How to configure Route Tagging

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 1.1.1.0 /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 1.1.1.0 /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 1.1.1.0 /24.

Of course the same thing applies to R2. I’m only showing you the example for R1:

R1#show ip route 1.1.1.0
Routing entry for 1.1.1.0/24
 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.