Multihomed route visibility
We are advertising the same prefix to ISP A & ISP B. When looking at various route servers across the world, I always see the path through ISP A in the BGP table, but most don't show a path at all through ISP B (some do, like Oregon IX). I would expect any router with the full table to show both routes. Is this a consequence of how the route servers are peered up? Are they only displaying the preferred path? ISP Issue? Am I missing something? Any help would be appreciated. Patrick Boyle Data Center Architect / Engineer Network Solutions Architecture and Engineering Section | Network Technology Services Bureau SITSD/Montana Department of Administration 406.444.2549 (D) SERVICE FIRST! Submit an Incident<https://montana.service-now.com/sp/?id=sc_category&sys_id=e15706fc0a0a0aa7007fc21e1ab70c2f> | Search our Knowledge Base<https://montana.service-now.com/sp/?id=kb_view2> | Request a Service<https://montana.service-now.com/sp/?id=sc_home>
On Mon, Nov 4, 2019 at 8:13 AM Boyle, Patrick via NANOG <nanog@nanog.org> wrote:
We are advertising the same prefix to ISP A & ISP B. When looking at various route servers across the world, I always see the path through ISP A in the BGP table, but most don’t show a path at all through ISP B (some do, like Oregon IX). I would expect any router with the full table to show both routes. Is this a consequence of how the route servers are peered up? Are they only displaying the preferred path? ISP Issue? Am I missing something?
Hi Patrick, Routers only propagate the *best* BGP route for the target network. You'll see both only where the AS received different best routes from its peers. Regards, Bill Herrin -- William Herrin bill@herrin.us https://bill.herrin.us/
On Oct 31, 2019, at 3:12 PM, Boyle, Patrick via NANOG <nanog@nanog.org> wrote:
We are advertising the same prefix to ISP A & ISP B. When looking at various route servers across the world, I always see the path through ISP A in the BGP table, but most don’t show a path at all through ISP B (some do, like Oregon IX). I would expect any router with the full table to show both routes. Is this a consequence of how the route servers are peered up? Are they only displaying the preferred path? ISP Issue? Am I missing something?
Any help would be appreciated.
You will generally only see the best path. Most route servers don’t collect any backup paths the routers may have, which could be quite extensive. - Jared
participants (3)
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Boyle, Patrick -
Jared Mauch -
William Herrin