The MPLS WG Archive

["Peter Ashwood-Smith" <petera@nortelnetworks.com>]

Title: RE: MPLS routing accross AS boundaries

All of it is correct and you are perfectly right that it is possible to do local constraint based routing between AS's but not over the AS. This is called stepwise or piecewise constraint based routing as opposed to hierarchical constraint based routing which requires that summarized constraint information cross AS boundaries.

My own opinion, for what it is worth, is that we will probably have to start with piecewise routing since this is relatively easy to do.  Then we can look at ways to make better gateway decisions based on constraints, possibly with more elaborate forms of inter AS feedback. We will have far more success getting something working reasonably well if we keep the solution simple and start with stepwise.

My guess is the BGP guys will like this idea and the PNNI guys will prefer full hierarchy. Like I said though some simulations are probably in order here to help put things properly in perspective.

This would be a great project for a masters student somewhere, simulate the two approaches for a set of real AS and come to PA with some results. Perhaps some of the George Mason Students from the advanced research lab would be interested in doing this?

Cheers,

Peter Ashwod-Smith

-----Original Message-----
From:   Abes, Andi [SMTP:aabes@quarrytech.com]
Sent:   Wednesday, March 29, 2000 5:28 PM
To:     MPLS mailing list
Subject:        RE: MPLS routing accross AS boundaries

Hi,
 
I'm probably out of my league here so please correct me.
 
One of the rules of BGP, and in general inter-domain routing protocols
is to advertise reachability to other AS's, while hiding as much as possible

the local AS's topology.
 
Such that when route Ax (AS A, route x) needs to send a packet to an address
D, the only thing it will learn from BGP is that there's a route to the
destination D, that travels through some AS's along the way. It will also
learn the egress router in his own AS that needs to get the packet.
How he needs to get to the egress router is a matter for an IGP to resolve.
 
Any of this correct ?
 
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.
 
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.
 
Please let me know if this makes some sense.