BGP and aggregation

In the referenced message, Andy Walden said:

Conditional Router Advertisement:

http://www.american.com/warp/public/459/cond_adv.pdf

andy

As it sounds like he's using a single AS, the above may not be
a fix, since a partitioned AS is still a failure condition.

One other solution, which could possibly be used along with the above,
is to build a GRE tunnel from the interface facing transitA to the interface
facing transitB, and exchanging routing information with a really high
metric. This would keep the AS non-partitioned during loss of the
inter-city link.

General rules regarding collapsing a tunnel by routing the tunnel endpoint
via the tunnel, do apply. This, however, is relatively easy to mitigate.

The benefit to a tunnel is that it adds no additional state in the DFZ,
and also is unlikely to run into the standard problems with deaggregation
(that people will undoubtedly eventually filter you).

One disadvantage is that you may have some amount of u-turn traffic, but
you also won't have traffic following a default route to unreachable
entities as you would likely have with any other solution.

Why?

If you announce one prefix via one circuit and announce a different
prefix via a different with the same source AS, I don't see a problem
since traffic will continue to reach its intended destination.

andy