BGP Backdoor Routes

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May 4th, 2007
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Hi Cisco Gurus,

can I extend the BGP Backdoor Routes functionality between iBGP and eBGP. In our scenario, we are trying to achieve load sharing between ebgp and ibgp. Currently we are learning default routes thru ebgp... we would now learn the same route as a backdoor route and do load sharing with ibgp.


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royalblues Fri, 05/04/2007 - 02:07
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I dont think i understood your question properly.

Backdoor routes are used as a redundancy option but not for load sharing. In certain network topologies, a BGP speaker might learn routes to the same prefix from an external BGP peer and via an IGP protocol.

EBGP routes have an administrative distance of 20 and are preferred over IGP routes, which have much higher administrative distances. Hence these routes are configured as backdoor so that the IGP route takes preference.

When you learn the same route via EBGP and IBGP, then it will always prefer the EBGP route as IBGP has a higher AD.

HTH, rate if it does


saimbt Fri, 05/04/2007 - 02:19
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Hi Narayan,

Whatever you said is abs right. As when we learn the same route via EBGP and IBGP, then it will always prefer the EBGP route as IBGP has a higher AD. In order to defeat this rule, we will learn the route (default) as a backdoor route, and learn the same route as a ibgp route.

I understand that bgp backdoor is predominantly used between IGP and eBGP.

royalblues Fri, 05/04/2007 - 02:31
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I would still prefer to use the BGP attributes rather than backdoor :-)


saimbt Fri, 05/04/2007 - 02:33
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usign the BGP attribute is always possible, we were trying to reinvent the wheel and wanted to do load sharing either by backdoor or by local-AS

saimbt Fri, 05/04/2007 - 02:43
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This is the exact scenario,

we have 2 links from two different providers one of them is an mpls link where as the other one is point-to-point link.

routes which we are learning through mpls are eBGP routes.

AS at both the location is same, now if we run BGP on this PP link the routes learned

through this route will be iBGP route. And thus this link will not be preferred.

If i specify all the network of other location as backdoor network, which i am learning through eBGP then finally it will show me as locally generated network and then i can do load balanicing between both the links (mpls & PP)


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