Superfluous advertisement (was: Friday's Random Comment)

Russ White 7riw77 at
Sat Apr 30 19:34:39 UTC 2016

> 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.



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