Unanswered Question
Dec 13th, 2007

How implicit NULL and Explicit NULL label are useful in MPLS netowrk? what is main function of both the labels?

I have this problem too.
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Harold Ritter Thu, 12/13/2007 - 14:32


implicit (label value of 3) and explicit (label value of 0) null labels are used by the router terminating an LSP to tell the penultimate hop how to behave when sending traffic along the LSP. An implicit null means that the penultimate hop router has to pop the label before forwarding to the egress router and explicit null means swap the incoming label with a label value of 0 (explicit null). The explicit null is sometimes used to convey the MPLS QOS value down to the egress router (sometimes refer to the pipe mode).

Refer to the following document for more information on the pipe mode.



mheusing Thu, 12/13/2007 - 14:32


A good portion of it is explained in my post

The usage of implicit-null is for Penultimate Hop Popping (PHP), whereas the explicit-null label is mainly used to transport QoS markings, i.e. experimental bits in some specific scenarios.

Regards, Martin

dangal.43 Thu, 12/13/2007 - 19:51

but who will propagate both type of LABEL and for what kind of prefix it will use this label?

mheusing Fri, 12/14/2007 - 03:27


imp-null label is for signalling purposes only, and will be assigned to directly connected networks on a LSR.

Explicit null label will be assigned to directly connected networks in a LSR, but is actually really sent in labeled packets.

Both are signalled f.e. to the LDP neighbors, who will use the information to forward traffic to the LSR with the connected network.

Hope this helps! Please use the rating system.

Regards, Martin


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