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Community Member

Label distribution in frame mode MPLS

Dear All,

I am reading up on the topic on ldp distribution and advertisement from 2 diff sources and it is telling me 2 diff things

Source 1

The allocated label is advertised to all neighbor LSR, regardless of whether the neighbors are upstream or downstream for the destination.

Source 2

2. In an MPLS domain running LDP, a label is assigned to a destination prefix found in the FIB, and it is distributed to upstream neighbors in the MPLS domain after session establishment.

If i understand correctly, sources 1 is saying that labels propagate to both upstream and downstream neighbor while sources 2 is reading label propagation is only to UPSTREAM neighbor only.

Can someone please help me to clarify




Re: Label distribution in frame mode MPLS

Hi birdy,

Source 1 is correct, every LDP neighbor will receive the chosen Label. The main reason is, that "upstream" or "downstream" are only making sense from a routing perspective. You need to know from where to where traffic is routed to define up- and downstream.

LDP, however, has nothing to do with IP routing, except that a label is chosen for every prefix in the IP routing table (except for BGP entries).

It would have been much more complex to first calculate f.e. in OSPF, whether the LDP peer could potentially need the label.

The design decision was to decouple LDP and IP routing.

Labels are sent to every neighbor, which will decide the label to use for a certain prefix from its own IP routing table. Simple rule: use the label from your current next hop calculated by the IP routing protocol.

Hope this helps! Please use the rating system.

Regards, Martin

Community Member

Re: Label distribution in frame mode MPLS

Hi Martin,

Thanks for the explanation.

I feel much clearer now.

Full steam ahead for CCIP......



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