Multihomed route visibility

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1

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)

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2

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

3

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