LDP FRR Configuration

The user enables Loop-Free Alternate (LFA) computation by SPF under the IS-IS or OSPF routing protocol level:

config>router>isis>loopfree-alternates config>router>ospf>loopfree-alternates

The above commands instruct the IGP SPF to attempt to pre-compute both a primary next-hop and an LFA next-hop for every learned prefix. When found, the LFA next-hop is populated into the RTM along with the primary next-hop for the prefix.

Next the user enables the use by LDP of the LFA next-hop by configuring the following option:

config>router>ldp>fast-reroute

When this command is enabled, LDP uses both the primary next-hop and LFA next-hop, when available, for resolving the next-hop of an LDP FEC against the corresponding prefix in the RTM. This results in LDP programming a primary NHLFE and a backup NHLFE into the IOM or XCM for each next-hop of a FEC prefix for the purpose of forwarding packets over the LDP FEC.

Because LDP can detect the loss of a neighbor/next-hop independently, it is possible that it switches to the LFA next-hop while IGP is still using the primary next-hop. To avoid this situation, it is recommended to enable IGP-LDP synchronization on the LDP interface:

config>router>if>ldp-sync-timer seconds