BGP and EIGRP routing

Unanswered Question
Jan 5th, 2008
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Is it possible to have 2 different protocols (bgp and eigrp) in the routing table with the same cost and the same destination? (maybe change bgp administrative distance to 170 same as eigrp) If so, how does the router determine which protocol to use?


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Overall Rating: 4.5 (2 ratings)
chschroe Sat, 01/05/2008 - 12:29
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If you have two equal cost routes, they are equal cost.

The thing is, the metric structure of each routing protocol is generally incompatible, so it is up to the protocol itself to offer its best route or routes to the routing table.

That being said, BGP only offers up the best route (routes) to the routing table, which it arrives at via its standard procedure. EIGRP will do the same. They will come up with completely different metrics, which aren't really comparable to each other.

The router will not determine which protocol to use, it will admit really whichever gets the first within the limits of the cef load balancing max-paths set. If a path doesn't get admitted it doesn't get admitted.

I'd like to hear more about why you'd like to do this. It is EXTREMELY dangerous and can lead to very unpredictable network behavior as well as routing loops and worse.

Within your network you still need IGP routes to get to BGP next-hops, so if everything is reachable via EIGRP you're going to have to be very creative to get them into bgp and complete to be sent around anyway, and that amount of work can be very counterproductive.

Bottom line, barring some really innovative reason for needing to do this, I see it as a very dangerous and risky move that will require a lot of extra effort just to get going anyway.

Looking forward to hearing more about what you want to do with this.



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