On May 24, 2008, at 9:15 AM, Marshall Eubanks wrote:
On May 23, 2008, at 8:15 PM, devang patel wrote:
Is that okay to use Same AS number for the two different site on
different
location?
To answer this specific question, Autonomous Systems should be
topologically convex.
This means, at the Internet interdomain routing (BGP) level, that
packets
cannot leave an AS in one place to get to locations in the same AS
in some other place.
So, to put two sites on one AS, there should be an internal
connection between them, which can be done
through your internal network, by a direct connection, or by a
tunnel. Traffic might come to
the AS at either site, and has to be routed internally to get to the
other.
I am afraid I have to disagree with Marshall.
The idea behind an AS when the routing protocols were written long ago
may have been a contiguous domain, but there are lots of things the
protocols did not originally envision.
If you have two islands, and they each have a prefix which is globally
routable, there is nothing wrong with the two islands sharing a single
ASN. Island A announces Prefix A, and Island B announces Prefix B.
Routing is done by prefix, not ASN, so there is no fear of Island A
getting packets for Island B, and therefore no requirement for
internal connectivity. And before anyone says anything about Island A
not having connectivity to Island B, these are obviously not "transit
free" networks, so each island can just point default. In fact,
cisco even has a knob to listen to paths with your own ASN in it so
you can do this without default (although I'm not sure I'd recommend
that).
It works fine and saves the community from burning an ASN.
--
TTFN,
patrick
