The SR: uLoop Avoidance feature will work only if the Topology-Independent Loop-Free Alternate (TI-LFA) feature is configured.
Restrictions for Segment Routing uLoop Avoidance
Segment routing uLoop avoidance feature does not support double fault at the same time.
Node up/down events are not supported in OSPF uLoop avoidance.
Information About Segment Routing uLoop Avoidance
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 time-to-live (TTL) expires. Eventually, the packets 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, or the packets might be out of order, and packets may get
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. Local uloops are usually seen in networks where
local loop-free alternate (LFA) path is not available. In such networks, remote LFAs provide backup paths for the network.
The information discussed above can be illustrated with the help of an example topology as shown in the following figure.
The assumptions in this example are as follows:
The default metrics is 10 for each link except for the link between Node 3 and Node 6, which has a metric of 50. The order
of convergence with SPF backoff delays on each node is as follows:
Node 3—50 milliseconds
Node 1—500 milliseconds
Node 2—1 second
Node 2—1.5 seconds
A packet sent from Node 3 to Node 9, the destination, traverses via Node 6.
If a link is established between Node 6 and Node 7, the shortest path for a packet from Node 3 to Node 9 would be Node 1,
Node 2, Node 7, and Node 6 before the packet reaches the destination, Node 9.
The following figure shows the Forwarding Information Base (FIB) table in each node before the link between Node 6 and Node
7 is established. The FIB entry contains the prefix of the destination node (Node 9) and the next hop.
When the link between Node 6 and Node 7 comes up, microloops occur for the links based on the order of convergence of each
node. In this example, Node 3 converges first with Node 1 resulting in a microloop between Node 3 and Node 1. Then, Node 1
converges next resulting in a microloop between Node 1 and Node 2. Next, Node 2 converges next resulting in a microloop between
Node 2 and Node 7. Finally, Node 7 converges resolving the microloop and the packet reaches the destination Node 9, as shown
in the following figure.
Adding the SPF convergence delay, microloop results in a loss of connectivity for 1.5 seconds, which is the convergence duration
specified for node 7.
How Segment Routing Prevents Microloops?
Using the example used to explain microloops, this section explains how to segment routing prevents microloops. Node 3 in
the example is enabled with the microloop avoidance segment-routing command. After the link between Node 6 and Node 7 comes up, Node 3 computes a new microloop on the network.
Instead of updating the FIB table, Node 3 builds a dynamic loop-free alternate (LFA) SR path for the destination (Node 9)
using a list of segments IDs, which include the prefix segment ID (SID) of Node 7, which is 16007, and the adjacency segment
ID (SID) of Node 6, which is 24076.
So, the SR path enables a packet from Node 3 reaches its destination Node 9, without the risk of microloop until the network
converges. Finally, Node 3 updates the FIB for the new path.
Use the protected keyword with the microloop avoidance segment-routing command, to enable microloop avoidance for protected prefixes only. The microloop avoidance rib-update-delaymilliseconds command can be used to configure the delay in milliseconds for a node to wait before updating the node’s forwarding table
and stop using the microloop avoidance policy. The default value for the RIB delay is 5000 milliseconds.
How to Enable Segment Routing uLoop Avoidance
Enabling ISIS - Microloop Avoidance
The following is a sample configuration code snippet to enable microloop avoidance in ISIS.
Use the show ip ospf segment-routing microloop-avoidance command to check if the repair path exists or not.
Router#show ip ospf segment-routing microloop-avoidance
OSPF Router with ID (188.8.131.52) (Process ID 1)
SR Microloop Avoidance is configured, delay 5000 msec
Area with ID (0)
Base Topology (MTID 0)
SR Microloop Avoidance is enabled and not running
Last topology change details:
Near end: 184.108.40.206
Far end: 220.127.116.11
Event: Link Up