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OSPFv2
Link-protection Topology Independent Loop Free Alternate Fast Reroute
This document
describes OSPFv2 implementation of IP Fast Re-Route Feature (IP FRR) using TI
-LFA (Topology Independent Loop Free Alternative).
Restrictions for
Topology Independent Loop Free Alternate Fast Reroute
TI-LFA is
supported only on OSPFv2.
TI-LFA tunnels are
created only if the router supports SR and it is configured with prefix SID.
The prefix (or) node SID can be configured as connected SID (or) advertised
using the SRMS (Segment Routing Mapping Server).
TI-LFA is not
supported on OSPF point to multi point interfaces.
TI-LFA does not
support Multi Topology Routing (MTR).
TI-LFA does not
create the repair path using virtual link, sham link (or) TE tunnels.
TI-LFA tunnel is constructed and programmed by explicitly specifying
the node (or) set of repair nodes through which the tunnel needs to traverse.
Information About
OSPFv2 Link-Protection Topology Independent Loop Free Alternate Fast
Reroute
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, such as RLFA (Remote Loop Free Alternative) cannot provide
protection. The goal of TI-LFA is to reduce the packet loss that results while
routers converge after a topology change due to a link 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 following are the
major benefits of using TI-LFA:
Provides 100%
coverage for all the prefixes and within 50-msec link and node protection.
Prevents transient
congestion and sub-optimal routing by leveraging on the post-convergence path.
Protects Label
Distribution Protocol (LDP) and IP traffic as well.
IP Fast Reroute and
Remote Loop Free Alternate
IP Fast Reroute (FRR)
is a set of techniques that allow rerouting the IP traffic around a failed link
or failed node in the network within a very short time (<50ms). One of the
techniques that is used is Loop Free Alternates (LFA), which is implemented
using OSPF protocol. OSPF currently supports per-prefix directly connected LFA
and remote LFA (RLFA). The problem with these LFA algorithms is the topology
dependency; the LFA algorithms cannot find a loop-free alternate path through
the network for all the topologies.
The per-prefix
directly connected LFA (also known as DLFA) provides loop-free alternate path
for most triangular topologies, but does not provide good coverage for
rectangular or circular topologies. The Remote LFA implementation (RLFA) which
uses MPLS forwarding with LDP signaling for tunneling the rerouted traffic to
an intermediate node, extends the IPFRR coverage in ring or rectangular
topologies. For each link, RLFA defines P-Space (set of nodes reachable from
calculating node without crossing the protected link) and Q-Space (set of nodes
that can reach the neighbor on the protected link without crossing the
protected link itself). The nodes that belong to both P and Q-Spaces are called
PQ nodes and can be used as the intermediate node for the protected traffic.
RLFA forms targeted LDP session to the PQ node and form the RLFA tunnel. But
for the topologies where P and Q-Spaces are disjoint, R-LFA does not provide
protection for those prefixes.
Topology
Independent Fast Reroute
Topology Independent
Fast Reroute (TI-FRR) is a technique which uses segment routing to provide link
protection in any topology assuming the metric on the links in the topology is
symmetrical. TI-LFA does not guarantee a backup in the cases where bandwidth on
a single link is asymmetrical. TI-LFA only considers loop-free repair paths
that are on the post-convergence path. It helps to do better capacity planning
of the network.
TI-LFA algorithm
allows to create a full explicit path through the network. Using fully
specified path may lead to issues in larger topologies due to the number of
segments along the path. Specifying the whole path is however not necessary,
only a subset of the path is needed to carry the traffic to an intermediate
node (release node) which does not loop the traffic back to the protecting
node. The TI-LFA algorithm constructs a SR tunnel as the repair path. TI-LFA
tunnel is constructed and programmed by explicitly specifying the node (or) set
of repair nodes through which the tunnel needs to traverse. The traffic is
carried on the tunnel (when the primary path fails) which is also on the post
convergence path.
Topology-Independent
Loop Free Alternate
When the local LFA
and remote LFA are enabled, there is a good coverage of the prefixes to be
protected. However, for some rare topologies that do not have a PQ intersect
node, both local and remote LFA will fail to find a release node to protect the
failed link. Furthermore, there is no way to prefer a post-convergence path, as
the two algorithms have no knowledge of the post-convergence characteristics of
the LFA.
To overcome the above
limitation, topology-independent LFA (TI-LFA) is supported on an SR-enabled
network and provides the following support:
Link
Protection—The LFA provides repair path for failure of the link.
Local LFA—Whenever a
local LFA on the post convergence path is available, it is preferred over
TI-LFA because local LFA does not require additional SID for the repair path.
That is, the label for the PQ node is not needed for the release node.
Local LFA for extended P
space—For nodes in the extended P space, local LFA is still the most
economical method for the repair path. In this case, TI-LFA is not chosen.
Tunnel to PQ
intersect node—This is similar to remote LFA except that the repair path is
guaranteed on the post convergence path using TI-LFA.
Tunnel to PQ
disjoint node—This capability is unique to the TI-LFA in the case when
local and remote LFA cannot find a repair path.
Tunnel to traverse multiple
intersect or disjoint PQ nodes—TI-LFA provides complete coverage of all
prefixes, up to the platform’s maximum supported labels.
P2P and Broadcast interfaces
for the protected link—TI-LFA protects P2P and broadcast interfaces.
Asymmetrical links—The
OSPF metrics between the neighbors are not the same.
Multi-homed
(anycast) prefix protection—The same prefix may be originated by multiple
nodes and TI-LFA protects the anycast prefixes also by providing post
convergence repair path.
Protected prefix
filtering—The route-map includes or excludes a list of prefixes to be
protected and the option to limit the maximum repair distance to the release
node.
Tiebreakers—A subset of
existing tiebreakers applicable to TI-LFA is supported.
Local and remote LFA
use default or user-configured heuristics to break the tie when there is more
than one path to protect the prefix. The attributes are used to trim down the
number of repair paths at the end of the TI-LFA link protection computation
before the load balancing.
Local LFA and remote
LFA support the following tiebreakers:
Linecard-disjoint—Prefers the line card disjoint repair path.
Load-sharing—Distributes
repair paths equally among links and prefixes.
When there are two
repair paths for a particular prefix, the path that the output port on
different line card than that of the primary port is chosen as the repair path.
LC-disjoint-index—If
both the repair paths are on the same line card as that of the primary path,
then both paths are considered as candidates. If one of the path is on a
different line card, then that path is chosen as the repair path.
SRLG-disjoint—Prefers the
SRLG disjoint repair path.
The SRLG ID can be
configured for each interface. When there are two repair paths for a prefix,
the configured SRLG ID for the repair path is compared with that of the primary
path SRLG ID. If the SRLG IDs for the secondary path is different than that of
the primary, that path is chosen as the repair path.
Effective with Cisco IOS-XE Release 3.18, node-protecting tie-breaker
is disabled by default. Tie-breaker default and explicit tie-breaker on the
same interface are mutually exclusive. The following tie-breakers are enabled
by default on all LFAs:
linecard-disjoint
lowest-backup-metric
SRLG-disjoint
P-Space
The set of routers
that can be reached from S on the shortest path tree without traversing S-E is
termed the P-space of S with respect to the link S-E.
Figure 1. A Simple Ring
Topology
Q-Space
The set of routers
from which the node E can be reached, by normal forwarding without traversing
the link S-E, is termed the Q-space of E with respect to the link S-E.
Post-Convergence
Path
Post convergence path
is the path that OSPF uses after the link failure. TI-LFA always calculates the
repair path which is the post convergence path. You can plan and dimension the
post-convergence path to carry the traffic in the case of failure. TI-LFA
enforces the post-convergence path by encoding it as a list of segments. The
following figure shows an example of TI-LFA using post convergence path:
Figure 2. TI-LFA Using
Post Convergence Path
It protects
destination Node 5 on Node 2 against failure of link 2-3.
Node 2
switches all the traffic destined to Node 5 via core links.
Per-Destination Link
Protection
TI-LFA implementation
provides per-destination link protection with the number of segments
(labels)supported by the underlying hardware. The following figures show the
implementation of TI-LFA:
Figure 3. TI-LFA: {
Prefix-SID(PQ) }
If PQ is a direct
neighbor of S, then no additional segment must be pushed.
TI-LFA backup
path is calculated only if TI-LFA protection is enabled on the primary
interface which is to be protected. By default all the interfaces are enabled
for protection.
TI-LFA repair path
is restricted by the number of labels supported by the hardware. If hardware
supports only 2 labels then TI-LFA repair path can protect only those prefixes
which can be protected by 2 or lesser segments. For those prefixes which need
more than 2 segment remain unprotected.
Prefix
Processing
Once TI-LFA path is
calculated for the all the links, prefix processing starts. By default only
intra and inter area prefixes are protected. For external prefixes to be
protected, you need to enable segment routing globally under the OSPF level.
The primary and repair
path should be of the same route type for the prefixes that are protected, that
means, if the intra area needs to be protected then the TI-LFA repair path also
calculates for the same intra area prefix whether the prefix is unique (or)
anycast prefix.
Anycast Prefix
Processing
OSPF TI-LFA also calculates the
repair path for the anycast prefixes. Anycast prefixes (or) dual homed prefixes
are the prefixes advertised by more than one routers. They could be intra,
inter (or), external prefixes. The calculation of TI-LFA repair path for
anycast prefixes is as below:
Assume the prefix P1 is
advertised by the routers R1 and R2. The prefix advertised by both the routers
should be of the same route type, that is, both R1 and R2 should advertise the
prefix as intra area prefix (or inter or external).
Take the primary path is calculated towards R1 due to the lesser
cost.
When TI-LFA calculates the back up path, it calculates the post
convergence path. So, post convergence path need not be towards R1. If the cost
to reach R2 (in the post convergence) is shorter, then TI-LFA algorithm chooses
the post convergence path towards R2. TI-LFA tunnel is formed towards R2.
When R2 un-advertises the prefix, then the TI-LFA algorithm is
re-calculated towards R1 for the repair path.
Per-Prefix Loop Free
Alternate Tie-Break
IP FRR has the
following tie break rules in the order given below. If you have more than one
repair path available to choose the best path from, the following tie-break
rules are applied. If more than one path matches all the tie break rules, then
all the paths are used as repair paths.
Post Convergence:
Prefers backup path which is the post convergence path. This is enabled by
default and user can not modify this.
Primary-path: Prefers
backup path from ECMP set.
Interface-disjoint: Point-to-point interfaces have no
alternate next hop for rerouting if the primary gateway fails. You can set the
interface-disjoint attribute to prevent selection of such repair paths, thus
protecting the interface.
Lowest-backup-metric:
Prefers backup path with lowest total metric. This is not applicable for TI-LFA
since TI-LFA always chooses the back up path which is lowest cost.
LC-disjoint: Prefers the
back up path which is in different line card than that of the primary path.
Broadcast-interface-disjoint
: LFA repair paths protect links when a repair path and a
protected primary path use different next-hop interfaces. However, on broadcast
interfaces if the LFA repair path is computed via the same interface as the
primary path and their next-hop gateways are different, in that case the node
gets protected, but the link might not be. You can set the
broadcast-interface-disjoint attribute to specify that the repair path never
crosses the broadcast network the primary path points to, that means, it cannot
use the interface and the broadcast network connected to it.
Load Sharing: When more
than one repair path matches the above rules, load share the backup paths. This
rule also can be modified by the user.
Note
The user can alter and define the tiebreak rules according to the
requirement. In this way, the user can re-prioritize the sequence and/or remove
some of the tie break indexes which are not needed.
Note
The
Lowest-backup-metric policy is not applicable for TI-LFA since TI-LFA always
chooses the lowest back up path only.
You can see the above
rules by using the following command:
R2#show ip ospf fast-reroute
OSPF Router with ID (2.2.2.200) (Process ID 10)
Microloop avoidance is enabled for protected prefixes, delay 5000 msec
Loop-free Fast Reroute protected prefixes:
Area Topology name Priority Remote LFA Enabled TI-LFA Enabled
0 Base Low No Yes
AS external Base Low No Yes
Repair path selection policy tiebreaks (built-in default policy):
0 post-convergence
10 primary-path
20 interface-disjoint
30 lowest-metric
40 linecard-disjoint
50 broadcast-interface-disjoint
256 load-sharing
OSPF/RIB notifications:
Topology Base: Notification Enabled, Callback Registered
Last SPF calculation started 17:25:51 ago and was running for 3 ms.
With the introduction
of TI-LFA, the following two tie-break rules are enhanced.
node-protection
srlg-protection
The above two
tie-break rules are not enabled by default. The user needs to configure the
above mentioned tie-break policies.
Node
Protection
TI-LFA node protection
provides protection from node failures. Node protecting TI-LFA attempts to
calculate the post conversion repair path that protects against the failure of
a particular next-hop, not just the link to that particular next-hop.
Node protection is
used as a tiebreaker in the implementation of the local LFA also. But when it
is combined with TI-LFA, the back up path calculated post convergences with
node protecting path. Per-Prefix TI-LFA node protection is disabled by default.
The IPFRR TI-LFA node protection features is enabled when the corresponding
tiebreak is enabled along with TI-LFA feature, that is,
When you enable node
protection, all the other tie break rules also need to manually configured. The
node protection is built over the link protection.
The difference
between
node-protecting and
node-protecting required is in selecting the backup
path. When you configure
node-protecting required, then back up which is chosen
has to be the path which does not go through the node (which is part of the
link which we are protecting). If no such path is available, then no path is
chosen as the backup path.
Shared Risk Link
Groups Protection
A shared risk link
group (SRLG) is a group of next-hop interfaces of repair and protected primary
paths that have a high likelihood of failing simultaneously. The OSPFv2
Loop-Free Alternate Fast Reroute feature supports only SRLGs that are locally
configured on the computing router. With the introduction of TI LFA, the post
convergence path which does not share the SRLG group id with the primary path
interface will be chosen. In that way, the user will be sure of the SRLG
protection whenever the primary link fails.
The IPFRR TI-LFA SRLG protection features is enabled when the
corresponding tiebreak is enabled along with Ti-LFA feature, that is,
When you enable SRLG protection, you need to manually configure all the
other tie break rules. The difference between
srlg-protecting and
srlg-protecting required is in selecting the backup
path. When you configure
srlg-protecting required, then back up which is chosen
has to be the path which does not share SRLG ID with the primary link which is
protected. If no such path is available, then no path is chosen as the backup
path.
Whereas, if you configure
srlg-protecting alone then if the SRLG protection path
is not available, the link protection path is chosen as the backup path. And
when the SRLG protection path is available, the switchover happens to the SRLG
protection path.
Node-Shared Risk
Link Groups Protection
You can configure both
node and SRLG protection tie breaks together. This means that the back up path
needs to fulfil both the criteria of node protection as well as SRLG
protection. In that case, an additional TI-LFA node-SRLG combination protection
algorithm is run. The TI-LFA node-SRLG combination algorithm removes the
protected node and all members of the interface with the same SRLG group when
computing the post-convergence shortest path tree (SPT).
To enable node and
SRLG protection tie breaks together, use the following command:
The following show
command is used to display the tie break policy:
R3#show ip ospf fast-reroute
OSPF Router with ID (3.3.3.33) (Process ID 10)
Loop-free Fast Reroute protected prefixes:
Area Topology name Priority Remote LFA Enabled TI-LFA Enabled
0 Base Low No No
1 Base Low No No
1000 Base Low No No
AS external Base Low No No
Repair path selection policy tiebreaks:
0 post-convergence
60 node-protecting
70 srlg
256 load-sharing
OSPF/RIB notifications:
Topology Base: Notification Disabled, Callback Not Registered
Last SPF calculation started 00:00:06 ago and was running for 2 ms.
How to Configure
Topology Independent Loop Free Alternate Fast Reroute
Enabling Topology
Independent Loop Free Alternate Fast Reroute
By default, TI-LFA is
disabled. You can use protocol enablement to enable TI-LFA.
Protocol enablement:
Enables TI-LFA in router OSPF mode for all the OSPF areas. Perform the
following steps to enable TI-LFA FRR.
[no] fast-reroute per-prefix ti-lfa [ area <area> disable]
router ospf <process>
fast-reroute per-prefix enable area <area> prefix-priority {low | high}
fast-reroute per-prefix ti-lfa [ area <area> disable]
You can also use
interface command to enable or disable IP FRR on specific interfaces.
interface <interface>
ip ospf fast-reroute per-prefix protection disable
ip ospf fast-reroute per-prefix candidate disable
ip ospf fast-reroute per-prefix protection ti-lfa [disable]
Note
When TI-LFA is
configured on the OSPF router and area wide, area specific configuration takes
precedence.
To protect
external prefixes, TI-LFA should be enabled globally.
Configuring Topology
Independent Loop Free Alternate Fast Reroute
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.
TI-LFA can be configured on instance or area level inherited by lower levels.
You can enable or disable per prefix FRR per interface level which is
applicable for TI-LFA also.
Before you begin to
configure, ensure that the following topology requirements are met:
Router interfaces
are configured as per the topology.
Routers are
configured with OSPF.
Segment routing is
enabled globally as well as under OSPF level.
Enables OSPF routing for the specified routing process and enters in
router configuration mode.
Configuring Topology
Independent Fast Reroute Tie-breaker
You need to enable
segment routing on all the routers with prefix SIDs configured for all the
nodes. Use the following topology as a reference to understand the
configuration.
Figure 5. Configuration
Example
Let us take the
device R2 which is protecting the link between R2 and R3. The configuration at
R2:
router ospf 10
fast-reroute per-prefix enable prefix-priority low
fast-reroute per-prefix ti-lfa
segment-routing mpls
segment-routing area 0 mpls
fast-reroute per-prefix enable prefix-priority low
fast-reroute per-prefix ti-lfa
fast-reroute per-prefix ti-lfa area 0
fast-reroute per-prefix tie-break node-protecting index 60
fast-reroute per-prefix tie-break srlg index 70
mpls traffic-eng router-id Loopback1
mpls traffic-eng area 0
interface GigabitEthernet4 //interface connecting to the router 4
ip address 100.101.4.4 255.255.255.0
ip ospf 10 area 0
ip ospf network point-to-point
srlg gid 10
negotiation auto
interface GigabitEthernet3 //interface connecting to the router 3
ip address 100.101.3.3 255.255.255.0
ip ospf 10 area 0
ip ospf network point-to-point
srlg gid 10
negotiation auto
interface GigabitEthernet5 //interface connecting to the router 2
ip address 100.101.5.5 255.255.255.0
ip ospf 10 area 0
ip ospf network point-to-point
srlg gid 20
negotiation auto
interface loopback2
ip address 2.2.2.2/32
ip ospf 10 area 0
Note
In all the other
devices, configuration of segment routing and assignment of connected prefix
SIDs need to be done.
How Node Protection
Works: Using the same topology as an example, let us take the case where
you are protecting the link between R2 and R3 and also the prefix which is
leant from R6. In that case, let us assume that the primary path for the prefix
is via R2-R3. So, our primary path is R2---R3---R6 and we are protecting the
link R2---R3.
In this scenario, only
link-protection is configured and enabled. When you enable TI-LFA under OSPF
process, then you get the following paths provided the cost for all the paths
are equal:
R2----R4----R5---R6
R2---R5----R3---R6
R2----R5---R6
If you have only link
protection configured, then all the three paths will be chosen and they will
share the load amongst them.
If you wish to
configure node protection, then the backup would be calculated in such a way
that the back up path does not contain the node that you are protecting. In
this example, the node R3 in the back up is not required. As a result, only the
following two paths would be chosen as the back up paths:
R2----R4----R5---R6
R2----R5---R6
It is possible that
R2---R5---R3---R6 have the lesser cost than the above two paths. But since the
node protection is configured, only the paths amongst the above two will be
considered.
How SRLG Protection
Works: SRLG protection further eliminates the back up paths in a such a way
that the primary path and the backup does not share the same SRLG ID. Suppose
the following back up paths are available:
R2----R4----R5---R6
R2----R5---R6
Then, the SRLG ID of
(R2----R4) and (R2----R5) are compared against the primary interface (R2----R3)
which is 10. It is noticed that only the interface R2----R5 has different SRLG
ID which is 20. So, only the backup path R2---R5---R6 will be chosen.
Verifying Topology
Independent Fast Reroute Tunnels
You can use the
following command, to check the TI LFA tunnels:
Device#show ip ospf fast-reroute ti-lfa tunnels
OSPF Router with ID (2.2.2.200) (Process ID 10)
Area with ID (0)
Base Topology (MTID 0)
Tunnel Interface Next Hop Mid/End Point Label
-------------------------------------------------------------------------------
MPLS-SR-Tunnel2 Et1/1 2.7.0.7 1.1.1.1 16020
MPLS-SR-Tunnel6 Et0/3 2.8.0.0 3.3.3.3 16003
MPLS-SR-Tunnel7 Et1/1 2.7.0.7 1.1.1.1 16020
5.5.5.5 16005
3.3.3.3 16003
MPLS-SR-Tunnel5 Et0/3 2.8.0.0 5.5.5.5 16005
MPLS-SR-Tunnel1 Et1/1 2.7.0.7 1.1.1.1 16020
5.5.5.5 16005
MPLS-SR-Tunnel3 Et1/1 2.7.0.7 6.6.6.6 16006
You can use the
following command, to check the route in OSPF routing table with primary and
repair path:
Device#show ip ospf rib 6.6.6.6
OSPF Router with ID (2.2.2.200) (Process ID 10)
Base Topology (MTID 0)
OSPF local RIB
Codes: * - Best, > - Installed in global RIB
LSA: type/LSID/originator
*> 6.6.6.6/32, Intra, cost 31, area 0
SPF Instance 19, age 02:12:11
contributing LSA: 10/7.0.0.0/6.6.6.6 (area 0)
SID: 6
CSTR Local label: 0
Properties: Sid, LblRegd, SidIndex, N-Flag, TeAnn
Flags: RIB, HiPrio
via 2.7.0.7, Ethernet1/1 label 16006
Flags: RIB
LSA: 1/6.6.6.6/6.6.6.6
PostConvrg repair path via 3.3.3.3, MPLS-SR-Tunnel6 label 16006, cost 81, Lbl cnt 1
Flags: RIB, Repair, PostConvrg, IntfDj, LC Dj
LSA: 1/6.6.6.6/6.6.6.6
You can use the
following command, to display the route in the IP routing table:
Device#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
Known via "ospf 10", distance 110, metric 31, type intra area
Last update from 2.7.0.7 on Ethernet1/1, 00:25:14 ago
SR Incoming Label: 16006
Routing Descriptor Blocks:
* 2.7.0.7, from 6.6.6.6, 00:25:14 ago, via Ethernet1/1, merge-labels
Route metric is 31, traffic share count is 1
MPLS label: 16006
MPLS Flags: NSF
Repair Path: 3.3.3.3, via MPLS-SR-Tunnel6
Debugging Topology
Independent Loop Free Alternate Fast Reroute
You can use the
following commands to debug TI-LFA FRR:
debug ip ospf fast-reroute spf
debug ip ospf fast-reroute spf detail
debug ip ospf fast-reroute rib
debug ip ospf fast-reroute rib [<access-list>]
Examples: OSPFv2
Link-Protection Topology Independent Loop Free Alternate Fast Reroute
The following are the examples for
the OSPFv2 Link-Protection TI-LFA FRR.
Example: Configuring
Topology Independent Loop Free Alternate Fast Reroute
This example shows how
to configure TI-LFA for segment routing TE tunnels using single or disjoint PQ
nodes. The following are the two topologies used:
Topology 1: A
single PQ Node and therefore has two SIDs from the source router, R1 through
the PQ Node to the destination router, R5.
Figure 6. Topology 1:
Single PQ Node
Topology 2:
Disjoint PQ Nodes and therefore consists of three SIDs from the source router
R1, through the P Node and the Q Node to the destination router, R5.
Figure 7. Topology 2:
Disjoint PQ Nodes
Configure TI-LFA for
OSPF on the source router (R1) interface connecting to the destination router
(R5).
Feature Information
for OSPFv2 Link-protection Topology Independent Loop Free Alternate Fast
Reroute
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature
Information for OSPFv2 Link-protection Topology Independent Loop Free Alternate
Fast Reroute
Feature Name
Releases
Feature
Information
OSPFv2
Link-protection Topology Independent Loop Free Alternate Fast Reroute
Cisco IOS XE
Everest 16.4.1
Cisco IOS XE Fuji 16.7.1
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. The
goal of TI-LFA is to reduce the packet loss that results while routers converge
after a topology change due to a link failure.
The following
commands were introduced or modified:
fast-reroute per-prefix
ti-lfa [area <area> [disable]],
fast-reroute per-prefix
tie-break node-protecting index <index>,
fast-reroute per-prefix
tie-break node-protecting required index <index>,
fast-reroute per-prefix
tie-break srlg index <index>,
fast-reroute per-prefix
tie-break srlg required index <index>,
ip ospf fast-reroute
per-prefix protection disable,
ip ospf fast-reroute
per-prefix candidate disable,
show ip ospf fast-reroute
ti-lfa tunnels.
In Cisco IOS XE Fuji 16.7.1, this feature is supported on Cisco
4000 Series Integrated Service Routers.
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