I am unable to understand from the drafts how the Layer 2 PDUs gets transported between all the PEs (i.e.the PWE3) in a VPLS domain.
If we compare this with 2547 based VPNs, the VPN label in 2547 VPNs is distributed by MP-iBGP. How does this happen in case of VPLS??? How is the VC label getting distributed between all the PEs since there's no routing protocol running in VPLS across the PEs. I am talking about the LDP based VPLS.
In the case of LDP-based VPLS, the labels are distributed using LDP, and not BGP. When a VPLS is configured using a full-mesh of pseudo-wires, each PE will establish a targeted LDP session with every other peer in the network. Since one requirement for using VPLS is that all PEs be configured with the same VPLS identifier, each PE is able to send LDP advertisements which associate a label with the VPLS identifer. The receiving PEs know which VPLS instance to associated the received label with, since they use the same VPLS identifier.
Let's consider a frame that enters the VPLS domain. The scenario here would be something like this.
A frame with VLAN tag enters the PE1 Ingress router. PE1 router is going to strip off the SOF,Premble,the .1q delimiter and is only going to keep the SA and DA in this frame thereby appending something called as a Tunnel label and a VC label. This frame traverses the MPLS Domain wherein the PE routers simply swaps the Tunnel Label wheres the PE2 egress is going to POP off the tunnel label and based on the VC label, it's going to forward to the proper destination.
From ur explanation, I do understand that Tunnel label would be the one generated using LDP, but what about the VC label???How is this VC label generated and distributed amongst all the PE routers??? So far I recall, LDP doesn't do any sort of routing for VC label exchange....And, we don't have a routing protocol as MP-iBGP in 2547 based VPNs. So how are these VC Labels distributed amongst all the PEs???
I read in draft that it happens through the Pseudowire (PWE3) which can carry the layer2 PDUs over MPLS circuit, but I am not exactly able to imagine how it happens. Would be really great if someone explains this in details...
I guess I need to be a bit clearer - the targeted LDP session I referred to is the one that distributes the VC label. The tunnel label can be distributed by LDP, RSVP or whatever - that is not relevant to this scenario. When the PEs establish the targeted LDP sesions to each other, it is for the purpose of exchanging the VC label. If you re-read my original post in that context, it should make a bit more sense now.
The draft that you should read that explains this stuff is:
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