Micro loop can be found in fast-rerouted networks. Fast reroute, as a proactive convergence mechanism, provides sub-second data plane convergence.
If any link fails, upstream node sends the traffic through the repaired path to the downstream device. For the downstream node to be used as a backup/repair node, it should be loop-free.
What do all these mean? The picture below provides further explanations.
In the topology diagram above, all the link routing costs – except D-E link, which is 30 – are 10.
Subnets behind Router A reaches over the top path A-B-E, since the total cost over the top path A-B + B-E is 20.
For the Router C to be used as a loop free alternate path for fast reroute purpose, Router C shouldn’t use Router A as its primary next hop.
However, in the diagram above, Router C also uses the top path C-A-B-E to reach the subnets behind Router E since the cost of the top path compared to that of the bottom is lower (30 vs. 40).
If the repaired path is installed in the hardware data plane of Router A (in case Router A-B link fails, since the repair path is already installed), within milliseconds Router A sends the traffic to Router B. But first, Router B should be informed about the failure associated with that link to send traffic to Router C.
When Router B converges sending traffic to Router C, Router B continues to send the traffic back to Router A since Router A is the primary next hop of Router B. The path goes back and forth between Router A and Router B until Router B starts to send the traffic to Router C. This process is known as micro loop.
You don’t need to do anything about the micro loop since it will be resolved after the routing converges. That’s why it is different from permanent loop such as spanning tree loops – the most known example.
In the permanent loop case, you need to react either by removing the lastly configured parameters or by adding links or features that could create a bug.
Permanent loops remain unchanged until you react and solve the problem. In addition, micro loop is automatically resolved after routing converges without manual intervention.