Subject: 2547 BGP VPNs

From: Sachin Kalra <[email-masked]>
Date: Tue, 10 Sep 2002 16:02:32 -0400

Dear MPLS Community:

Please consider the following simple but strange scenario.
Lets say we have two directly connected PEs. Traffic is flowing from CE1 to 
CE2. According to VRF table entry on PE1, for destination CE2 bottom label 
is "X" and top label is "Y". And, we are doing penultimate-hop-popping 
(PHP) for the LSP between PE1 and PE2.

                             LSP
                        ========>
[CE1]------[PE1]-----------------[PE2]------[CE2]

So, my question is who does push and who does pop here? Will top label "Y" 
be ever used in this scenario?

Thanks for your response,
Sachin Kalra

===========================================================
Sachin Kalra
Modeling Engineer
OPNET Technologies, Inc.
(240)-497-3000 x2796
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Register for OPNET's Online Technology Workshops
(http://www.opnet.com/TechWorkshops/)
===========================================================

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From: Rahul Aggarwal <[email-masked]>
Date: Tue, 10 Sep 2002 13:14:55 -0700

> Please consider the following simple but strange scenario.

Its not all that strange :) 

> be ever used in this scenario?

The top label Y in this case is an implicit null (value=3) label. PE1 will
send a packet on the wire with only the one label in the label stack : label X.
Label Y is used in the signaling control plane by PE2 to indicate to PE1 to
do PHP for the LSP.

The label actions on PE1 are implemenation dependent. The simplest approach
is to regard this as a push X on PE1 as the encapsulation will be programmed
to push only one label.