BGP with Maximum-paths

Unanswered Question
May 21st, 2009

Hi Everybody,

I need a clarification on the behaviour of BGP with equal cost path (maximum-paths 2)

I have 1841 Router with Dual leased line named my ISP named ISP-1. I was runnning BGP between my router(AS 500) & ISP-1 (AS100). Since the two links are equal cost, by using maximum-paths command i was loadbalancing the traffic. "show ip route" command was showing that destination was having two paths one via ISP-2 & another via ISP-1

I surrendered one of the link to ISP-1 and procured a new link from ISP-2(200) and running BGP as like previous configuration.

But after this inspite of "maximum-paths 2" command only one route is appearing in the routing table of my router.

Why & What is the meaning of this behaviour of the router when connected to different AS. Why it is not load balancing or taking multiple paths when connected to different AS. But it is working if the BGP peering is same AS

can you please help me in understanding this concept and solution

RBK

I have this problem too.
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Laurent Aubert Thu, 05/21/2009 - 18:32

Hi RBK,

It means the routes you received from ISP2 are not identical as the ones received from ISP1 in terms of BGP attributes.

By default it's working when you are peering with the same ISP on both links as you are receiving exactly the same routes from both BGP neighbors.

To install several eBGP routes, all the attributes (AS-PATH, MED, origin,..) must match.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#bgpmpath

HTH

Laurent.

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