MPLS VPN without Signalling Protocol in CORE

Unanswered Question
Sep 1st, 2007


I heard its possible to run L3 MPLS VPN between two sites across SP core without having any Signalling protocol (TDP/LDP)enabled on the core,the only constraint is running two TE tunnels between the two PE routers connected to CE. Is it possible. Can someone explain elaborately, pls?

I have this problem too.
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mohammedmahmoud Sat, 09/01/2007 - 23:48


When doing L3 MPLS VPN TDL/LDP role is to get the packet to the egress PE, and thus the LSP is dynamically built (using the IGP), but when using TE, the LSP is statically configured and no need for TDP/LDP in this case, but be aware that you can only use OSPF and ISIS as the IGP in this case in conjunction with RSVP with traffic engineering extensions.

Kindly check out this document:


Mohammed Mahmoud.

swaroop.potdar Sun, 09/02/2007 - 23:19

Some more details regarding the behavior as to why LDP/TDP is not required in case of end-to-end TE tunnel between the PE's.

Using TE also the LSP is dynamically built untill and unless you are using explicitly defined TE tunnels.

Also do note that when you have TE tunnels end to end your egress PE receives the packet with the VPN label only and then takes the appropriate action as per the VPN forwarding table.

In case you dont have end to end TE tunnels you will have to enable LDP on the tunnels to carry the VPN labels untouched till the egress PE.( As in case if the tunnels are not end to end and are terminating on a P' which doesnt have any VPN information the packet would be dropped, so enabling LDP becomes a must.)

Here is a detailed document explaining the beahaviour in more detail and explains when LDP should be enabled or disabled with illustrations.



arun kumar Mon, 09/03/2007 - 09:53

Hi Swaroop,

That document was very much helpful.. I have one more query.. I have end to end TE tunnels enabled between PE routers and TDP/LDP running on my core. From PE1, i'm learning the labels for PE2 via two modes

1) via LDP from 'P' router

2) via RSVP from end to end TE tunnel

Which one PE1 chooses to forward?

Thanks in advance...

swaroop.potdar Mon, 09/03/2007 - 12:54

Hi Arun, the label for your best IGP next hop is choosen. So which means that when you do a show ip route for the egress PE's BGP nexhtop, if you see the outgoing interface as your physical then it will choose the LDP label, and if it shows as the BGP next hop is reachable through the TE tunnel then it will use the TE label as the outer label.



swaroop.potdar Mon, 09/03/2007 - 13:57

Devang do note that if the next hop is not reachable through the TE tunnel, the TE (RSVP) label would not be used. ( For eg: when you have not announced the existence of the Tunnel to the IGP)



arun kumar Mon, 09/03/2007 - 18:40

The reason i asked is i read somewhere, the AD of RSVP (7) is lesser than the AD of LDP (9). So the router would be using labels with lower AD signalling protocols. Is this true, any concept of AD in LDP/RSVP?


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