Configuring IS-IS TI-LFA FRR

This section describes how to configure IS-IS TI-LFA FRR.

Usage Scenario

With the development of networks, VoIP and on-line video services require high-quality real-time transmission. Nevertheless, if an IS-IS fault occurs, multiple processes, including fault detection, LSP update, LSP flooding, route calculation, and FIB entry delivery, must be performed to switch traffic to a new link. As a result, the traffic interruption time is longer than 50 ms, leading to a failure to satisfy real-time requirements.

TI-LFA FRR provides link and node protection for Segment Routing (SR) tunnels. If a link or node fails, traffic is rapidly switched to a backup path, which minimizes traffic loss.

In some LFA or RLFA scenarios, the P space and Q space do not share nodes 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 can be used.

Pre-configuration Tasks

Before configuring IS-IS TI-LFA FRR, complete the following tasks:

Configure IP addresses for interfaces to implement connectivity at the network layer.
Configure basic IPv4 IS-IS functions.
Globally enable the Segment Routing capability.
Enable the Segment Routing capability in an IS-IS process.

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 frr
The IS-IS FRR view is displayed.
Run loop-free-alternate [ level-1 | level-2 | level-1-2 ]
IS-IS LFA is enabled, and LFA links can be generated.
Run ti-lfa [ level-1 | level-2 | level-1-2 ]
IS-IS TI-LFA is enabled.
(Optional) Run inter-level-protect level-1 [ prefer ]
Inter-level protection is enabled in IS-IS Level-1.

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

If the prefer parameter is specified, an inter-level TI-LFA backup path is preferentially selected even if there is a 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 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 completing the preceding configuration, IS-IS TI-LFA is enabled on all IS-IS interfaces. If you do not want to enable IS-IS TI-LFA on some interfaces, perform the following operations:
1. Run quit
  Quit the IS-IS FRR view.
2. Run quit
  Quit the IS-IS view.
3. Run interface interface-type interface-number
  The interface view is displayed.
4. Run isis [ process-id process-id ] ti-lfa disable [ level-1 | level-2 | level-1-2 ]
  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
  Quit the interface view.
2. Run isis [ process-id ]
  
  The IS-IS process is created, and the IS-IS view is displayed.
3. Run avoid-microloop frr-protected
  IS-IS local microloop avoidance is enabled.
4. (Optional) Run avoid-microloop frr-protected rib-update-delay rib-update-delay
  The delay after which IS-IS delivers routes is configured.
5. Run avoid-microloop segment-routing
  
  IS-IS remote microloop avoidance is enabled.
6. (Optional) Run avoid-microloop segment-routing rib-update-delay rib-update-delay
  
  The delay in delivering IS-IS route in an SR scenario is set.
Run commit
The configuration is committed.

Verifying the Configuration

After configuring IS-IS TI-LFA FRR, run the display isis route [ process-id ] [ level-1 | level-2 ] [ verbose ] command to check information about the primary and backup links.