Establishing LDP LSPs to PW Endpoints

Establishing a PW requires both an LSP and a targetted LDP session between the two endpoints.

The local PW endpoint is the MPLS (Multiprotocol Label Switching) LSR ID. The remote PW endpoint is identified using an IP address configuration parameter.

When using LDP to establish the LSPs, each endpoint needs to advertise a label mapping for an LSP to its local endpoint address. To ensure that its LDP peers use the label mapping, a corresponding IGP route should also be advertised for the address. The IGP route can come from any of the supported routing protocols, such as OSPF (Open Shortest Path First), or IS-IS. For example, when using OSPF, an OSPF route with prefix length 32 should be advertised for the configured IP address.

We recommend that you configure a loopback VLAN (Virtual LAN) using the IP address of the local endpoint (the MPLS LSR ID). Use prefix length 32 for the IP address configured for the loopback VLAN. When you configure a loopback VLAN, the IP address used to identify the endpoint remains active, even when one or more of the LSR VLAN interfaces go down. Should a remote peer normally use one of the down interfaces, the normal IGP and LDP recovery procedures allow the PW to use one of the remaining up interfaces to minimize the network outage.

You should also configure the loopback VLAN for MPLS using the configure mpls add vlan vlan_name command. The addition of the loopback VLAN to MPLS causes LDP to include the IP address in LDP address messages. Some implementations (including the ExtremeXOS software) require this information to determine the correct LDP session over which to advertise label mappings for VC FECs (see Using LDP to Signal PW Label Mappings).

Note

Neither MPLS nor LDP have to be enabled on the loopback VLAN.

There are two options to initiate the LDP advertisement of an LSP to the local MPLS LSR ID when a loopback VLAN has been configured for that IP address:

Configure MPLS LDP to advertise a direct interface whose IP address matches the LSR ID and has prefix length 32. Use the configure mpls ldp advertise direct lsr-id command to do this.
Configure MPLS LDP to advertise direct interfaces using the configure mpls ldp advertise direct all command.

Note

This causes LDP to advertise label mappings for all VLANs that have an IP address configured and have IP forwarding enabled.

While both of the above methods initiate the advertisement of a label mapping for an LSP to the local endpoint, the first method is the preferred method.

Published March 2020prev | next

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