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advertising routes in BGP

Unanswered Question
Feb 15th, 2012
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Hi,


I am new to BGP, so need some help with BGP routing.


all our 10 sites are connected via MPLS. i have dedicated link between two sites which are in the same geographical area and is backup link between these 2 sites only.I want to use this link as a backup to MPLS so that all other locations can reach to that site using this link via my site. The primary connectivity is via MPLS. My question is, can i publish the same routes in BGP with a higher metric i.e weight, same will be published at the original site so that if the MPLS link is down for that particular site, all other traffic should reach that site via my MPLS link and then it will take dedicated link to reach the destination.


Thanks,

Sridhar

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JohnTylerPearce Wed, 02/15/2012 - 13:08
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Well, I don't have all the experience in the world incorporating MPLS and BGP, I would as long as routers that are connected via this backup link can advertise prefixes out so that the other sites can receive the prefixes then you should be able to do this. As far as which attribute to modify that would depend on how you have things currently configured which only you would know. You don't need to use weight since that's just local to the router, I would use the Local_Preference attribute in your case. As long as the defaults are in place, I would give the routers that are throwing out the prefixes via the primary link, a higher preference and the others just leave at the default. That way the primary link prefixes have higher preference and are therefore used over the lower preference backup link.


I hope that helped you some.

james-worley Thu, 02/16/2012 - 02:26
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When you say your sites are connected via MPLS I assume you mean your SP provides you with an MPLS cloud but you yourself do not opporate MPLS? Assuming you are using a standard CE->PE model and connect to your SP via eBGP then this should work.


If the CE for site A has an active route to site B over this private link the simplist thing to do is re-originat those prefixes into the SP cloud with a AS-prepend. This is easily done with an export route-map. Attributed like weight and local-preference will not be passed over eBGP sessions.


If I understand correctly you have two hub sites connect via a private circuit. I assume traffic between these sites flows over these links when the WAN is UP? Do you run dynamic routing between these sites already?


For an exact answer it would help to know the ASs involved. Do all your site CEs use the same AS number? Do you have access to the CEs?

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