Subject: Bridging on an MPLS network....

From: "Arun M. Thomas" <[email-masked]>
Date: Wed, 20 Feb 2002 13:46:48 -0800

This question is focused more at actual use of MPLs rather than at what the
specs state about how it should be used.  Please let me know if this is an
inappropriate forum for this question.

I've been trying to understand the requirements as far as bridging and MPLS
from a practical perspective.  Suppose, for example, that the following LSR
exists on an Ethernet network:

	(Ingress) A --> B --> C (Egress)

For any particular packet traversing this LSR, is it reasonable to expect
that it will only touch three hosts enroute from A to C, or is it possible
that there could be a multitude of intervening hosts?  e.g. Could the actual
path the packet traverses look like:

	A --> 1 --> 2 --> B --> 3 --> C

where 1, 2, and 3 represent switches performing L2 bridging?

Thanks in advance for any comments.  They're much appreciated!

-AMT

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From: Sukanta ganguly <[email-masked]>
Date: Wed, 20 Feb 2002 14:35:47 -0800 (PST)

On a pure MPLS network that is correct. If the LSR is
A--->B--->C, then thats how the packet is enrouted.
However there is a lower layer VLAN like tunnel that
is setup for policy purposes or so then you may have
redirections but at the MPLS level they won't be seen.
Hope it helps...
SG

--- "Arun M. Thomas" <[email-masked]>
> Thanks in advance for any comments.  They're much

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From: [email-masked]
Date: Thu, 21 Feb 2002 07:11:47 -0500 (EST)

Hello Arun,

Yes it is possible.

Assuming that the ethernet bridges are doing 802.1 transparent bridging,
it could transit multiple intervening bridges.  This should have an
effect on transmission propagation delays and nothing else.

An 802.1 bridge will not alter the ethernet frames that are in transit
(unless the frame is bad according to 802.3, in this case it will be
dropped).

-Ben

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From: "Sukanta Ganguly" <[email-masked]>
Date: Thu, 21 Feb 2002 07:36:02 -0800

Arun,
  IF you have an LSR, as you mentioned, A ---> B ---> C, then the packets under MPLS will be enroute this exact path. However, if there is a lower layer VLAN setup, which for policy reason redirects the packet through different paths. Even then at the MPLS layer the other path would be unknown.

SG

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