Configuring IS-IS SRv6 TI-LFA FRR

IS-IS SRv6 topology-independent loop-free alternate (TI-LFA) fast reroute (FRR) uses an explicit path to represent a backup path, which poses no topology constraints and provides more reliable FRR.

Usage Scenario

In some LFA or RLFA scenarios, the P space and Q space do not intersect or have direct neighbors. If a link or node fails, no backup path can be calculated, causing traffic loss and resulting in a failure to meet reliability requirements. In this situation, TI-LFA FRR can be used to resolve the issue.

SRv6 TI-LFA FRR provides link and node protection in SRv6 scenarios. If a link or node fails, traffic is rapidly switched to a backup path, minimizing traffic loss.

SRv6 TI-LFA FRR applies to both SRv6 BE and SRv6 TE Policy scenarios. In normal cases, SRv6 BE packets do not need to carry SRH information. However, in TI-LFA FRR scenarios, to encapsulate repair list information, SRv6 BE packets are required to carry SRH information.

Pre-configuration Tasks

Before configuring IS-IS SRv6 TI-LFA FRR, enable IS-IS SRv6.

Procedure

Run system-view
The system view is displayed.
Run isis [ process-id ]
An IS-IS process is created, and the IS-IS view is displayed.
Run ipv6 enable topology ipv6
IPv6 is enabled for the IS-IS process.
Run ipv6 frr
The IPv6 IS-IS FRR view is displayed.
Run loop-free-alternate [ level-1 | level-2 | level-1-2 ]
IPv6 IS-IS FRR is enabled, so that a loop-free backup link is generated using the LFA algorithm.
Run ti-lfa [ level-1 | level-2 | level-1-2 ]
IS-IS SRv6 TI-LFA is enabled.
(Optional) Run inter-level-protect level-1 [ prefer ]
Inter-level protection is enabled in IS-IS Level-1.

By default, SRv6 TI-LFA computes backup paths only in the same IS-IS level. After the inter-level-protect level-1 command is run, if no SRv6 TI-LFA backup path exists in IS-IS Level-1, inter-level SRv6 TI-LFA backup path computation is performed.

If the prefer parameter is specified, an inter-level SRv6 TI-LFA backup path is preferentially selected even if there is an SRv6 TI-LFA backup path in IS-IS Level-1.
(Optional) Run tiebreaker { node-protecting | lowest-cost | srlg-disjoint | hold-max-cost } preference preference [ level-1 | level-2 | level-1-2 ]
An IS-IS SRv6 TI-LFA FRR tiebreaker is configured for backup path computation.

A larger value indicates a higher preference.

Before configuring the srlg-disjoint parameter, you need to run the isis srlg srlg-value command in the IS-IS interface view to configure the IS-IS SRLG function.
(Optional) After the preceding configurations are complete, IS-IS SRv6 TI-LFA is enabled on all IPv6 IS-IS interfaces. If you do not want to enable IS-IS SRv6 TI-LFA on some interfaces, perform the following steps:
1. Run quit
  Exit the IS-IS FRR view.
2. Run quit
  Exit the IS-IS view.
3. Run interface interface-type interface-number
  The interface view is displayed.
4. Run isis ipv6 ti-lfa disable [ level-1 | level-2 | level-1-2 ]
  IPv6 IS-IS TI-LFA is disabled on the interface.
If a network fault occurs or is rectified, an IGP performs route convergence. A transient forwarding status inconsistency between nodes results in different convergence rates on devices, posing the risk of microloops. To prevent microloops, perform the following steps:
1. Run quit
  Exit the interface view.
2. Run isis [ process-id ]
  The IS-IS process is enabled, and the IS-IS view is displayed.
3. Run avoid-microloop frr-protected
  Local IS-IS anti-microloop is enabled.
4. (Optional) Run avoid-microloop frr-protected rib-update-delay rib-update-delay
  A delay in IS-IS route delivery is configured.
5. Run segment-routing ipv6 locator locator-name
  IS-IS SRv6 is enabled.
6. Run ipv6 avoid-microloop segment-routing
  
  Remote IS-IS anti-microloop is enabled.
7. (Optional) Run ipv6 avoid-microloop segment-routing rib-update-delay rib-update-delay
  
  A delay in SRv6 IS-IS route delivery is configured.
Run commit
The configuration is committed.

Verifying the Configuration

After configuring IS-IS SRv6 TI-LFA FRR, verify the configuration.

Run the display isis [ process-id ] route ipv6 [ level-1 | level-2 ] [ verbose ] command to check information about the primary and backup links after IS-IS SRv6 TI-LFA FRR is enabled.