The MPLS WG Archive

[David Charlap <dcharlap@fore.com>]

"Abes, Andi" wrote:
> 
> Now my question is, why can't this model be followed under traffic
> engineering conditions ?
> 
> Router Ax will learn from BGP what is the local AS egress point
> towards D.  It will then consult the IGP-TE database and construct an
> explicit route to that router, and tuck at the end a loose route to D.
> It is then the egress router's job to "fix" the explicit route such
> that the ERO now reflects the route to the next AS. The Ingress router
> in transit AS's will do just the same - it will change the ERO (that
> now contains 2 entries - one that indicates the current AS, and one
> for the end destination) such that it contains a strict route accross
> the new current AS and a loose route to the destination.  and so on
> till the destination AS.

Actually, you should be able to do better than this (if RSVP-TE is used,
anyway.  I don't know about CR-LDP)

According to draft-ietf-mpls-rsvp-lsp-tunnel-05.txt, the explicit route
object supports an AS-number subobject.

So, the ingress router should be able to get (from BGP) an explicit
route of AS numbers from source to destination.  It can make an ERO that
begins with the hops through the local net, and ends with the list of AS
numbers for ASes beyond the local network.

Upon arrival in the next AS, the router there can replace the
AS-subobject for itself with a per-hop route to the next AS on the list.

And so on.

So the ingress router can't specify the complete path, but it can
specify which ASes it wants the path to go through on its way to the
destination AS.

> Each BGP router can consult what ever TE constraits it has to deceid
> what BGP next hop to use, but it will only construct a strict route
> through the local AS and append a loose route to the final
> destination.

It could do this also.

A loose route from the last local router to the final destination could
work, but that means the actual path chosen is left completely up to the
routers in other ASes.

A route that explicitly lists ASes may be better.  There is still some
looseness, since each AS will determine its own route through, but you
can keep the route going through the ASes you want.

The first way would definitely be easier to implement, but the second
way might also be desirable under certain circumstances.

-- David