tiebreaker (IS-IS FRR view)

Usage Scenario

The solution of selecting a backup path for IS-IS Auto FRR is node-protection path first. In actual networking scenarios, the solution may need to be changed to smallest-cost path first due to considerations such as interface forwarding capacity and link cost.

Scenario 1: If the primary path is Link-1=DeviceS-> DeviceE-> DeviceD, the next hop of the backup path can be Link-2=DeviceS->DeviceE or Link-3=DeviceS->DeviceN. The cost of Link-1 is 10, the cost of Link-2 is 20, and the cost of Link-3 is 100. By default, Link3=DeviceS-> DeviceN-> DeviceD is preferentially selected as the backup link. To change the solution of selecting a backup path to the path with the smallest cost, run the tiebreaker lowest-cost preference preference [ level-1 | level-2 | level-1-2 ] command. That is, Link-2 is preferentially selected as the backup path.

In scenario 2, if traffic is forwarded from Device S to Device E, load balancing is implemented between Device S and Device E through Link-1 = Device S -> Device E and Link-2 = Device S -> Device E. Device S and Device N are connected through Link-3. The cost of Link-1 is 10, the cost of Link-2 is 10, and the cost of Link-3 is 15. Link-1 selects Link-2 as the backup path, and Link-2 selects Link-1 as the backup path. To change the solution of selecting a backup path to non-load balancing, run the tiebreaker non-ecmp preference preference [ level-1 | level-2 | level-1-2 ] command. After the command is run, Link-3 is preferentially selected as the backup path.

In scenario 3, if traffic is forwarded from Device S to Device E, Link-1 and Link-2 implement load balancing between Device S and Device E. Device S and Device N are connected through Link-3. The cost of Link-1 is 10, the cost of Link-2 is 10, and the cost of Link-3 is 10. In this case, IS-IS LFA auto FRR provides protection for the load-balancing links. That is, Link-1 and Link-2 back up each other. If Link-1 and Link-2 have the same risk of link failures, you can run the tiebreaker srlg-disjoint preference preference [ level-1 | level-2 | level-1-2 ] command to change the solution of selecting a backup path to a non-intersecting SRLG. That is, Link-3 is preferentially selected as the backup path.

To use this solution, run the isis srlg srlg-value command to add the interfaces with the same risk of link failures to the same SRLG.

Scenario 4: If traffic is forwarded from Device S to Device E, there are two links, Link-1 and Link-2, between Device S and Device E. Link-1 is 10 and Link-1 is configured with the isis peer hold-max-cost command. The cost of Link-2 is 30, device S and Device N are connected through Link-3, and Device N and Device E are directly connected. The cost of Link-3 is 10, and the cost of the link between Device N and Device E is 10. In normal cases, the link from Device S to Device E is Link-1, and the backup link is Link-3 (node protection is preferred). After Link-1 is disconnected, the primary path from Device S to Device E becomes Link-3, and the backup path becomes Link-2. After the undo showdown command is run on Link-1, Link-1 advertises the maximum cost. In this case, if you want to use Link-1 as the backup path, run the tiebreaker hold-max-cost preference preference [ level-1 | level-2 | level-1-2 ] command to change the cost to the maximum value in the preferred solution, link-1 can be used as the backup path.

Prerequisites

IS-IS Auto FRR has been enabled using the loop-free-alternate command.

Precautions

By default, the preference value is 40 for the node-protection path first solution, 20 for the smallest-cost path first solution, 15 for the maximum cost path first solution, 10 for the non-ECMP first solution, and 5 for the SRLG disjoint first solution.