Only Label Distribution? without Label Revocation?

[yunsog]

The books I read about MPLS only said Label Distribution without Label Revocation.

I suppose, without revocation, the label space will be enomous.

Is there label revocation process in MPLS? If yes, why most of the book do not mention it?

[romccallum]

what does revocation mean :-)

[yunsog]

As MPLS Router generate Label for each ip prefix and and regenerated when the topology change.....

The number of label will be huge.....

For example, if there was a 10.1.1.0/24 network and it was disappeared... and 10.1.2.0/24 network was exist and disappeared...

Even though they are disappered for long time the label for 10.1.1.0 and 10.1.2.0 will be reserved and the label can not be reused for other prefixes....

At last, the resource of the label will be scarce... and the MPLS router can not allocate label for newly discovered prefixes after all...

Therefore, I suppose, the MPLS router should revoke the unused label for future use... (so, I said revocation)

Isn't there similar mechanism in MPLS?

[b-watkins]

I believe you have a misconception regarding the retention of labels. If, in your example, the prefix 10.1.2.0/24 were to disappear, the labels are implicitly "revoked". There is no explicit message sent out by the LDP/TDP neighbors, but once the label is gone from the LIB and LFIB (and again, this is triggered by the removal of the route from the route table) it will no longer exist and that label would be available for reuse.

At least, that is my understanding. Someone please correct me if I am wrong.

[Harold Ritter]

Yunsog is correct. Once a label has been allocated locally to a prefix, the association between the prefix and the label will not changed even if the downstream LDP peer withdraws the label it previously advertised. The label is quite large (over 1,000,000 labels) and I have never seen a customer getting anywhere close to it. Bear in mind that labels are not allocated for BGP leant routes, only to IGP routes (static included).

The default label pool is from 16 to 1048575.

Hope this helps,

[b-watkins]

Thanks for the correction. Clearly it was I who had the misconception.

So even if a router loses a route, it won't remove the label that's assigned to that prefix from its local label space?

It's interesting that there is no way to reclaim the label. I guess I don't understand why, if the prefix is removed from the route table, that the label would not be available. Is this perhaps a conservative measure to ensure that the label is not reused too early, possibly causing unintended consequences to both the new LSP and the former LSP should it come back up?

[handsomeamit1]

Thats why we have RSVP, Resource Reservation Protocol.

[romccallum]

nope but what it does do is cause mpls reconvergence on the way back up to be rather fast ;-) (if configured properly with your IGP)