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VPLS Labelling

(Tunnel and Service Labels)

As we discussed in the MPLS articles before, for MPLS we need labels. VPLS is also a MPLS Service, so we need to use labels in VPLS too.

There are two MPLS labels used in VPLS. These labels are:

• Outher Label (Transport Label)
• Inner Label (Servise Label / VC LAbel)

vpls-labels-ipcisco

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.

vpls, single-tldp-session

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.

In VS, MAC Learning is very important. Traffic are flooding to the LSPs (Label Switch Path) according to these learned MAC addresses. Here, LSP is an MPLS term we have talked about before. Basically, it is the path of MPLS packets.

The learning process is similar to a Ethernet Switch. The learned addresses are stored in the Forwarding Database (FDB). During forwarding, if the destination is known then it is forwarded to there. If not, it is forwarded as broadcast in the switching throught all the LSPs.

Other VPLS Articles…

VPLS (Virtual Private LAN Service) – VPLS Overview
VPLS (Virtual Private LAN Service) – VPLS Topologies
VPLS (Virtual Private LAN Service) – VPLS MAC Learning
VPLS (Virtual Private LAN Service) – VPLS Labelling
VPLS (Virtual Private LAN Service) – Alcatel-Lucent(Nokia) VPLS Configuration
VPLS (Virtual Private LAN Service) – (h-VPLS)
VPLS (Virtual Private LAN Service) – VPLS versus h-VPLS
VPLS (Virtual Private LAN Service) – Cisco VPLS Configuration

Alcatel-Lucent L2 VPN and L3 VPN Articles…

Alcatel-Lucent L2 VPN and L3 VPN Services – Part 1 (Service Terms and Service Creation Steps)
Alcatel-Lucent L2 VPN and L3 VPN Services – Part 2 (Service Types)
Alcatel-Lucent L2 VPN and L3 VPN Services – Part 3 (Network Preparation)
Alcatel-Lucent L2 VPN and L3 VPN Services – Part 4 (Tunnel Structure)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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