Using LDP to Signal PW Label Mappings

Just as LDP advertises label mappings for LSPs, it can also advertise label mappings for Layer 2 VPNs.

In this case, the signaled FEC information describes a particular Layer 2 VPN. This FEC is often called a Virtual Circuit FEC, or VC FEC. The VC FEC information includes a PWid that is a 32-bit numeric field. Unlike LSP label advertisements that are usually sent to all possible upstream peers, the VC FEC information is sent only to the configured remote endpoint.

When the first Layer 2 VPN is configured to a remote peer, MPLS (Multiprotocol Label Switching) automatically creates a targeted hello adjacency entity for establishing an LDP session. Once the session is established, LDP passes the VC FEC label mapping associated with the Layer 2 VPN. Once VC FECs for the same PW ID have been exchanged in each direction, MPLS is ready to associate the PW with an LSP to the remote endpoint as described in Message Types.

To determine the correct LDP session over which to send a VC FEC, MPLS checks the IP addresses learned from its LDP peers via LDP address messages. The ExtremeXOS software MPLS expects to find the IP address of a remote PW peer among the addresses received over the LDP session to that peer.

To ensure that the local endpoint IP address is included in LDP address messages, it is highly recommended to configure MPLS on a loopback VLAN as described in Establishing LDP LSPs to PW Endpoints.

Use the command configure mpls add vlan vlan_name to configure MPLS on the loopback VLAN. It is not required that LDP or MPLS be enabled on the VLAN for the associated IP address to be advertised in LDP address messages.