribute it back into Routing Protocol A. This is something you want to avoid and we can do it with route tagging…here’s an example:
When R1 redistributes network 22.214.171.124 /24 into routing protocol B it should tag it. When R2 is going to redistribute routing information from routing protocol B into routing protocol A it will notice the tag and skip redistribution for network 126.96.36.199 /24.
Of course the same thing applies to R2. I’m only showing you the example for R1:
R1#show ip route 188.8.131.52 Routing entry for 184.108.40.206/24 Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2 Last update from 192.168.12.2 on FastEthernet0/0, 00:00:04 ago Routing Descriptor Blocks: * 192.168.12.2, from 192.168.45.4, 00:00:04 ago, via FastEthernet0/0 Route metric is 20, traffic share count is 1
You can use show ip route to check if a route has been tagged or not. Nothing has been tagged so far. Let’s configure route tagging:
R1(config)#route-map TAG deny 10 R1(config-route-map)#match tag 1 R1(config-route-map)#exit R1(config)#route-map TAG permit 20 R1(config-route-map)#set tag 1
You can tag routes using a route-map. I created a simple route-map called TAG with two sequence numbers:
- Sequence number 10 says that when it matches tag number 1 that it should be denied.
- Sequence number 20 says that we need to set tag number 1. There’s no “match” statement so EVERYTHING will match.
R1(config)#router rip R1(config-router)#redistribute ospf 1 metric 5 route-map TAG
R1(config)#router ospf 1 R1(config-router)#redistribute rip subnets route-map TAG
Now I need to make sure we use the route-map when redistributing. The example above is for redistributing into RIP and OSPF. At the end of your redistribution command you need to specify the route-map. Everything that is redistributed INTO RIP or OSPF will have a tag of 1. Once again I’m only showing R1 but you need to do this on R2 as well.