Using other provider's ASN (was Re: Sprint and peering points)

Sun Apr 1 12:57:04 UTC 2001

Sean Donelan writes:

| If Sprint isn't pre-pending its routes with Sprint's ASN, who is?

Who cares?

Maybe more interesting is how is this done?   Curious minds want to know.

| It is generally considered ill-mannered to use someone else's ASN w
| without their permission.

Sez who?  AS_PATH is a mostly-simple attribute which is used for
loop avoidance.  Using it as a trail of breadcrumbs is a convenience,
but in reality, the only important thing is that eBGP-speakers 
check "that the autonomous system number of the local system does
not appear in the AS path" without discarding the route, although
the current draft does point out that eBGP talkers may be configured
to use such routes but such a router's "Operations ... are outside the
scope of this document".  (I would have said "sane analysis", but...)

The loops that are avoided are ROUTING ANNOUNCEMENT loops, not
forwarding loops.   The AS_PATH  need have no relation to the
way actual traffic will flow when a BGP-speaker selects that route.

| Traditionally, pre-pending is done with the pre-pender's own ASN.

So, if tradition is so important, why aren't you runing rcp_routed for IDR?

| If nothing else, to help track down the party when evil things happen.

Sorry, all you can glean from an AS_PATH alone is that your neighbour
wants to avoid having the route seen by the things in that AS_PATH.

Attributes like AGGREGATOR exist to track parties down, since the
AS_PATH cannot be, in some cases.

What's your worry here?  That someone can stuff your AS into an
AS_PATH (personally I'd do it by forming an AS_SET), and thus you 
can't see a given route?   Didn't you follow the ptomaine discussion
where exactly that was done to an aggregate?

	Sean.