route-map tag

[suthomas1]

bgp is redistributed under ospf as below. i am trying to catch the meaning of route-map & its statements which is used here.

router ospf 5

redistribute bgp 1234 subnets route-map redistribute_bgp-to-ospf

route-map redistribute_bgp-to-ospf

route-map redistribute_bgp-to-ospf, deny, sequence 10

  Match clauses:

    tag 41

  Set clauses:

  Policy routing matches: 0 packets, 0 bytes

route-map redistribute_bgp-to-ospf, permit, sequence 20

  Match clauses:

  Set clauses:

    tag 41

  Policy routing matches: 0 packets, 0 bytes

Please help me to understand what does the tag 41 refer to and what does this entire statements do.I am unable to locate this tag in the configuration.

TIA

[Richard Burts]

This route map is using the tag to avoid redistributing routes which have already been redistributed (which would contribute to a routing loop). The first instance in the route map says that if a route has a tag with value 41 then it should be denied redistribution. The second instance in the route map redistributes routes and assigns the redistributed route a tag with value of 41.

You will not find these tags anywhere else in the configuration. The tags are only used within the route map.

HTH

Rick

[suthomas1]

Thanks,

But how does the route-map know which routes are carrying tag 41 , so that it does the action of deny / permit as in the configurations.

TIA

[cadet alain]

Hi,

In BGP  routes are tagged with most recent AS number but when redistributing into OSPF the tag is maintained.

So the first sequence of your route-map will not redistribute the routes having  the tag 41 so announced by  AS 41 and the second sequence will tag all the other BGP routes when redistributing them into OSPF. Are you multihomed? If not then you won't have any routes into OSPF if next hop is  AS 41 and if you are then you will redistribute only routes from one ISP only.

Can someone confirm my reasoning is right?