Tag Route!

Unanswered Question
mheusinger Wed, 01/31/2007 - 03:44


One example among others:

assume you have f.e. OSPF to EIGRP and vice versa on two routers for redundancy. To avoid routing loops you want to avoid redistributing back into OSPF, what originally came from OSPF ... and the other way round. One option is to LIST all networks with a prefix-list. This might not be feasible in a very dynamic environment or where many routes are to be handled.

So one can set a tag during OSPF to EIGRP redistribution and filter all routes with that tag when redistributing from EIGRP to OSPF.

This way you get a flexible configuration independant on the number of routes or networks in use.

Regards, Martin

mheusinger Wed, 01/31/2007 - 04:17


An example is given in


The main idea is: a tag is nothing else as a specific "marking" to identify special routes. You could also say a tag is like an attribute, lets say "blue". Maybe the task is not to announce all "blue" routes to a certain location. The meaning of a tag is defined through the route-map and conditions therein to set the tag.

So in the redistribution case, a tag can mean "everything inserted into OSPF from EIGRP on a specific router doing redistribution".

In another router you could say: "If routes in EIGRP came from OSPF I do not want to reinject those routes back into OSPF". The question remains: How can a router know which networks where redistributed somewhere else in the network? Answer: The router should look for the specific tag which has this special meaning in your environment.

One router is setting the tags based on criteria, which you can define through a route-map. Another router will search for the tag and perform some actions based on another route-map.

Regards, Martin


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