Limitations

Only two backup labels are supported.

Usage Guidelines and Limitations

The TI-LFA guidelines and limitations are listed below:

TI-LFA Functionality IS-IS1 OSPFv2
Protected Traffic Types
Protection for SR labeled traffic Supported Supported
Protection of IPv4 unlabeled traffic Supported (IS-ISv4) Supported
Protection of IPv6 unlabeled traffic

Unsupported

 N/A
Protection Types
Link Protection Supported Supported
Node Protection Supported Supported
Local SRLG Protection Supported Supported
Weighted Remote SRLG Protection

Unsupported

Unsupported
Line Card Disjoint Protection

Unsupported

 Unsupported
Interface Types
Ethernet Interfaces Supported Supported
Ethernet Bundle Interfaces

Unsupported

Unsupported
TI-LFA over GRE Tunnel as Protecting Interface

Unsupported

Unsupported
Additional Functionality
BFD-triggered Unsupported Unsupported
BFDv6-triggered Unsupported

N/A
Prefer backup path with lowest total metric

Unsupported

Unsupported
Prefer backup path from ECMP set Supported Supported
Prefer backup path from non-ECMP set Supported Supported
Load share prefixes across multiple backups paths

Unsupported

Unsupported
Limit backup computation up to the prefix priority Supported Supported
1 Unless specified, IS-IS support is IS-ISv4 and IS-ISv6

Configuring TI-LFA for IS-IS

This task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computation to converge traffic flows around link, node, and SRLG failures.

Before you begin

Ensure that the following topology requirements are met:

  • Router interfaces are configured as per the topology.

  • Routers are configured with IS-IS.

  • Segment routing for IS-IS is configured. See Enabling Segment Routing for IS-IS Protocol.

  • Enter the following commands in global configuration mode:

    Router(config)# ipv4 unnumbered mpls traffic-eng Loopback0
Router(config)# mpls traffic-eng
Router(config-mpls-te)# exit
Router(config)#

Procedure

  Command or Action Purpose
Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters mode.

Step 2

router isis instance-id

Example:


RP/0/RP0/CPU0:router(config)# router isis 1

Enables IS-IS routing for the specified routing instance, and places the router in router configuration mode.

Note 
You can change the level of routing to be performed by a particular routing instance by using the is-type router configuration command.
Step 3

interface type interface-path-id

Example:


RP/0/RP0/CPU0:router(config-isis)# interface GigabitEthernet0/0/0/1

Enters interface configuration mode.
Step 4

address-family ipv4 [ unicast]

Example:


RP/0/RP0/CPU0:router(config-isis-if)# address-family ipv4 unicast

Specifies the IPv4 address family, and enters router address family configuration mode.

Step 5

fast-reroute per-prefix

Example:


RP/0/RP0/CPU0:router(config-isis-if-af)# fast-reroute per-prefix

Enables per-prefix fast reroute.

Step 6

fast-reroute per-prefix ti-lfa

Example:


RP/0/RP0/CPU0:router(config-isis-if-af)# fast-reroute per-prefix ti-lfa

Enables per-prefix TI-LFA fast reroute link protection.

Step 7

fast-reroute per-prefix tiebreaker { node-protecting | srlg-disjoint } index priority

Example:


RP/0/RP0/CPU0:router(config-isis-if-af)# fast-reroute per-prefix tie-breaker srlg-disjoint index 100

Enables TI-LFA node or SRLG protection and specifies the tiebreaker priority. Valid priority values are from 1 to 255. The lower the priority value, the higher the priority of the rule. Link protection always has a lower priority than node or SRLG protection.

Note 
The same attribute cannot be configured more than once on an interface.
Note 
For IS-IS, TI-LFA node protection and SRLG protection can be configured on the interface or the instance.

TI-LFA has been successfully configured for segment routing.

Configuring TI-LFA for OSPF

This task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computation to converge traffic flows around link, node, and SRLG failures.


Note
TI-LFA can be configured on the instance, area, or interface. When configured on the instance or area, all interfaces in the instance or area inherit the configuration.

Before you begin

Ensure that the following topology requirements are met:

  • Router interfaces are configured as per the topology.

  • Routers are configured with OSPF.

  • Segment routing for OSPF is configured. See Enabling Segment Routing for OSPF Protocol.

  • Enter the following commands in global configuration mode:

    Router(config)# ipv4 unnumbered mpls traffic-eng Loopback0
Router(config)# mpls traffic-eng
Router(config-mpls-te)# exit
Router(config)#

Procedure

  Command or Action Purpose
Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters mode.

Step 2

router ospf process-name

Example:


RP/0/RP0/CPU0:router(config)# router ospf 1

Enables OSPF routing for the specified routing process, and places the router in router configuration mode.

Step 3

area area-id

Example:


RP/0/RP0/CPU0:router(config-ospf)# area 1

Enters area configuration mode.

Step 4

interface type interface-path-id

Example:


RP/0/RP0/CPU0:router(config-ospf-ar)# interface GigabitEthernet0/0/0/1

Enters interface configuration mode.
Step 5

fast-reroute per-prefix

Example:


RP/0/RP0/CPU0:router(config-ospf-ar-if)# fast-reroute per-prefix

Enables per-prefix fast reroute.
Step 6

fast-reroute per-prefix ti-lfa

Example:


RP/0/RP0/CPU0:router(config-ospf-ar-if)# fast-reroute per-prefix ti-lfa

Enables per-prefix TI-LFA fast reroute link protection.

Step 7

fast-reroute per-prefix tiebreaker { node-protecting | srlg-disjoint } index priority

Example:


RP/0/RP0/CPU0:router(config-ospf-ar-if)# fast-reroute per-prefix tie-breaker srlg-disjoint index 100

Enables TI-LFA node or SRLG protection and specifies the tiebreaker priority. Valid priority values are from 1 to 255. The lower the priority value, the higher the priority of the rule. Link protection always has a lower priority than node or SRLG protection.

Note 
The same attribute cannot be configured more than once on an interface.

TI-LFA has been successfully configured for segment routing.

TI-LFA Node and SRLG Protection: Examples

The following examples show the configuration of the tiebreaker priority for TI-LFA node and SRLG protection, and the behavior of post-convergence backup-path. These examples use OSPF, but the same configuration and behavior applies to IS-IS.

Example: Enable link-protecting and node-protecting TI-LFA


router ospf 1
 area 1
  interface GigabitEthernet0/0/2/1
    fast-reroute per-prefix 
    fast-reroute per-prefix ti-lfa
    fast-reroute per-prefix tiebreaker node-protecting index 100

Both link-protecting and node-protecting TI-LFA backup paths will be computed. If the priority associated with the node-protecting tiebreaker is higher than any other tiebreakers, then node-protecting post-convergence backup paths will be selected, if it is available.

Example: Enable link-protecting and SRLG-protecting TI-LFA


router ospf 1
 area 1
  interface GigabitEthernet0/0/2/1
    fast-reroute per-prefix 
    fast-reroute per-prefix ti-lfa
    fast-reroute per-prefix tiebreaker srlg-disjoint index 100

Both link-protecting and SRLG-protecting TI-LFA backup paths will be computed. If the priority associated with the SRLG-protecting tiebreaker is higher than any other tiebreakers, then SRLG-protecting post-convergence backup paths will be selected, if it is available.

Example: Enable link-protecting, node-protecting and SRLG-protecting TI-LFA


router ospf 1
 area 1
  interface GigabitEthernet0/0/2/1
    fast-reroute per-prefix 
    fast-reroute per-prefix ti-lfa
    fast-reroute per-prefix tiebreaker node-protecting index 100
    fast-reroute per-prefix tiebreaker srlg-disjoint index 200

Link-protecting, node-protecting, and SRLG-protecting TI-LFA backup paths will be computed. If the priority associated with the node-protecting tiebreaker is highest from all tiebreakers, then node-protecting post-convergence backup paths will be selected, if it is available. If the node-protecting backup path is not available, SRLG-protecting post-convergence backup path will be used, if it is available.