Unanswered Question
Mar 27th, 2007

I have a query regarding configuration of Traffic engineering.Do we require to enable LDP/TDP or enable IBGP label carrying features?If not so then how the labels are distributed within the MPLS nodes.Does RSVP has the capability to distribute labels?Can anyone explain me the lablel assignment and distribution process relevant to RSVP configuration.

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Harold Ritter Tue, 03/27/2007 - 06:37


RSVP is fully capable of signaling the LSP end to end, which includes label distribution. This means that you could have an MPLS core solely based on MPLS TE (RSVP).

I would suggest you take a look at the following presentation to learn more about MPLS TE and how it is used to distribute labels.

Hope this helps,

ranjeetba Thu, 04/12/2007 - 23:22

Thanks hritter for explanation .Here are few more questions which I need to understand..

Where the first label allocation does starts? Is it the tail end router?

Is the RSVP allocated label at tail end always the inner label as in the typical MPLS case?

Does a Single RESV message contain all the label mappings allocated by series of routers from tail end to head end [piggybacking]?

Is label distribution process an atomic transaction i.e. if one of the routers fails to allocate labels the RSVP path establishment stalls?

Can I deploy traffic engineering with no LDP/TDP or label distribution protocol? I have doubt on this because RSVP assumes that there is label switch path in place from the given head end to tail end.

Harold Ritter Fri, 04/13/2007 - 04:58


- The tail end uses the RESV message to send either an implicit or explicit null label to the penultimate hop router, which in turn allocates a label and send it to the upstream router also using the RESV message. This process continues till the RESV message reaches the head end router.

- The RSVP allocated label is generally used as the outter label or in other words the label that is used to get the traffic from the ingress to egress PE.

- If the route-record option is used then the RESV message will contain all the labels for the LSRs that have been traversed. This option is on by default when an LSP is protected using TE FRR.

- If one of the nodes fails to allocate a label then the LSP can't be established.

- RSVP doesn't assume that there is LSP in place between the ingress and egress PE. You could deploy an entire MPLS network solely using MPLS TE to build the LSPs between all of your PEs. There no dependancy between RSVP and LDP.

Hope this helps,


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