MPLS LDP label advertisement

[g.rodegari]

Hi all,

please consider this network:

|R1|------|R2|-------|R3|

three router, in a point-to-point scenario with any IGP, MPLS and LDP.

R1 has 2 routes in routing table and, by default, advertise thorugh LDP a label-3 for both two prefix (PHP).

In my experience, I know, that R2 advertise the two prefix with 2 different label. Am I correct?

Than R3 has 2 LSP to reach two networks that actually follow the same path...

Is it possible to configure R2 to advertise both network with the same label value?

Thank you

And Kind Regards,

Graz.

[Harold Ritter · Accepted solution]

LDP allocates one label for each and every prefix in the RIB (except for BGP routes). The only way to heve one label allocated for the two routes in your case would be to advertise them as one aggregated route if possible.

Hope this helps,

[g.rodegari]

Thank you Harold,

in my opinion is a pretty strange behaviour...

but does not matter.

Regards,

Graz.

[olorunloba]

Hmmmm, Harold, using MPLS static labels, would it be possible to assign the same label to more than one prefix?

[Harold Ritter]

IOS will not let you use the same label to different prefixes (FEC). Why bother anyway and not just let LDP alocate 1 label per prefix. The label space is large enough to accomodate even the largest IGP I have come across.

Hope this helps,

[Harold Ritter]

It doesn't seem strange to me that if these two prefixes are considered independent from one another from a routing point of view that they each get assigned a different LSP. What if prefix 1 and prefix 2 shared the same LSP but all of a sudden they are not preferred via the same egress LSR.

Hope this helps,