Hello all! I am going to try and attempt to clarify this concept. To
enable MPLS on a Juniper, you must enable MPLS forwarding on each interface
you want to participate and on the router itself. Everything else is
optional. Every interface you enable MPLS, there are 2 special routes
installed in the label forwarding table. One route has a label of 0 and the
other one a label of 1. Label 0 means IPv4 explicit NULL. When the router
sees label 0, the router automatically decapsulates the MPLS header and
performs the IPv4 lookup.
The penultimate LSR inserts label 0 in the MPLS header as it forwards the
packet to the egress router. The label 0 tells the router that is the
egress LSR and therefore it should perform the IPv4 lookup. Label 3 as
stated already is an Implicit NULL label and it may be assign by an LSR.
When the LSR receives the the Implicit NULL label, it will pop the stack
instead of replacing it. In this case, the penultimate router is
responsible for removing the MPLS header instead of the egress router which
would otherwise and under normal situations receive a label of 0, it will
remove the header and route the packet. Label 3 will never apper in the MPLS
header. Juniper supports both methods (label 0 or 3)
Hope this helps!
Mario Puras
SoluNet Technical Support
Network Support Engineer
-----Original Message-----
From: dan martin [mailto:dmartin@micromuse.com]
Sent: Tuesday, April 03, 2001 7:41 AM
To: Lane Patterson; juniper-nsp@puck.nether.net; mpls-ops@mplsrc.com
Subject: RE: Juniper as egress LSR: to penultimate hop pop, or not?
its not a won't, its a shouldn't.
i think the standardards goal was to keep a single router on the edge from
doing two operations on the same packet, popping the label and forwarding
the ip frame.
i think asking a router to process a packet twice is the equivalent of the
double sided copier, a simple design idea that leads to a tortuously long
paper path.
-----Original Message-----
From: Lane Patterson [mailto:lpatterson@equinix.com]
Sent: Monday, April 02, 2001 4:43 PM
To: 'juniper-nsp@puck.nether.net'; 'mpls-ops@mplsrc.com'
Subject: Juniper as egress LSR: to penultimate hop pop, or not?
It appears that if I want to use a Juniper as an egress LSR, I MUST
use penultimate hop popping? For example, using signalled LSPs,
juniper will only send RESV's with a "3" label (implicit NULL), and
I don't see knobs for it to use an IPv4 explicit NULL "0" label.
Is this because the juniper forwarding path is incapable of doing
both a label pop and an IPv4 forwarding decision at line rate on
the egress LSR?
Cheers,
-Lane
Lane Patterson <lane@equinix.com>
Member Research Staff
Equinix, Inc.
(650) 316-6012
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