TI-LFA FRR
Topology-independent loop-free alternate (TI-LFA) fast reroute (FRR) protects links and nodes on segment routing tunnels. If a link or node fails, TI-LFA FRR rapidly switches traffic to a backup path, minimizing traffic loss.
Related Concepts
Concept |
Definition |
|---|---|
P space |
The P space contains a set of nodes reachable to the root node on links, not the protected link, along the SPF tree that originates from the protected link's source node functioning as the root node. |
Extended P space |
The extended P space contains nodes reachable to the root nodes on links, not the protected link, along the SPF trees originating from the root nodes that are neighbors of protected link's source node. |
Q space |
The Q space contains nodes reachable to the root node on links, not the protected link, along the reverse SPF tree originating from the protected link's destination node functioning as the root node. |
PQ node |
A PQ node resides in both the extended P space and Q space. The PQ node functions as the destination node of a protected tunnel. |
LFA |
The loop-free alternate (LFA) algorithm computes a standby link. A root node that can provide a standby link runs the Shortest Path First (SPF) protocol to compute the shortest path to a destination node. The root node then computes a loop-free standby link with the smallest cost. For more information about LFA, see IS-IS Auto FRR. |
RLFA |
Remote LFA (RLFA) computes a PQ node based on a protected path and establishes a tunnel between the source and PQ nodes to provide next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability. For more information about RLFA, see IS-IS Auto FRR. |
TI-LFA |
In some LFA or RLFA scenarios, the P space and Q space do not share nodes or have directly connected neighbors. Consequently, no backup path can be calculated, which does not meet reliability requirements. In this situation, TI-LFA can be used. The TI-LFA algorithm computes the P space and Q space based on a protected path, a shortest path tree (also called a post-convergence tree), and a repair list. The algorithm establishes a segment routing tunnel between the source node and PQ node to provide backup next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability. |
Background
Conventional LFA requires that at least one neighbor be a loop-free next hop to a destination. RLFA requires that there be at least one node that connects to the source and destination nodes along links without passing through any faulty node. Unlike LFA or RLFA, TI-LFA uses an explicit path to represent a backup path, which poses no requirements on topology constraints and provides more reliable FRR.
Benefits
- Meets basic requirements of IP FRR rapid convergence.
- Theoretically supports all protection scenarios.
- Uses an algorithm with moderate complexity.
- Selects a backup path over a converged route and has no intermediate state, compared with the other FRR techniques.
TI-LFA FRR Principles
In Figure 2-73, PE1 is a source node, P1 is a faulty node, and PE3 is a destination node. Link costs are marked.
TI-LFA traffic protection involves link and node protection.
Link protection: protects traffic passing through a specific link.
Node protection: protects traffic passing through a specific node. Node protection takes precedence over link protection.
In the following example, the process of node protection is as follows. In Figure 2-73, traffic travels along a path PE1->P1->P5->PE3. If P1 fails, TI-LFA computes the P space, Q space, SPF tree (also called the post-convergence tree), backup outbound interface, and repair list. Traffic is forwarded along the backup path to the destination PE3, which implements rapid protection to prevent traffic loss.
- Computes the P space. It contains the set of nodes reachable to the root node on links, not the protected link, along the SPF tree that originates from the protected link's source node functioning as the root node.
- Computes the space Q. It contains the set of nodes reachable to the root node on links, not the protected link, along the reverse SPF tree that originates from the protected link's destination node functioning as the root node.
- Computes the post-convergence SPF tree. It excludes the primary next hop.
- Computes a backup outbound interface and a repair list.
Backup outbound interface: In some scenarios, if the P space and Q space do not share nodes or have directly connected neighbors, the post-convergence next-hop outbound interface functions as a backup outbound interface.
- Repair list: a constrained path that directs traffic to the Q node. The repair list consists of a P node label and adjacency labels of the P-to-Q path. In Figure 2-73, the repair list consists of P3's node label 100, and P3-to-P4 adjacency label 9304.
- A node SID advertised by the repair node is preferentially selected.
- A prefix SID advertised by the repair node is preferentially selected. The smaller the SID, the higher the priority.
- A node that does not support segment routing or a node that does not advertise a prefix or node SID cannot function as a repair node.
TI-LFA FRR Backup Path Forwarding
After a TI-LFA backup path is computed, if the primary path fails, traffic switches to the backup path, preventing packet loss.
In Figure 2-74, Device F is a P node, and Device H is a Q node. The primary next-hop B fails, which triggers FRR switching. Traffic switches to the backup path.
Device |
TI-LFA FRR Backup Path Forwarding Process |
|---|---|
Device A |
Device A encapsulates a label stack to a packet based on the repair list from outer to inner: Node label of the P node (Device F) = Start label in next-hop Device D's SRGB + Label offset of the P node = 720 P-to-Q adjacency labels of 130 and 240 Destination node label = Start label of the Q node's SRGB + Label offset of the destination node (Device C) = 310 |
Device D |
Upon receipt of the packet, Device D searches the label forwarding table based on the outgoing label and finds a matching entry with the outgoing label of 120 and next hop at Device F. Device D swaps the outgoing label for 120 and forwards the packet to Device F. |
Device F |
Upon receipt of the packet, Device F searches the label forwarding table based on the outgoing label. Device F is the egress so that it removes the label. It finds a matching entry with a routed path label of 130, the outgoing label as empty, and the next hop at Device G. Device F removes label 130 and forwards the packet to Device G. |
Device G |
Upon receipt of the packet, Device G searches the label forwarding table based on the outgoing label, removes label 240, and forwards the packet to Device H. |
Device H |
Upon receipt of the packet, Device H searches the label forwarding table based on the outgoing label and finds a matching entry with the outgoing label of 510 and the next hop at Device E. Device H swaps the outer label for 510 and forwards the packet to Device E. Device E forwards the packet to Device C. The packet travels along the shortest path. |
TI-LFA FRR Protection Usage Scenarios
TI-LFA FRR Protection |
Description |
Deployment |
|---|---|---|
TI-LFA FRR protects IP forwarding. |
Traffic is transmitted over an IP routed primary path, and a TI-LFA FRR backup path is computed. |
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TI-LFA FRR protects traffic on a segment routing tunnel. |
Traffic is transmitted over a primary segment routing tunnel, and a TI-LFA FRR backup path is computed. |
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