Micro-loop avoidance for link addition, restoration, or metric decrease

The following figure shows an example of link addition or restoration in a network topology.

Figure: Micro-loop avoidance in link addition or restoration

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

1. Link 7-6 is added to the topology.

2. Router 3 detects a single link addition between remote nodes 7 and 6.

3. Router 3 runs the main and LFA SPFs.

   • All nodes downstream of the added link in Dijkstra tree (in this case, nodes 6 and 9) see a next-hop change.

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

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

4. For all nodes downstream from the added link, the algorithm computes and activates an SR tunnel that forces traffic to remote endpoint 6 of the added link.

   The algorithm pushes node SID 7 and adjacency SID of link 7-6 in the SR IS-IS tunnel for these nodes.

5. The use of the adjacency SID of link 7-6 skips the FIB state on node 7 and traffic to all nodes downstream of 6 are not impacted by micro-loop convergence.

6. The same method applies to metric decrease of link 7-6 that causes traffic to be attracted to that link.