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PeterBLAdmin-5508 asked SaiKishor-MSFT edited

BGP Path Selection with 2 VPN tunnels

Hi,

I can see within documentations that 2 ExpressRoute links could use BGP AS Prepending to select the primary link. I also see that 1 ExpressRoute and 1 VPN would also work with BGP by choosing Expressroute routes over VPN. What I haven't found was, what happens with 2 VPN links, how can I influence BGP to prefer routes over one link compared to the other? Your answers are appreciated.

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SaiKishor-MSFT answered SaiKishor-MSFT edited

@PeterBLAdmin-5508 Thank you for reaching us on Microsoft Q&A.

I understand that you would like to know how you can influence route selection using BGP with 2 VPN link as this is not documented anywhere. You would still use AS Path Prepending for this too and the one with the lower path will be preferred. Hope this helps.

Please let us know if you have any further questions and we will be glad to assist you further. Thank you!

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@SaiKishor-MSFT

The following document states, "You need to peer with a public AS and append public AS numbers in the AS PATH to influence routing for Microsoft Peering."

https://docs.microsoft.com/en-us/azure/expressroute/expressroute-optimize-routing#solution-use-as-path-prepending

If peering with private AS numbers how can we influence routing such that traffic to the site A uses the VPN tunnel to site A and not site B? At what point are private AS numbers removed as we can still see them in Learned Routes of our Virtual network gateway?


Thanks.

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@DuncanEdwards-0696 Apologize for the delay in response to your question, I am looking into it and will get back to you soon.

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