There are two MPLS labels used in VPLS. These labels are:
• Outher Label (Transport Label)
• Inner Label (Servise Label / VC LAbel)
Again in the MPLS lessons, we have talked about P and PE routers. Briefly, P routers are the core MPLS routers that carries MPLS traffic and PE routers are the border routers of MPLS that services are defined.
Outher Label (Transport Label) is the label that only P routers are use. In other words, this label is used in the swap process of P routers. Think about this label as the package of your delivery. This package changes on every P router.
Inner Label (Servise Label / VC Label) is the label that carries the original customer VPLS information. At P routers, it remains unchanged. But in PE routers this labels stripped and according to VPLS information inside, VPLS process is done. Think this label as a Secret Key of the customer. It is only belong to him.
VPLS Label Signalling
What about signalling? The signalling of these two labels are done differently.
- Outher Label (Transport Label) signalling is done via LDP, RSVP or GRE
- Inner Label (Servise Label / VC Label) signalling is done via T-LDP (Targeted LDP)
For VPLS service, T-LDP session is very important. Here, Inner labels signalling is done via T-LDP. This is done with SDP configuration on Alcatel-Lucent (Nokia) Routers. This is Pseudowire for Cisco products. For VPLS service between different nodes, there is an ID that must much for the same service. This is VC ID (Virtual Circuit Identifier). Do not mix VC ID with VC Label.
Beside VC ID, there is also a Service ID for VPLS configuration. Service ID is a local parameter. It is not need to configure same Service ID for both end. For both end, VC ID must be same.
Here, there is an important point. If you use Mesh-SDP Service ID is equal to VC ID. If you use Spoke-SDP, VC ID is determined manually.
By the way, for label signalling, only one T-LDP is required between two nodes. The service numbers running on these system is not important. One T-LDP session work for all VPLS services.
In PE routers some swicthing facilities are needed. To do this a Virtual Switch (VS) is defined in PE routers. And all the switching facilities needed like MAC learning is done with this VS. You can think this like classicall switch activities.