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Ask the Expert- SD-WAN


Understanding BGP Best Path Selection & Manipulation

I am looking at BGP route manipulation within our global MPLS for several 3rd party connections which are connected at various sites and am looking for some advice about path manipulation.


In my scenario, we have a route injected by a 3rd party in the London Office and then the same route injected into Paris office. Both offices are connected to the MPLS and an office in New York would like to get to that 3rd party network. Each internal site has its own AS number.


I would like to be able to control that users in New York go via London first and then Paris as a backup. I am thinking that a simple Routemap change to import the routes in New York with a higher localpref would be suitable. I was thinking that I could then either locate the site specific route based on the AS number of the originating site, but then looked into communities and am not sure on the best approach.


I was expecting that I could simply match on the London AS number using Regex on the path, along with the targeted subnets and then set the localpref higher. Is this too simple, or would it be better to tag the routes when they are imported at the source and then match on this community value to set the localpref?


Could anyone advise on which one is best to use and which one would be best in this scenario?

Everyone's tags (2)
VIP Collaborator

Re: Understanding BGP Best Path Selection & Manipulation

If I understand you correctly, your network is a single AS. New York, London and Paris are their own AS.

Setting local preference with the route map will work within your AS, yet this isn't an attribute that carries across AS boundaries.

You could advertise the route to New York and they could set the local preference themselves. Communities are also a way to go, yet once again the New York side would have to associate the community value with the proper local preference.

Sounds like you are on the right track, just have to hash out the details.

Hope this helps

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