Topology-Independent Loop-Free Alternate with Segment Routing for OSPF
Topology-Independent Loop-Free Alternate with Segment Routing for OSPF Overview
This section describes the TI-LFA feature for OSPF.
When used with OSPF, TI-LFA provides protection against link failure, node failure., fate-sharing failures, and shared risk link group failures. In link failure mode, the destination is protected if the link fails. In node protection mode, the destination is protected if the neighbor connected to the primary link fails. To determine the node-protecting post-convergence path, the cost of all the links leaving the neighbor is assumed to increase by a configurable amount.
Configure fate-sharing protection in TI-LFA networks for segment routing to choose a fast reroute path that does not include fate-sharing groups in the topology-independent loop-free alternate (TI-LFA) backup paths to avoid fate-sharing failures. With fate-sharing protection, a list of fate-sharing groups are configured on each PLR with the links in each fate-sharing group identified by their respective IP addresses. The PLR associates a cost with each fate-sharing group. The fate-sharing-aware post-convergence path is computed by assuming that the cost of each link in the same fate-sharing group as the failed link has increased the cost associated with that group.
Configure Shared Risk Link Group (SRLG) protection in TI-LFA networks for segment routing to choose a fast reroute path that does not include SRLG links in the topology-independent loop-free alternate (TI-LFA) backup paths. SRLGs share a common fibre and they also share the risks of a broken link. When one link in an SRLG fails, other links in the group might also fail. Therefore, you need to avoid links that share the same risk as the protected link in the backup path. Configuring SRLG protection prevents TI-LFA from selecting backup paths that include a shared risk link. If you have configured SRLG protection then OSPFv2 computes the fast reroute path that is aligned with the post convergence path and excludes the links that belong to the SRLG of the protected link. All local and remote links that are from the same SRLG as the protected link are excluded from the TI-LFA back up path. The point of local repair (PLR) sets up the label stack for the fast reroute path with a different outgoing interface. Currently you cannot enable SRLG protection in IPv6 networks and in networks with multitopology.
In order to construct a backup path that follows the post-convergence path, TI-LFA can use several labels in the label stack that define the backup path. If the number of labels required to construct a particular post-convergence backup path exceeds a certain amount, it is useful in some circumstances to not install that backup path. You can configure the maximum number of labels that a backup path can have in order to be installed. The default value is 3, with a range of 2 through 5.
It is often the case that the post-convergence path for a given failure is actually a set of equal-cost paths. TI-LFA attempts to construct the backup paths to a given destination using multiple equal-cost paths in the post-failure topology. Depending on the topology, TI-LFA might need to use different label stacks to accurately construct those equal-cost backup paths. By default, TI-LFA only installs one backup path for a given destination. However, you can configure the value in the range from 1 through 8.
Benefits of Using Topology-Independent Loop-Free Alternate with Segment Routing
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Loop-free alternate (LFA) and remote LFA (RLFA) have been used to provide fast-reroute protection for several years. With LFA, a point of local repair (PLR) determines whether or not a packet sent to one of its direct neighbors reaches its destination without looping back through the PLR. In a typical network topology, approximately 40 to 60 percent of the destinations can be protected by LFA. Remote LFA expands on the concept of LFA by allowing the PLR to impose a single label to tunnel the packet to a repair tunnel endpoint from which the packet can reach its destination without looping back through the PLR. Using remote LFA, more destinations can be protected by the PLR compared to LFA. However, depending on the network topology, the percentage of destinations protected by remote LFA is usually less than 100 percent.
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Topology-independent LFA (TI-LFA) extends the concept of LFA and remote LFA by allowing the PLR to use deeper label stacks to construct backup paths. In addition, TI-LFA imposes the constraint that the backup path used by the PLR be the same path that a packet takes once the interior gateway protocol (IGP) has converged for a given failure scenario. This path is referred to as the post-convergence path.
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Using the post-convergence path as the backup path has some desirable characteristics. For some topologies, a network operator only needs to make sure that the network has enough capacity to carry the traffic along the post-convergence path after a failure. In these cases, a network operator does not need to allocate additional capacity to deal with the traffic pattern immediately after the failure while the backup path is active, because the backup path follows the post-convergence path.
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When used with OSPF, TI-LFA provides protection against link failure and node failure.
Configuring Topology-Independent Loop-Free Alternate with Segment Routing for OSPF
Before you configure TI-LFA for OSPF, be sure you configure SPRING or segment routing.
Junos supports creation of OSPF topology-independent TI-LFA backup paths where the prefix SID is learned from a segment routing mapping server advertisement when the PLR and mapping server are both in the same OSPF area.
To configure TI-LFA using SPRING for OSPF, you must do the following: