BGP to EIGRP redistribution

Unanswered Question
Mar 10th, 2009

Here is the strange config. We have two site running BGP. one site is primary..now-both site learn same routes from the cloud but backup site prepands AS# to all routes learned from the cloud -in (yes in not out) than redistributes that to EIGRP. Primary site also redistributes BGP into EIGRP. Both sites use same metrics while redistributing. Please if you can tell me if that prepanding done on backup router influences how the BGP is redistributed? how ? I'm looking why backup site to get to subnets learned from BGP gets to them through the primary site - EIGRP..so it takes the route from primary as better route for that subnet. Did that prepand affected how backup router redistributes?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
Loading.
Harold Ritter Tue, 03/10/2009 - 15:33

Mateusz,

So you mean that both routers are connected to the core via BGP and are also learning EIGRP routes through backdoor links, correct?

The as path prepending on the BGP routes has no bearing on the EIGRP redistributed routes.

Without knowing the configuration of these two routers, I would say that the reason why the backup site prefers the EIGRP routes rather than the BGP learned routes is probably a question of timing. The backup router learned the EIGRP routes first and then redistributed them into BGP. This would lead to the locally sources BGP prefix being preferred over the one learned from the core.

Regards

mike-greene Tue, 03/10/2009 - 19:34

Hi,

If I understand you correctly I've seen this before and I've corrected it by raising the weight above 32768 on the MPLS BGP peer.

Routes learned from the MPLS would have a weight of 0 because they did not originate from the router. Routes redistributed from EIGRP to BGP on the same router would have a weight of 32768 and EIGRP would install the routes in the routing table. Once I raised the weight to 64000 on the MPLS BGP peer all the routes from the MPLS were installed in the routing table.

Hope that helps.

Harold Ritter Wed, 03/11/2009 - 06:12

Mike,

Another way to solve this might be not to redistribute the EIGRP external routes (redistributed from BGP) back to the core.

Regards

mike-greene Wed, 03/11/2009 - 06:45

True. There is always more then one way to skin a cat, especially in this game we call routing. The two environments I've had to do BGP redistribution have been running EIGRP as there IGP. The MPLS circuits were BGP and the backup T1's to regions or customer equipment ran EIGRP. With this topology I did route summarization on the EIGRP links so the BGP learned routes would have the best path and get installed in the routing table at the local site.

The second was another MPLS migration but the backup circuits were Internet connections using GRE/IPsec tunnels (EIGRP) to the remote sites for backup. On these, I raised the distance in EIGRP on the Internet routers to 180 so the external routes being redistributed from BGP to EIGRP on the MPLS router would be better routes with 170.

Sorry for the long winded post but I enjoy reading about how people do things with routing to accomplish networks goals Seems like there a many ways to accomplish issues with most of them not being write or wrong.

Mike

Matt qomat Wed, 03/11/2009 - 07:17

But we want the other site to know if it looses MPLS link how to get to the MPLS cloud, so it will learn those subnets through EIGRP. I kinda see now why backup preferrs primary MAN connection rather than BGP-MPLS.EIGRP got 32768 weight while redistributing into BGP.

Matt qomat Wed, 03/11/2009 - 07:08

Yes.I think I've seen similar explanation. I'm not sure how to check how those routes got weighted. Routing will only say it's EIGRP learned route. I know Cisco says that they get by default 32768. I can see the solution to put a rute-map on a eBGP neighbor to weight routes learned from that neighbour.

Harold Ritter Wed, 03/11/2009 - 07:12

Mateusz,

You can do a "show ip bgp" to see the BGP routes and verify the behavior that Mike and I explained.

Regards

Matt qomat Wed, 03/11/2009 - 10:38

ok.

so I did show ip bgp neighbor x.x.x.x advertised-routes. I checked one of the routes that we learn through bgp. I did a traceroute to that route. It goes through my primary site(not desirable) I do sh ip route x.x.x.x and it shows learned throguh EIGRP distance 170.So it got redistributed from BGP to EIGRP on my primary location and when it came to this location..it is injected it got weighted ? is that true? why my router goes through primary. First I thought it's because of AS Prepand that we do "in". but that should not affect how this router gets to those routes. It starts coming to me, that it may be because primary site learned those routes first..injected in EIGRP..second site learned that from primary..while redistributing to BGP it weighed those routes with 32768 and that's how second site goes through primary.But I do not know how to confirm that. Do you know the way to confirm that injected route get that weight? sh ip bgp | include x.x.x.x . I do not see that weight. Can this have anything with iBGP relationship between them ?

Actions

This Discussion