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EIGRP routes vs eBGP routes

 

https://supportforums.cisco.com/document/29361/preferring-mpls-vpn-bgp-path-igp-backup

I am having a very similar issue to the one documented above here. In my case, the Primary MPLS (BGP) fails and backup link (EIGRP) takes over. Once the Primary MPLS link comes up, the traffic remains routing over backup due to the redistributed EIGRP route staying in the routing table for the Primary router. One thing that I don't quite understand is WHY this happens.

SCENARIO 1:

If EIGRP route is recieved first for a prefix then its entered to the routing table. If BGP establishes and eBGP learns the same prefix, the EIGRP route remains in the routing table becuase it has a higher weight of 32768 vs the default eBGP weight of 0 i,e Best Path Selection Process is used to calculate the best path and AD is NOT even considered.

 

SCENARIO 2:

However, if eBGP route enters the routing table first, and then EIGRP route is recieved, the BGP route remains in the routing table because the AD of eBGP is better than EIGRP (AD 20 vs 90).

How come Best Path Selection is not used in that case? Why does AD get used in this case and not like in the first scenario? 

 

1 REPLY

Hi, in Scenario 2:The eBGP

Hi,

 

in Scenario 2:

The eBGP route gets to the RIB as the first one.

Then the same prefix comes via EIGRP and the router compares the ADs of both prefixes (the best ones per routing protocol) to decide which should go to the RIB.

So the eBGP route remains in the RIB and the EIGRP prefix is not redistributed to BGP (note: Only routes which are in RIB can be redistributed to other routing protocols!).

 

In Scenario 1:

The EIGRP route comes to the RIB as the first one.

Then it gets redistributed to BGP. Then the same prefix comes via eBGP. It's compared by the BGP Best Path selection Algorithm with the same prefix already present in the BGP database (redistributed from EIGRP originally) to choose the best route within BGP protocol. As the redistributed prefix has got better weight attribute, it remains the best within BGP. So no new best BGP route and no reason to compare the ADs!

 

If you want the eBGP prefix to win again in Scenario 1, you need to configure a route-map used for EIGRP to BGP redistribution which would change the weight attribute to 0 for the redistribiuted prefixes. Then other BGP attributes would get compared and the eBGP prefix might win (if you want to be sure, increase its local preference, e.g.).

 

Best regards,

Milan

 

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