Hi All,

I was preparing some content on MPLS for a training sessionΒ  and as a part of it, was going through LDP. The interesting aspect is very obvious

-> LDP is dependent on IGP

-> What ever Draw-backs IGP has will be inherited by LDP

-> LDP has to be enabled on the Interface to exchange Labels, else it wont consider the exit-interface from IGP and hence there will be no LSP’s

So far so good and makes sense as well


I will not be boring with command line outputs in this case

-> I have disabled the interface between R3/R4 so if R3 Has to reach R1, it will use R3-R2-R1 path

All good, Am going to just tweak the metric of the interface on R3 -> R2 before I enable back the R3 – R4

Now let me enabled the interface between R3-R4

-> It has a Better cost

-> It has not been enabled for LDP



If we go back to R3, to examine the result

This is dangerously familiar for me πŸ™‚ , There is a LDP neighbor, but No routes are present in Inet.3 (neither for R1 or R2) as Routes are learned from R4 for its best path but since R4 is not exchanging labels, R3 will not have any Inet.3 LSP’s inspite of having LDP neighbor.

What to do. ?

-> Troubleshoot – Obvious

-> Tie LDP to IGP

-> T-LDP Session

We all know the reason why LDP is no there – I have not explicitly not enabled it

We will explore the second option

What this does – Well, it simply increases the cost of the interface if the LDP adjacency is not seen on the interface while you have IGP on the interface.

R3—-no ldp -Yes IGP —- R4

As we see above, since there was NO LDP on R3—-R4, the metric is increased so that the other available path is choosen by Router which in-turn let LDP choose it