Subject: Implicit NULL label/Penultimate hop popping and LDP

From: Suresh Boddapati <[email-masked]>
Date: Sun, 22 Apr 2001 17:59:58 -0700

Hi,
	Section 4.1.5 of "MPLS Architecture" states 
	"However, if the penultimate LSR is an ATM switch, it may not have
the capability to pop the label stack. Hence a binding of Implicit NULL may
be distributed only to LSRs which can support that function".

	Elsewhere in section 3.16 it states,
	"Initial label distribution protocol negotiations MUST allow each
LSR to determine whether its neighboring LSRs are capable of popping the
label stack. A LSR MUST NOT request a label distribution peer to pop the
label stack unless it is capable of doing so".

	But the LDP specification does not have anything in the
initialization procedures in order to determine if a neighboring LSR
supports penultimate hop popping or not. Am I missing something? Thanks in
advance.

Regards,

Suresh

---

From: Ajay Simha <[email-masked]>
Date: Sun, 22 Apr 2001 22:41:41 -0400 (Eastern Daylight Time)

> supports penultimate hop popping or not. Am I missing something? Thanks in

From RFC 3036:

6. Areas for Future Study

   The following topics not addressed in this version of LDP are
   possible areas for future study:

      -  Section 2.16 of the MPLS architecture [RFC3031] requires that
         the initial label distribution protocol negotiation between
         peer LSRs enable each LSR to determine whether its peer is
         capable of popping the label stack.  This version of LDP
         assumes that LSRs support label popping for all link types
         except ATM and Frame Relay.  A future version may specify means
         to make this determination part of the session initiation
         negotiation.

-ajay

-- 
Ajay Simha
MPLS Deployment Engineer
IOS Technology Division
(919) 392-3141

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