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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
IS-IS Auto FRR

IS-IS Auto FRR

Background

IS-IS Auto fast re-route (FRR) is a dynamic IP FRR technology that minimizes traffic loss by immediately switching traffic to the alternate link pre-computed by an IGP based on the LSDBs on the entire network and stored in the FIB if a link or adjacent node failure is detected. As IP FRR implements route convergence, it is becoming increasingly popular with carriers.

Major Auto FRR techniques include loop-free alternate (LFA), U-turn, Not-Via, Remote LFA, and MRT, among which IS-IS supports only LFA and Remote LFA.

Related Concepts

LFA

LFA is an IP FRR technology that calculates the shortest path from the neighbor that can provide an alternate link to the destination node based on the Shortest Path First (SPF) algorithm. Then, LFA calculates a loop-free alternate link with the smallest cost based on the inequality: Distance_opt (N, D) < Distance_opt (N, S) + Distance_opt (S, D).

In the preceding inequality, S, D, and N indicate the source node, destination node, and a neighbor of S, respectively, and Distance_opt (X,Y) indicates the shortest distance from node X to node Y.

Remote LFA

LFA Auto FRR cannot be used to calculate alternate links on large-scale networks, especially on ring networks. Remote LFA Auto FRR addresses this problem by calculating a PQ node and establishing a tunnel between the source node of a primary link and the PQ node. If the primary link fails, traffic can be automatically switched to the tunnel, which improves network reliability.

P space

P space consists of the nodes through which the shortest path trees (SPTs) with the source node of a primary link as the root are reachable without passing through the primary link.

Extended P space

Extended P space consists of the nodes through which the SPTs with neighbors of a primary link's source node as the root are reachable without passing through the primary link.

Q space

Q space consists of the nodes through which the SPTs with the destination node of a primary link as the root are reachable without passing through the primary link.

PQ node

A PQ node exists both in the extended P space and Q space and is used by Remote LFA as the destination of a protection tunnel.

IS-IS LFA Auto FRR

IS-IS LFA Auto FRR protects against both link and node-and-link failures.
  • Link protection: Link protection applies to traffic transmitted over specified links.

    In the example network shown in Figure 8-21, traffic flows from Device S to Device D, and the link cost meets the preceding link protection inequality. If the primary link (S -> Device D) fails, Device S switches the traffic to the alternate link (S -> Device N -> Device D), minimizing traffic loss.

    Figure 8-21 Networking for IS-IS LFA Auto FRR link protection

  • Node-and-link protection: Node-and-link protection applies to traffic transmitted over specified nodes or links. Figure 8-22 illustrates a network where LFA Auto FRR node-and-link protection is used. Node-and-link protection takes precedence over link protection.

    Node-and-link protection takes effect when the following conditions are met:
    1. The link cost satisfies the inequality: Distance_opt (N, D) < Distance_opt (N, S) + Distance_opt (S, D).

    2. The interface cost of the device satisfies the inequality: Distance_opt (N, D) < Distance_opt (N, E) + Distance_opt (E, D).

    Figure 8-22 Networking for IS-IS LFA Auto FRR node-and-link protection

IS-IS Remote LFA Auto auto FRR

Similar to LFA Auto FRR, Remote LFA protects against both link and node-and-link failures. The following example shows how Remote LFA works to protect against link failures:

In Figure 8-23, traffic flows through PE1 -> P1 -> P2 -> PE2, and the primary link is between P1 and P2. Remote LFA calculates a PQ node (P4) and establishes a Label Distribution Protocol (LDP) tunnel between P1 and P4. If P1 detects a failure on the primary link, P1 encapsulates packets into MPLS packets and forwards MPLS packets to P4. After receiving the packets, P4 removes the MPLS label from them and searches the IP routing table for a next hop to forward the packets to PE2. Remote LFA ensures uninterrupted traffic forwarding.

Figure 8-23 Networking for Remote LFA
On the network shown in Figure 8-23, Remote LFA calculates the PQ node as follows:
  1. Calculates the SPTs with all neighbors of P1 as roots. The nodes through which the SPTs are reachable without passing through the primary link form an extended P space. The extended P space in this example is {PE1, P1, P3, P4}.

  2. Calculates the SPTs with P2 as the root and obtains the Q space {PE2, P4}.

  3. Selects the PQ node (P4) that exists both in the extended P space and Q space.

NOTE:

IPv6 IS-IS remote LFA auto FRR protects IPv6 traffic and uses IPv4 LDP LSPs. The principle of IPv6 IS-IS remote LFA auto FRR is similar to that of IPv4 IS-IS remote LFA auto FRR.

IS-IS FRR in a Multi-Source Routing Scenario

IS-IS LFA FRR uses the SPF algorithm to calculate the shortest path to the destination node, with each neighbor that provides a backup link as the root node. The backup next hop is node-based, which applies to single-source routing scenarios. With the diversification of networks, multi-source routing scenarios appear, where multiple nodes advertise the same route. Such multi-source routing scenarios do not meet single-source LFA conditions. As a result, the backup next hop cannot be calculated. IS-IS FRR for multi-source routing scenarios can address this problem by using a routing source to protect the primary routing source and improve network reliability.

Figure 8-24 Networking for IS-IS FRR in a multi-source routing scenario

In Figure 8-24 (a), the cost of the link between Device A and Device B is 5, whereas the cost of the link between Device A and Device C is 10. Both Device B and Device C advertise the route 10.1.1.0/24. IS-IS FRR is enabled on Device A. However, single-source LFA conditions are not met. As a result, Device A fails to calculate the backup next hop of the route 10.1.1.0/24. IS-IS FRR for multi-source routing scenarios can address this problem.

In Figure 8-24 (b), a virtual node is simulated between Device B and Device C and is connected to Device B and Device C. The cost of the link from Device B or Device C to the virtual node is 0, whereas the cost of the link from the virtual node to Device B or Device C is the maximum value. After the virtual node advertises the route 10.1.1.0/24, Device A uses the LFA algorithm to calculate the backup next hop of the virtual node. Then the route 10.1.1.0/24 inherits the backup next hop from the virtual node. In this example, the primary link to the virtual node is the one from Device A to Device B, and the backup link is the one from Device A to Device C.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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