So guys,

The brain teazer for the day.

Have a BGP core service provider network.

Prefixes that come from an external BGP peer on my router get redist into EIGRP and the natural BGP ASN (well the carrier ASN) flows naturally from BGP into the EIGRP tag field into the topology table.

Now.

Lets say mutual redist is on and I want to stop the BGP prefixes that have now come into EIGRP from going back out into BGP. Yes, I can have a route-map to deny any tag with the carriers ASN going back into BGP. This works.

BUT. Say, I want an extra level of protection, outside of redistribution route-maps, lets just say the IOS gotcha of the following thread applies :-

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&type=Subscriptions&loc=.2cc05b15/4&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing

And there is a mis configuration, and redistribution starts to happen unconditionally.

Would there be a way of preserving the EIGRP tag from EIGRP to BGP (rather than setting the tag on a route map :- Im getting confused writing this now so example below :-

prefix 1.1.1.1/32 comes in from BGP AS1

gets redist into EIGRP with tag of 1

now gets redist back into BGP with tag of 1 (without setting tag manually)

have an outbound route-map on the external peer saying if this bgp prefix 1.1.1.1/32 has as tag of 1, deny

Can you pass a tag from eigrp into BGP? Is there this concept?

A good anaolgy of this would be "the bgp as-path" loop prevention, where it sees its own ASN and will not advertise it further, but doing this naturally between an EGP to IGP back to EGP?

I hope I have not confused everyone, but if anyone knows of a way or method to do this? its an interesting one.

Thx to all,

Ken