Simpler, with B and C peered: F / \ B---C \ / A If B does not send the /24 to F, then F will send all the traffic to C, even if A wanted a load balance. Maybe I could ask the community: Why do you advertise longer prefixes with the same nexthop as the shoter prefix? Is it this use case, or something else? Thanks, Jakob.
-----Original Message----- From: Russ White [mailto:7riw77@gmail.com] Sent: Saturday, April 30, 2016 12:35 PM To: Jakob Heitz (jheitz) <jheitz@cisco.com>; nanog@nanog.org Subject: RE: Superfluous advertisement (was: Friday's Random Comment)
A use case for a longer prefix with the same nexthop:
F / \ D E | | B C \ / A
Suppose A is a customer of B and C.
This is possible, but only remotely probable. In the real world, D and E are likely peered, as are B and C. Further, it's quite possible for F to choose the path through E anyway, regardless of A's wishes, or even to load share over to the two paths. If it's really a backup path, and you don't want traffic on it unless the primary is completely down, then you need to not advertise it until you actually need it. One of the various principles of packet based routing is that if you advertise reachability, it means someone, someplace, might just choose the path you've advertised. You can't control what other people choose.
:-)
Russ