Harold Ritter Mon, 06/04/2007 - 11:01
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You generally have a label stack when you deliver services over your MPLS backbones, such as L2VPN, L3VPN, 6PE, 6VPE (L3VPN for IPv6).

The first label (AKA IGP label) is used to forward the packet from the ingress to the egress PE and the second one (AKA service label) is used to demultiplex at the PE level and to forward the packet to the appropriate egress interface.

Hope this helps,

mohammedmahmoud Mon, 06/04/2007 - 11:12
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The first label in the stack is called the top label, and the last label is called the bottom label. In between, you can have any number of labels.

Two examples of such MPLS applications are MPLS VPN and AToM. Both MPLS VPN and AToM put two labels in the label stack.

For example, in ordinary MPLS VPN, a label stack of 2 labels is attached to the packet. The Top label should be the LDP label for the egress PE router (for the packet to reach the Egress PE router). The second label should be the VPN label assigned by the egress PE router (for the packet to go to the right VRF or the outgoing interface) which is exchanged between PE routers via MPBGP.

Please do refer to the attached diagram.

I hope that i've been informative.

HTH, please do rate all helpful relpies,

Mohammed Mahmoud.

dangal.43 Mon, 06/04/2007 - 11:22
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i tried to understand the implicite Null and Explicit Null label but i am not getting it very clearly so will you please explain me it in litttle bit details?

as well as what are the label forwarding methods?

Harold Ritter Mon, 06/04/2007 - 12:09
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The implicit null is advertised as a value of 3 by the egress PE, to the penultimate hop router to tell it to pop the outer label before forwarding the packet.

The explicit null is advertised as a value of 0 by the egress PE, to the penultimate hop router to tell it not to pop the outer label but rather to do a label swap with the label value of 0. This is sometimes useful when you want to preserve MPLS QOS caracteristics down to the egress PE.

As for your second question, I am not sure what you are referring to. Can you please clarify.

Hope this helps,


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