The MPLS WG Archive

["Abes, Andi" <aabes@quarrytech.com>]

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.