Gaynor, Jonathan wrote:
My institution has a single /16 spread across 2 sites: the lower /17 is
used at site A, the upper /17 at site B. Sites A & B are connected
internally. Currently both sites have their own ISPs and only advertise
their own /17's. For redundancy we proposed that each site advertise
both their own /17 and the whole /16, so that an ISP failure at either
site would trigger traffic from both /17s to reconverge towards the
unaffected location.
My worry/question: will carriers down the line auto-summarize my
advertisements into a single /16, resulting in a 'load sharing' while
both sites are active? If you're a backbone carrier and you saw x.x/16
and x.x/17 (or x.x/16 and x.x.128/17) being advertised from the same
peer would you drop the longer match?
No, BGP does not work this way. But you may force some carriers to have
only /16. First, you may try to announce the /17's with the community
no-export, so they will be seen only by your direct ISP, not by the rest
of the world. Or you may try to use some other communities to limit
announcements of your shorter prefixes, only to some part of the world.
--
Grzegorz Janoszka
