# LDP Signaling for VPLS Overview

When you configure VPLS with LDP signaling, LDP supports a full mesh of pseudowires among the participating PE routers. This is analogous to BGP signaling, in which BGP builds a full mesh of label-switched paths (LSPs) among all of the VPLS instances on each of the PE routers participating in a particular VPLS domain.

- Targeted Sessions
- PWid FEC Element TLV

## Targeted Sessions

LDP establishes targeted sessions to the remote PEs configured at the edge of the service provider’s MPLS core network. The number of targeted sessions supported for a local PE router is equal to the total number of other PE routers that participate in the VPLS instances configured on the local PE router. As is the case with Martini encapsulation for Ethernet layer 2 services over MPLS, a targeted session to a remote PE router can have many pseudowires that terminate at the same remote PE router.

To enable LDP to establish targeted sessions with remote PEs across the MPLS core, you must issue both the mpls ldp vpls-id command to configure a VPLS identifier for the VPLS instance, and the mpls ldp vpls neighbor command to configure a list of neighbor (peer) addresses to which LDP can send or from which LDP can receive targeted hello messages.

## PWid FEC Element TLV

LDP signaling information for VPLS is carried in a label mapping message. The label mapping message contains the Generic Label type-length-value (TLV), and the pseudowire identifier (PWid) forwarding equivalence class (FEC) element. A FEC is a group of IP packets forwarded over the same path with the same path attributes applied.

The PWid FEC element (FEC Type 128 or 0x80) contains the VPLS identifier information configured for your VPLS instance with the mpls ldp vpls-id command. Taken together, the pseudowire type field and the PWid field in the TLV represent a unique VPLS instance. The pseudowire type field is Ethernet to identify the pseudowires that carry Ethernet traffic for multipoint connectivity between the local and remote VEs. The PWid field is a nonzero 32-bit integer that contains the VPLS identifier, which is a globally unique identifier for a VPLS domain. All VEs that participate in the same VPLS domain must use the same VPLS identifier.

Martini encapsulation for Ethernet layer 2 services over MPLS also uses the PWid FEC Element TLV. As a result, the PWid for Martini configurations must not be the same as the VPLS identifier configured for a VPLS instance. To prevent this conflict from occurring, the JunosE Software displays an error and rejects the configuration if you attempt to configure the same value for the Martini PWid and the VPLS identifier.

#### Related Documentation

- Configuring LDP Signaling for VPLS
- Configuring MPLS.