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NE20E-S V800R010C10SPC500 Feature Description - Value-Added-Service 01

This is NE20E-S V800R010C10SPC500 Feature Description - Value-Added-Service
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TI-LFA FRR

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

Table 2-24 TI-LFA FRR 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.

In Figure 2-53, there are packets that need to be sent from Device A to Device F. If the P space and Q space do not intersect, RLFA requirements fail to be fulfilled, and RLFA cannot compute a backup path, that is, the Remote LDP LSP. If a fault occurs on the link between Device B and Device E, Device B forwards data packets to Device C. Device C is not the Q node and doe not have the destination IP address directly to the destination IP address. In this situation, Device C has to recompute a path. The cost of the link between Device C and Device D is 1000. Device C considers that the optimal path to Device F passes through Device B. Device C loops the packet to Device B, leading to a loop and resulting in a forwarding failure.
Figure 2-53 RLFA networking
TI-LFA can be used to solve this problem. In Figure 2-54, if a fault occurs on the link between Device B and Device E, Device B enables TI-LFA FRR backup entries and adds new path information (node label of Device C and adjacency label for the C-D adjacency) to the packets to ensure that the data packets can be forwarded along the backup path.
Figure 2-54 TI-LFA networking

Benefits

Segment routing-based TI-LFA FRR has the following advantages:
  1. Meets basic requirements of IP FRR rapid convergence.
  2. Theoretically supports all protection scenarios.
  3. Uses an algorithm with moderate complexity.
  4. 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 2-55, 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.

Figure 2-55 Typical TI-LFA networking

In the following example, the process of node protection is as follows. In Figure 2-55, traffic travels along a path PE1->P1->P5->PE3. If P1 fails, TI-LFA computes the P space, the Q space, and the SPF tree (also called the post-convergence tree), and a backup outbound interface and a repair list. Traffic is forwarded along the backup path to the destination PE3, which implements rapid protection to prevent traffic loss.

TI-LFA FRR computation is as follows:
  1. 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.
  2. 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.
  3. Computes the post-convergence SPF tree. It excludes the primary next hop.
  4. 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, for example, interface 1 in Figure 2-55.

    • 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 2-55, the repair list consists of P3's node label 100, and P3-to-P4 adjacency label 9304.
Rules for selecting a SID on a repair node are as follows:
  • A node SID advertised by the repair node is preferentially selected.
  • The smallest prefix SID of a single source on a repair node is preferentially selected.
  • A non-multiple-source prefix on a repair node is preferentially selected.
  • 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 2-56, 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.

Figure 2-56 TI-LFA FRR backup path forwarding
Table 2-25 TI-LFA FRR backup path forwarding process

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

Table 2-26 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.

  1. Establish an IS-IS neighbor relationship between each pair of directly connected nodes on a network. Enable segment routing on all nodes. Set a prefix SID on the P node.
  2. Enable TI-LFA FRR on the source node.

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.

  1. Establish an IS-IS neighbor relationship between each pair of directly connected nodes on a network. Enable segment routing on all nodes. Set a prefix SID on each of the P and destination nodes.
  2. Enable TI-LFA FRR on the source node.

Anti-Micro-Loop Switchover

In Figure 2-57, if Device B fails, traffic is switched to a TI-LFA FRR backup path. After Device A completes route convergence, traffic is switched from the TI-LFA FRR backup path to a converged path. If Devices D and F do not complete route convergence, they transmit traffic over the path established before convergence is performed. As a result, a loop emerges between Devices A and F and is broken after route convergence finishes on Devices D and F.

To prevent the loop-induced problem, the implementation is modified. After Device B fails, traffic is switched to the TI-LFA backup path. Device A delays convergence. After Devices F and D finish path convergence, Device A starts path convergence. After path convergence is complete, traffic is switched from the TI-LFA backup path to the converged path.

Figure 2-57 Anti-micro-loop switchover

Configure the anti-micro-loop switchover function on the source node.

The delayed route switchover must meet the following conditions:
  • The interface directly connected to the local interface fails, or local BFD goes Down.
  • No network topology change occurs during the delay time.
  • A backup next hop for a route is available.
  • The primary next hop of the route is the faulty interface.
  • The primary and backup next hops are different after the path convergence is complete.
  • During the delay of the multi-source route convergence, the route source change event occurs, and the delay stops.

Anti-Micro-Loop Switchback

In Figure 2-58:
  1. Data is being transmitted along the backup path before the link between Device B and Device C recovers.
  2. After the link between Device B and Device C recovers, if Device A converges earlier than Device B, Device A forwards traffic to Device B that does not finish convergence. Upon receipt of the traffic, Device B forwards traffic along the original path to Device A, causing a loop.
  3. To prevent a micro loop, after a traffic switchback is performed on Device A, configure an explicit path to forward packets. Device A adds E2E path information (for example, a B-to-C adjacency label) to data packets. Upon receipt of the data packets, Device B forwards packets to Device A, C based on the carried path information.
Figure 2-58 Anti-micro-loop switchback

After Device B finishes convergence, Device A deletes explicit path information from data packets so that the data packets can be forwarded to Device C using normal SR.

Anycast FRR

Anycast SID

The anycast SID is the same SID advertised by all routers within a group. On the network shown in Figure 2-59, Device D and Device E reside on the egress of an SR area. Traffic can reach the non-SR area through either Device D or Device E. The two devices can back up each other. In this situation, Device D and Device E can be configured in the same group and advertise the same prefix SID, so called anycast SID.

An anycast SID's next hop directs to Device D that has the smallest IGP cost in the router group. Device D is called the optimal source that advertises the anycast label, and the other device in the router group is the backup source. If the primary next-hop link or direct neighbor node of Device D fails, traffic can reach the anycast label device through the other protection path. The anycast label device can be the source that has the same primary next hop or another source. When VPN traffic passes through an SR LSP, the same VPN private-network label must be configured for anycast.

Figure 2-59 Anycast SID
Anycast FRR

In anycast FRR, multiple nodes advertise the same prefix label. In other words, anycast FRR is multi-source prefix label FRR. The common FRR algorithms use the SPT to compute the backup next hops. This applies to single-source route scenarios but not to multi-source route scenarios.

Before a device computes the backup next hop of a prefix label in a multi-source route scenario, the multi-source route must be converted to a single-source route. Anycast FRR constructs a virtual node to convert multi-source routes to single-source routes and uses the TI-LFA algorithm to compute a backup next hop of the virtual node. The anycast prefix label inherits the backup next hop from the created virtual node. This solution does not involve any modification of the backup next hop algorithm. The solution retains the loop-free trait so that no loop occurs between the computed backup next hop and the primary next hop of the peripheral node before convergence.

Figure 2-60 IGP FRR networking in a multi-source route scenario

On the network shown in Figure 2-60 (a), the cost of Device A-to-Device B link is 5, and that of Device A-to-Device C is 10. Device B and Device C advertise the route source of 10.1.1.0/24 simultaneously. TI-LFA FRR is enabled on Device A. Because the single-source TI-LFA condition is not met, Device A cannot compute the backup next hop of the route destined for 10.1.1.0/24. To address this problem, TI-LFA FRR in a multi-source route scenario can be used. Implementation is as follows:

On the network shown in Figure 2-60 (b), a virtual node is constructed between Device B and Device C. The virtual node is connected to both Device B and Device C. The costs of the link from Device B and Device C to the virtual node are 0. The costs of links from the virtual node to Device B and Device C is infinite. The virtual node advertises a prefix of 10.1.1.0/24, converts the multi-source route to a single-source route, and uses TL-LFA to compute a backup next hop to the virtual node. The multi-source route destined for 10.1.1.0/24 inherits the computation result. On the network shown in Figure 2-60 (b), Device A computes two links to the virtual node. The active link is Device A to Device B, and the standby link is Device A to Device C.

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Updated: 2019-01-03

Document ID: EDOC1100055132

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