Subject: pernultimate LSR

From: "Chang, Chong Yie" <[email-masked]>
Date: Thu, 18 Apr 2002 07:29:05 +0800

Dear David:

David Charlap wrote:

"Chang, Chong Yie" wrote:

I got some different view from NULL and implicit NULL label.
From my view, if a downstream LSR is not capable of handling
MPLS encoded packets but it needs the upstream LSR utilize the
function of label forwarding, it will distribute implicit NULL label
to its upstream.

If the downstream LSR is incapable of forwarding labeled traffic, then
it has no business participating in MPLS signaling ;-)

If an egress router can not pop the label stack, then it may send an
implicit NULL upstream - requesting that the upstream router pop the
stack before sending the packet.  This may result in the egress router
receiving a labeled packet (if it's terminating the innermost LSP of a
hierarchical LSP) or an unlabeled packet (if the next-hop is beyond the
edge of the MPLS cloud.)

I agree your explanation and thanks.

If the downstream need the upstream forward packets according
to packet layer 3 address, it will distribute explicit NULL label
(0).

>From the standpoint of packet forwarding, implicit NULL is the same as
explicit NULL.  In both cases, the egress router must forward the packet

based on its contents after popping the label stack.  With implicit
NULL, the previous-hop router does the pop.  With explicit NULL, the
next-hop router does the pop.  There is no other significant difference.

The penultimate may pop out the last label to reduce the effort of
egress LSR
(according to Uyless Black's book). If the penultimate knows that it can
pop out
the last label and it can find out the outgoing port from the incoming
label as
the index. The penultimate LSR may also knows that it must pop out the
label and forward to the specific port (This is defined in NHLFE).
I believe that it can reduce the effort of penultimate LSR and egress
LSR.
It is more like the name label switching.

    Best regards,

                                            Chang, Chong Yie

-- David
.