IP fast reroute (IPFRR) provides rapid convergence during the link-down events by moving the traffic to a pre computed backup path until the regular convergence mechanisms move the traffic to the newly found best path referred to as the post-convergence path.
Once the traffic is moved to the post-convergence path, it is inclined to a microloop. Microloops are formed as a result of the fact that each node on the path does its calculation at different times and independently of other nodes. If certain nodes converge and sends traffic to a neighbor node, which has not converged yet, traffic may be looped between these two nodes.
Microloops are formed between the router where the failure is detected and its neighbors. Local microloops are created in cases where there is no local loop-free alternate (LFA) backup available in ring or square topologies. In such topologies, remote LFA provides a backup, but the fast-convergence benefit of the remote LFA cannot be completely utilized due to the high probability of the local microloop creation. Avoiding the local micro loop provides a significant improvement in the fast convergence in the ring and square topologies.
Microloop avoidance is automatically enabled as soon as remote LFA (rLFA) is enabled.
When using microloop avoidance for prefixes (for which a repair path has been installed in the forwarding plane), the OSPFv2 IP FRR Local Microloop Avoidance feature is enabled when the forwarding plane is triggered to switch to using a pre installed repair path. The local microloop avoidance for the link-down event supports the following triggers:
- Interface down event.
- Adjacency down event due to the Bidirectional Forwarding Detection (BFD) session down.
If microloop avoidance is used regardless of whether a repair path has been installed in the forwarding plane, then in addition the third trigger is used:
- Adjacency down event due to neighbor hold time expiration.
When the neighbor reports loss of adjacency to the local system in its link state neighbor advertisements, the value of using microloop avoidance depends on whether the remote event that caused loss of adjacency on the neighbor is detectable by the local forwarding plane (that is, whether the forwarding plane will react and switch to using pre programmed repair paths).