Implicit-Null Label Value

[dknov]

Hi,

A quick question. Does anyone know if when said that last hop router signals implicit null to penultimate router it actually sends the advertisement (LDP for example) with a label value of 3 or is there another way to signal that?

If so, I assume that with MPLS-TE LSPs it's the same thing but with RSVP Resv message...

Thanks,

David

[mheusinger]

Hello David,

in fact Label value 3 has the meaning "implicit null". So whatever protocoll is used (LDP or RSVP) this is the signaled value to activate PHP (penultimate hop popping).

From RFC 3032 section 2.1

" iv. A value of 3 represents the "Implicit NULL Label". This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is "Implicit NULL", the LSR will pop the stack instead of doing the replacement. Although this value may never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved."

Hope this helps! Please rate all helpful posts!

Martin

[dknov]

Hi,

So in essence IOS just shows implicit-null (in "show mpls forward" for example) , but in fact value of 3 is actually being advertised.

David

[mheusinger]

Hello David,

yes either you see "imp-null" or "pop" in various show commands as a result of an advertisement of label 3.

Adn I am quite happy about this, because it is easier interpreting the output and not having to know/remember what Label 3 means.

Hope this helps! Please rate all posts

Martin