Segment Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.3.x
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Topology-Independent Loop-Free Alternate (TI-LFA) uses segment routing to provide link, node, and Shared Risk Link Groups (SRLG) protection in topologies where other fast reroute techniques cannot provide protection.
Classic Loop-Free Alternate (LFA) is topology dependent, and therefore cannot protect all destinations in all networks. A
limitation of LFA is that, even if one or more LFAs exist, the optimal LFA may not always be provided.
Remote LFA (RLFA) extends the coverage to 90-95% of the destinations, but it also does not always provide the most desired
repair path. RLFA also adds more operational complexity by requiring a targeted LDP session to the RLFAs to protect LDP traffic.
TI-LFA provides a solution to these limitations while maintaining the simplicity of the IPFRR solution.
The goal of TI-LFA is to reduce the packet loss that results while routers converge after a topology change due to a link
or node failure. Rapid failure repair (< 50 msec) is achieved through the use of pre-calculated backup paths that are loop-free and
safe to use until the distributed network convergence process is completed.
The optimal repair path is the path that the traffic will eventually follow after the IGP has converged. This is called the
post-convergence path. This path is preferred for the following reasons:
Optimal for capacity planning — During the capacity-planning phase of the network, the capacity of a link is provisioned while
taking into consideration that such link with be used when other links fail.
Simple to operate — There is no need to perform a case-by-case adjustments to select the best LFA among multiple candidate
LFAs.
Fewer traffic transitions — Since the repair path is equal to the post-convergence path, the traffic switches paths only once.
The following topology illustrates the optimal and automatic selection of the TI-LFA repair path.
Figure 1. TI-LFA Repair Path
Node 2 protects traffic to destination Node 5.
With classic LFA, traffic would be steered to Node 4 after a failure of the protected link. This path is not optimal, since
traffic is routed over edge node Node 4 that is connected to lower capacity links.
TI-LFA calculates a post-convergence path and derives the segment list required to steer packets along the post-convergence
path without looping back.
In this example, if the protected link fails, the shortest path from Node2 to Node5 would be:
Node2 → Node6 → Node7 → Node3 → Node5
Node7 is the PQ-node for destination Node5. TI-LFA encodes a single segment (prefix SID of Node7) in the header of the packets
on the repair path.
TI-LFA Protection Types
TI-LFA supports the following protection:
Link protection — The link is excluded during the post-convergence backup path calculation.
Node protection — The neighbor node is excluded during the post convergence backup path calculation.
Shared Risk Link Groups (SRLG) protection — SRLG refer to situations in which links in a network share a common fiber (or
a common physical attribute). These links have a shared risk: when one link fails, other links in the group might also fail.
TI-LFA SRLG protection attempts to find the post-convergence backup path that excludes the SRLG of the protected link. All
local links that share any SRLG with the protecting link are excluded.
When you enable link protection, you can also enable node protection, SRLG protection, or both, and specify a tiebreaker priority
in case there are multiple LFAs.
The following example illustrates the link, node, and SRLG protection types. In this topology, Node2 applies different protection
models to protect traffic to Node7.
Figure 2. TI-LFA Protection Types
Behaviors and Limitations of TI-LFA
The behaviors and limitations of TI-LFA are listed below:
TI-LFA protects unlabeled IPv4 traffic.
TI-LFA does not protect unlabeled IPv6 traffic.
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.
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.
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.