[189015] in North American Network Operators' Group
RE: Superfluous advertisement (was: Friday's Random Comment)
daemon@ATHENA.MIT.EDU (Jakob Heitz (jheitz))
Sat Apr 30 16:05:16 2016
X-Original-To: nanog@nanog.org
From: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
To: Russ White <7riw77@gmail.com>, "nanog@nanog.org" <nanog@nanog.org>
Date: Sat, 30 Apr 2016 20:05:11 +0000
In-Reply-To: <008a01d1a317$56e33080$04a99180$@gmail.com>
Errors-To: nanog-bounces@nanog.org
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)
>=20
>=20
> > 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.
>=20
> 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 choo=
se
> the path through E anyway, regardless of A's wishes, or even to load shar=
e
> 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.
>=20
> :-)
>=20
> Russ
