The MPLS-OPS Archive

["Akhilesh" <akhilesh@trinc.com>]

Leticia

Penultimate Hop Popping (PHP) is the optional implemetation in MPLS.  Using
PHP, v can reduce the overhead of the LERs.
In PHP, Egress(LER) will give IMPLICIT NULL LABEL in label mapping msg to
its Upstream LSR(Penultimate Hop) .  As, this LSR has IMPLICIT NULL LABEL as
a out label, it will blindly pop the Label stcak entry, and forwards
remaining packet to the LER.
    If one does not do PHP, then when the LSP egress receives a packet,   it
first looks up the top label, and determines as a result of that  lookup
that it is indeed the LSP egress.  Then it must pop the stack,   and examine
what remains of the packet.  If there is another label on  the stack, the
egress will look this up and forward the packet based on this lookup.  (In
this case, the egress for the packet's level m LSP is also an intermediate
node for its level m-1 LSP.)  If there is  no other label on the stack, then
the packet is forwarded according  to its network layer destination address.
Note that this would  require the egress to do TWO lookups, either two label
lookups or a label lookup followed by an address lookup.

Cheers
Akhilesh Yede
Hyderabad, India.

----- Original Message -----
From: Leticia Cozart <lcozart@technicacorp.com>
To: <mpls-ops@mplsrc.com>
Sent: Tuesday, September 25, 2001 5:35 PM


> Hello-
>
> Is anyone familiar with the term Penultimate Hop Popping?  What is its
> purpose?
>
> Thanks!
>
> Leticia...
>
>
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