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 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, or the packets might be out of order, 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. 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 7—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.

The ISIS - SRV6: uLoop Avoidance feature supports the following scenarios, with only one event supported at a time:

• link-up

• link-down

• link cost increase

• link cost decrease

• overload bit set

• overload bit cleared

Note	Node up/down are not supported as these are multiple events.

From Cisco IOS XE 17.15.1a, the ISIS - SRv6: uLoop Avoidance feature is supported on the following platforms:

• Cisco ASR1000 RP3 + ESP100-X, ASR1001-HX, ASR1002-HX

• Cisco Catalyst 8000V Edge Software

• Cisco Catalyst 8200 Series Edge platforms

• Cisco Catalyst 8300 Series Edge platforms

• Cisco Catalyst 8500 and 8500L Series Edge platforms

Use the microloop avoidance command under address-family ipv6 to enable SRv6 uLoop avoidance.

Note	SRv6 uLoop avoidance is enabled by default when SRv6 TILFA is enabled.

router(config-router-af)# microloop avoidance ?
  	disable           Disable Microloop avoidance
  	rib-update-delay  Microloop avoidance RIB update delay
  	segment-routing   Configuring Segment-Routing Microloop Avoidance

The following is a sample configuration to enable SRv6 uLoop avoidance:

router isis
address-family ipv6
 microloop avoidance segment-routing 
 microloop avoidance rib-update-delay 3000

Use the show isis ipv6 rib command to verify the uLoop configuration.

router# show isis ipv6 rib FCCC:CCC1:F1::/48

* FCCC:CCC1:F1::/48
    via ULOOP_EP SRv6-Fwd-Id 25165888, type L1  metric 120 tag 0 
        P node: R5 SID FCCC:CCC1:E1:: uN (PSP/USD)
        Q node: R6 SID FCCC:CCC1:E1:E001:: uA (PSP/USD)
    prefix attr: X:0 R:0 N:0
     (installed)
             - - - - - -
    via FE80::A8BB:CCFF:FE00:9D21/Ethernet1/2, type L1  metric 120 tag 0 
    prefix attr: X:0 R:0 N:0

Use the show ipv6 microloop-avoidance commands to display the current uLoop status.

router# show isis ipv6 microloop-avoidance
Tag: 1

Algo 	State 		   Delay 	      Running(L1/L2)
0 	   Segment-Routing 	3000 		FALSE/FALSE
router#

router# show isis ipv6 microloop-avoidance log

Tag:1 LVL:1
Timestamp Algo Event Reason
*Jul 18 09:49:44.885 CST 0 EXPIRED NA
*Jul 18 09:49:29.885 CST 0 STARTED LINK UP
*Jul 18 09:46:46.064 CST 0 ABORTED NA
*Jul 18 09:46:44.942 CST 0 STARTED LINK DOWN
*Jul 18 09:46:17.536 CST 0 EXPIRED NA
*Jul 18 09:46:02.536 CST 0 STARTED LINK DOWN
*Jul 17 19:39:47.748 CST 0 EXPIRED NA
*Jul 17 19:39:32.748 CST 0 STARTED LINK UP
*Jul 17 19:28:42.973 CST 0 EXPIRED NA