Micro-loop avoidance for link removal, failure, or metric increase

The following figure depicts an example of link removal or failure in a network topology.

Figure: Micro-loop avoidance in link removal or failure

The micro-loop avoidance algorithm performs the following steps in the preceding network topology example.

  1. Link 6-9 is removed or fails.

  2. Router 3 detects a single link event and runs main and LFA SPFs.

    • All nodes downstream of the removed link in the Dijkstra tree (in this case, nodes 9, 10, 11, and 12) see a next-hop change.

    • Nodes 10, 11, and 12 are no longer downstream of node 9.

    • All nodes upstream of the removed link (in this case, nodes 1, 2, 7, and 6) see no route change.

    • Nodes 1', 2', and 7' are not using node 6 or 9 as parent nodes and are not impacted by the link removal event.

  3. For each impacted node, the algorithm computes and activates a loop-free SR tunnel to the farthest node in the shortest path that did not see a next hop change, then uses the adjacency SIDs to reach destination node.

    • For the SR IS-IS tunnel of node 12, push the SID of node 7, then the SIDs of adjacencies 7-11 and 11-12.

    • This loop-free SR tunnel computation is similar to the P-Q set calculation in TI-LFA (see Topology independent LFA), but the P node is defined as the farthest node in the shortest path to the destination in the new topology with no next-hop change.

    • The maximum number of labels used for the P-Q set is determined as follows:

      • If TI-LFA is enabled, use the configured value of the max-sr-frr-labels parameter.

      • If TI-LFA is disabled, use the value of 3, which matches the maximum value of TI-LFA max-sr-frr-labels parameter.

      In both cases, this value is passed to MPLS for checking against the max-sr-labels [additional-frr-labels] parameter for all configured SR-TE LSPs and SR-TE LSP templates.

    • The path to the P node may travel over an RSVP-TE LSP used as an IGP shortcut. In this case, the RSVP-TE LSP must have CSPF enabled. This is to avoid churn in IGP and to avoid micro-loops in the path of the RSVP control plane messages that are generated following the convergence of IGP, because the next hop in the ERO is looked up in the routing table.

    • When SR-LDP stitching is enabled and the path to the P node or the path between the P and Q nodes is partly on the LDP domain, no loop-free SR tunnel is programmed and IGP programs the new next hop or hops.

  4. The same method (steps2 and 3)applies to a metric increase of link 6-9 that causes traffic to move away from that link; for example, a metric change from 1 to 200.

Parent topic: Micro-loop avoidance using loop-free SR tunnels for IS-IS