LFA protection option applicability

About this task

Depending on the configured options of the loopfree-alternates command, the LFA SPF in an IGP instance runs algorithms in the following order:

Procedure

  1. The LFA SPF computes a regular LFA for each node and prefix. In this step, a computed backup next hop satisfies any applied LFA policy. This backup next hop protects that specific prefix or node in the context of IP FRR, LDP FRR, SR FRR, and SR-TE FRR.
  2. The LFA SPF follows with the TI-LFA, if the ti-lfa option is enabled for all prefixes and nodes, regardless of the outcome of the first step.

    A prefix or node for which a TI-LFA backup next hop is found overrides the result from the first step in the context of LDP FRR in SR FRR and in SR-TE FRR if the LDP fast-reroute backup-sr-tunnel option is enabled.

    With SR FRR and SR-TE FRR, the TI-LFA next hop protects the node-SID of that prefix and any adjacency-SID terminating on the node-SID of that prefix.

    The prefix or node continues to use the backup next hop found in step1 in the context of LDP FRR (if the LDP fast-reroute backup-sr-tunnel option is disabled), or in IP FRR.

  3. The LFA SPF runs remote LFA only for the next hop of prefixes and nodes that remain unprotected after step1 and step 2 if the remote-lfa option is enabled. A prefix or node for which a remote LFA backup next hop is found uses it in the context of LDP FRR in SR FRR and in SR-TE FRR when the LDP fast-reroute backup-sr-tunnel option is enabled.

What to do next

To protect an adjacency SID, the LFA selection algorithm uses the following preference order:

  1. adjacency of an alternate parallel link to the same neighbor.

    If more than one adjacency exists, select one as follows:

    1. adjacency with the lowest metric

    2. adjacency to the neighbor with the lowest router ID (OSPF) or system-id (IS-IS), and the lowest metric

    3. with the lowest interface index and the lowest router ID (OSPF) or system-id (IS-IS)

  2. an ECMP next hop to a node-SID of the same neighbor that is different from the next hop of the protected adjacency.

    If more than one next hop exists, select one as follows:

    1. next hop with the lowest metric

    2. next hop to the neighbor with the lowest router ID (OSPF) or system-id (IS-IS) if same lowest metric

    3. next hop to the lowest interface index if same neighbor router ID (OSPF) or system-id (IS-IS)

  3. LFA backup outcome of a node SID of the same neighbor. The following is the preference order:

    1. TI-LFA backup

    2. LFA backup

    3. RLFA backup

Parent topic: TI-LFA link-protect operation