Subject: MPLS domain - AS

From: "Naveen Seth" <[email-masked]>
Date: Thu, 25 May 2000 16:16:07 +0530

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?

2. Is an MPLS domain contained within an AS?.

3. Can we have more than one MPLS domains within an AS?

Naveen

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From: Curtis Villamizar <[email-masked]>
Date: Thu, 25 May 2000 14:09:11 -0400

In message <[email-masked]>, "Naveen Seth" wri
tes:
> 
> 1. Does it imply that we cannot have 2 AS(Autonomous System) within the =
> same MPLS domain?

A common IGP is needed to exchange link state and traffic engineering
information.  If the two AS did share a common IGP, they would more
likely be sub-AS within BGP confederations within a common AS.

> 2. Is an MPLS domain contained within an AS?.

Generally yes.  Strictly speaking it doesn't have to be.

> 3. Can we have more than one MPLS domains within an AS?

Yes and this is expected to be quite common as MPLS TE capability
moves closer to the edges of large provider networks and those
providers want to limit the number of LSPs in the core.

> Naveen

Curtis

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From: Jeremy Lawrence <[email-masked]>
Date: Fri, 26 May 2000 07:02:53 +1000

>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

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From: "Brijesh Kumar" <[email-masked]>
Date: Fri, 26 May 2000 10:25:52 -0400

In his last mail, Jeremy Lawrence writes

> >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.
> >
> traffic usually will be either IP forwarded by the ASBR, or

Well... This is inconsistent with the MPLS Arch document.

A MPLS domain is defined in MPLS architecture document as "a contiguous set
of nodes which operate MPLS routing and forwarding and WHICH ARE IN *ONE
ROUTING OR ADMINISTRATIVE DOMAIN*".

This means that a MPLS Domain, at most, can cover one Administrative Domain.
Any other implementation (i.e., an MPLS domain being defined as part of two
administrative domain) is not in line with this definition. Whether this
should be the case or not, is all together different issue.

Cheers,

--brijesh
Ennovate Networks Inc.

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From: Jeremy Lawrence <[email-masked]>
Date: Sat, 27 May 2000 13:22:25 +1000

Someone else has privately pointed out that Brijesh and I are in
violent agreement on this issue. :-)

This is not quite true: we are generally in agreement, apart from
one detail. Comments inline...

>Well... This is inconsistent with the MPLS Arch document.

No, it is consistent but unlikely - see below.

>This means that a MPLS Domain, at most, can cover one Administrative Domain.

Agreed.

>administrative domain) is not in line with this definition.

Agreed.

It is possible for two ASes to be in the same administrative domain.
There are a small number of large SP networks which consist of more
than one registered AS, and there are probably many more which use
private ASes. In the unlikely case that an MPLS domain does extend
across two ASes, the two ASes must have tightly coordinated routing,
which would require that they are in the same administrative domain
(at least in effect).

Regards,

Jeremy