Summary of requirements and procedures for IGP shortcut

The main difference between IGP shortcut and LDP over RSVP-TE is that the LSP is installed in the RIB with IGP shortcut. The T-LDP signaling messages are resolved through IGP shortcut and go over the P2P RSVP-TE. In the case of LDP over RSVP-TE, the P2P RSVP-TE is not installed in the RIB, so the T-LDP signaling is not over RSVP.

The tunneling command is required to ensure the label mapping is generated through T-LDP.

For upstream FEC resolution:

• mcast-tunneling must be enabled under the targeted peer for the mcast FEC to resolve over an IGP shortcut or using T-LDP.

• If prefer-mcast-tunnel-in-tunnel is disabled, the preference is given in the following order:

  1. non-tunneled NHs of IGP route

  2. IGP shortcut (tunneled NHs) of IGP route

  3. direct T-LDP to the root address

  4. indirect T-LDP to an intermediate node using the tunnel endpoints provided by routing; for IGP to compute the tunnel endpoints, the ldp-over-rsvp command must be enabled under the IGP context

• If prefer-mcast-tunnel-in-tunnel is enabled, the preference is given in the following order:

  1. direct T-LDP to root address

  2. indirect T-LDP to an intermediate node using the tunnel endpoints

  3. IGP shortcut (tunneled NHs) of IGP route

  4. non-tunneled NHs of IGP route

For the downstream direction, a direct RSVP LSP to the downstream peer address is required for T-LDP sessions to be considered.

When the downlstream peer is reachable through both a link LDP and a T-LDP, the mcast FEC next-hop is programmed as follows:

• The link LDP session is preferred if prefer-mcast-tunnel-in-tunnel is disabled.

• The T-LDP session is preferred if prefer-mcast-tunnel-in-tunnel is enabled.