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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.

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Tags: IP Routing, Redistribution

Forum Replies

djpooley says:
Can you confirm if this is my correct reading of this route-map
Networks from Protocol B ‘Jack’ are untagged until they enter Protocol A where they are Tagged - (The first statement is ignored because they are not Tagged)
```
Jack(config)#route-map TAG permit 20
Jack(config-route-map)#set tag 1 
```
When these now Tagged routes hit ‘John’ Still in Protocol A they are denied from re-entering Protocol B via the First statement.
```
 route-map TAG deny 10
    John(config-route-map)#match tag 1
```
But as you say the route-map has to deny the Tagged routes first or it will allow
... Continue reading in our forum
rtsask says:

Hello Rene,

I have intlo0 with ip address of 1.1.1.1/24

I have access-list 1 permit 1.1.1.1 0.0.0.0 and route map route-map TST permit
route-map TST
set tag 222

match ip address 1
set tag 111

and redistribute into ospf from eigrp

redistribute eigrp 100 subnets route-map TST

but at the destination router I only see tag 222 for the prefix 1.1.1.1 instead of 111

as son as I change the access list wild card mask to 0.0.0.255 the destination router sees the correct tag of 111

Could you please explain this why with wilcard 0.0.0.0 the correct tag is not shown but wit
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asattareee says:

Hi Rene,
Please please help me out regarding redistribution. when redistributing mutually between ospf and bgp by tagging and i m getting an error

% “TAG1” used as redistribute ospf into bgp route-map, set tag not supported"

route-map TAG1 deny 10
match tag 1
route-map TAG1 permit 20
set tag 1

//cdn-forum.networklessons.com/uploads/default/original/1X/429954a1ab1f680c3a4470abd54ddb55ec9239b6.png

according to the topology, I have to redistribute mutually bet OSPF and BGP at R3 and R4. I have to advertise 10.2.0.0/16 from R3 and R4 towards R1 and R2. simple
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ReneMolenaar says:
Hi Abdus,

Route tagging when redistributing into BGP is not supported. That’s why you get that error:
```
R1(config)#route-map SET_TAG permit 10
R1(config-route-map)#set tag 10

R1(config)#router bgp 1
R1(config-router)#redistribute ospf 1 route-map SET_TAG
% "SET_TAG" used as redistribute ospf into bgp route-map, set tag not supported
```
You can use something else in BGP to “tag” redistributed routes, like a community value. Between OSPF and EIGRP, you can use tags to prevent routing loops.

Let’s think about the things that could go wrong in a topology like this if
... Continue reading in our forum
wilder7bc says:

This would have been a lesson that a functioning topology would have been really nice. I had created one myself and had some issues and had a ten mile post but I figured out what my issue was. I basically didnt have a completed route-map configuration on R2. I have fixed that now and everything is working.

Basically I was interested in the path of the traffic and the incomplete configuration on R2 was making it where nothing made sense but after I added the exact same configuration route-map wise on R2 everything was good.

//cdn-forum.networklessons.com/
... Continue reading in our forum

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