The MPLS WG Archive

[Jeremy Lawrence <jlawrenc@cisco.com>]

At 16:16 05/25/2000 +0530, Naveen Seth wrote:
>Hi,
>The definition of MPLS domain in the draft-ietf-mpls-arch-06.txt says that the MPLS nodes are in one routing or administrative domain.
>  
>1. Does it imply that we cannot have 2 AS(Autonomous System) within the same MPLS domain?

This is possible only under very restricted circumstances. Consider
the ASBRs of two adjacent ASes. If either or both ASBRs summarize
eBGP routes before distributing them into their IGP, or if there is
any other set-up where the IGP routes cover a set of FECs which
differs from that of the eBGP routes (and this would almost always
be the case), then the ASBRs cannot forward traffic based on the
top-level label. A similar argument applies to TE tunnels. Some
traffic usually will be either IP forwarded by the ASBR, or forwarded
based on a non-top-level label (sections 10(a), 10(b), and the later
part of 10(c) of
http://www.ietf.org/internet-drafts/draft-rosen-rfc2547bis-01.txt
describe MPLS applications where forwarding based on a non-top-level
label will occur at an ASBR).

So there would usually be 2-3 MPLS forwarding domains if there were
two ASes: one for each of the two ASes, and possibly one for the link
between the two ASBRs (in the case that labelled packets instead of
Ip packets are forwarded between the two ASBRs).

Also, it's likely that the ASBRs could not be ATM-LSRs, as ATM-LSRs
typically have limited or no capability of manipulating label stacks
or forwarding unlabelled IP traffic.

Regards,

Jeremy Lawrence