how to worsen iBGP routes

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May 8th, 2009
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I have two sites which are connected via eBGP to a provider. These links are the main connection to the headquarter.

For backup purpose we use a transparent L2 link between both sites; where an iBGB neighbor ship is setup.

In case of a failure of one of the main links the traffic of the affected site should go through the iBGP neighbor and it's eBBP partner on the other site.

This works - but the fallback (when the failed router comes back) doesn't work.

Then the routing gets asymmetric.

The problem is to worsen the iBGP routes against the one from the provider.

Tried to local pref, AS path prepend etc. - but nothing works.

Isn't there a simple solution?

Thanks for your help

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Laurent Aubert Fri, 05/08/2009 - 05:55
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eBGP routes are preferred by default over iBGP routes so it should work.

could you post the sh ip bgp of the router when the link comes back and the eBGP session is UP again ? Also the routers BGP configuration could help as well



Giuseppe Larosa Fri, 05/08/2009 - 06:59
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Hello Frank,

I guess you mean that after the failed router comes back you see traffic from HQ to the multihomed site flowing only on the other link.

And for this reason the routing becomes asymmetric.


if instead it is the previously failed router that still prefers the IBGP routes to the eBGP routes coming from the provider I would suggest to use an high value of weight.


router bgp your.ASN

neigh ebgp-peer weight 40000

this should provide correct restore of eBGP routes whatever config you have.

In any case I agree with Laurent, to understand why this happens you should provide the configurations.

Hope to help


lamav Fri, 05/08/2009 - 14:32
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No extra configuration should be needed. As laurent pointed out, eBGP is always preferred over iBGP. So once the eBGP neighboir is restored, that edge router should automatically select the ISP over the iBGP crosslink.

Post the configs.



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