When changes occur in a network topology because of the failure or
restoration of a link or a network device, IP Fast Reroute enables rapid
network convergence by moving traffic to precomputed backup paths until regular
convergence mechanisms move traffic to a newly computed best path, also known
as a post-convergence path. This network convergence may cause short microloops between two directly or indirectly connected
devices in the
topology. Microloops are caused when different nodes in the network calculate
alternate paths at different times and independently of each other. For
instance, if a node converges and sends traffic to a neighbor node, which has
not converged yet, traffic may loop between the two nodes.
Microloops may or may not result in traffic loss. If the duration of a
microloop is short, that is the network converges quickly, packets may loop for
a short duration before their TTL expires. Eventually, the packets will get forwarded to
the destination. If the duration of the microloop is long, that is one of the
routers in the network is slow to converge, packets may expire their TTL or
the packet rate may exceed the bandwidth, and packets may get dropped.
Microloops that are formed between a failed device and its neighbors are called
local uloops, whereas microloops that are formed between devices that are
multiple hops away are called remote uloops. The ISIS Local Microloop
Protection feature helps networks avoid local uloops. Local uloops are usually
seen when there is no local loop-free alternate (LFA) path available,
especially in ring or square topologies. In such topologies, remote LFAs
provide backup paths for the network. However, the fast-convergence benefit of
the remote LFA is at risk because of the high probability of uloop creation.
The ISIS Local Microloop Protection feature can be used to avoid microloops or
local uloops in such topologies.