Route tagging

Unanswered Question
Aug 22nd, 2009

All,

Am I correct in my assumption that route tagging is primarily used in redistributing scenarios, and the tags are usually set on the router that's doing the redistributing?

Thanks,

John

I have this problem too.
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Jon Marshall Sat, 08/22/2009 - 05:42

John

Yes is the short answer.

Edit - of course the router that tags the packets may not be the one that then matches the tag and applies a policy/filter based on these tags.

Jon

Giuseppe Larosa Sat, 08/22/2009 - 10:34

Hello John,

route tags are very useful when you have multiple ASBR routers performing mutual redistribution between two routing protocols/domains.

This is to avoid unwanted feedback of routes learned from protocol A at ASBR1, (injected by ASBR1 in protocol B) to be reinjected in protocol A at ASBR2 from protocol B.

So as Jon has noted the router that sets the route tags does this to help other devices to do the right decisions in redistribution scenarios.

To be noted that working with route redistributions some combinations of protocols allow to propagate a route tag from protocol A into protocol B other don't do this.

To be more specific RIPv2 supports route tags but its route tag field is 16 bit wide. OSPF and EIGRP have 32 bits route tags.

In another thread some mounths ago this issue was discussed because the OP noted that it was not possible to pass a route tag from OSPF or EIGRP to RIP but it needed to be manually set in the route map used to decide what to redistribute into RIP.

Also to be noted that like BGP communities route tags are a form of dynamic filtering that is they can adapt to a change in the network:

if a new subnet is advertised by protocol A it can be redistributed into protocol B with the appropriate route tag with no change to configuration (unless needed to allow it).

Hope to help

Giuseppe

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