Route aggregation w/o AS-Sets

Wed Apr 15 18:24:35 UTC 2020

Suppose you had a set of customers than all announced to you a set of routes
and all those routes complete an aggregate
and you announce only the aggregate to those customers
and you include an AS_SET with it
then those customers will drop your aggregate, thinking there is an AS-loop
and those customers will not be able to reach each other.

An AS_SET does not prevent routing loops and can prevent correct routing.

But you must include the ATOMIC_AGGREGATE attribute, so that someone else
does not disaggregate your aggregate that does not have the AS_SET.

Regards,
Jakob.

-----Original Message-----
Date: Tue, 14 Apr 2020 02:32:37 -0700
From: Matthew Petach <mpetach at netflight.com>

I generally would use the atomic-aggregate knob to
generate aggregate routes for blocks I controlled,
when the downstream ASN information was not
necessary to propagate outside my network
(usually cases where I had multiple internal ASNs,
but all connectivity funneled though a single upstream pathway.)

If you have discrete downstream ASNs with potentially
different external pathways, you shouldn't be generating
aggregate routes that cover them; that's just bad routing 101.

Thus, my rules for aggregation always came down to:
1) is there more than one external/upstream pathway for the ASN and prefix?

    If so, don't aggregate.
2) is redundant, reliable connectivity between all the external gateway
routers that would be announcing the aggregate?
    If not, don't generate a covering aggregate.
3) If there's only a single upstream pathway through you for the ASN and
prefix,
and that won't be changing any time soon (eg, you have a collection of
downstream
datacenter with their own ASNs and prefixes, but they all route through a
common
backbone), then use the atomic-aggregate option to suppress the more
specific
AS_PATH information, and simply announce the space as a single aggregate
coming
from your backbone ASN.

That way, there's no confusion with RPKI and AS_SETS; all you're ever
announcing
are simple AS_PATHs for a given prefix.

Best of luck!

Matt