BGP best path

[rsavena14]

Hi,

I have experienced that when I do a show ip bgp command in a public internet routeserver I only see one available path which is the best path for a specific block. Since I am doing multihoming with two ISPs I was expecting two paths incase best path goes down. But When I try to remove the advertisement to the best path ISP the new path becomes visible in the show ip bgp command. Do you guys have any  explanation?

[Peter Paluch]

Hi,

After a BGP speaker runs its bestpath algorithm, it selects only a single best path and it advertises it to its neighbors (where appropriate). This is an important fact to remember - a BGP speaker always advertises only a single best path to its neighbors, not multiple best/alternative paths.

Looking into a public route server's BGP database may therefore be inconclusive - it may have only a single neighbor, thus there is only one choice to choose from, or all its neighbors agree on a single best path (even if they do see some alternatives), so the public route server learns only about this single best path from its multiple neighbors despite the neighbors may (but may not!) be aware of alternative routes.

This is how I understand it.

Best regards,

Peter

[rsavena14]

Hi Peter,

Thank you for your explanation. I forgot to mention that when I removed the advertisement in the Main ISP the route server sees my prefix but a different next hop (or Neighbor). Not the original neighbor, so this confirms that the route server is seeing two possible routes.