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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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OSPF IP Fast Reroute (FRR) is dynamic IP FRR in which a backup link is pre-computed by an OSPF based on the LSDBs on the entire network. The backup link is stored in the forwarding table to protect traffic in the case of failures. In this manner, the failure recovery time can be reduced to less than 50 ms.

OSPF IP FRR complies with RFC 5286, that is, Basic Specification for IP Fast Reroute Loop-Free Alternates, which protects traffic when links or nodes become faulty.


With the development of networks, Voice over IP (VoIP) and online video services require high-quality real-time transmission. Nevertheless, if an OSPF fault occurs, multiple processes, including fault detection, LSP update, LSP flooding, route calculation, and FIB entry delivery, must be performed to switch traffic to a new link. As a result, the fault recovery time is much greater than 50 ms, the time for users to sense traffic interruption, which cannot meet the requirement for real-time services.

Implementation Principle

OSPF IP FRR pre-computes a backup link by using the Loop-Free Alternate (LFA) algorithm, and then adds the backup link and the primary link to the forwarding table. In the case of failures, OSPF IP FRR can fast switch traffic to the backup link before routes on the control plane converge. This prevents traffic interruption and thus protects traffic and improves reliability of an OSPF network. The Router supports IPv4 OSPF IP FRR.

In the LFA algorithm, considering a neighbor that can provide a backup link as the root node, the neighbor computes the shortest path from itself to the destination of the primary link by using the SPF algorithm. The neighbor then computes a loop-free backup link with the smallest cost by using the inequality defined in RFC 5286.

OSPF IP FRR can filter backup routes that need to be added to the IP routing table. Only the backup routes that are filtered through the filtering policy are added to the IP routing table. In this manner, users can flexibly manage the addition of OSPF backup routes to the IP routing table.

Application Environment

OSPF IP FRR is classified into link protection and link-node dual protection. Distance_opt(X,Y) indicates the shortest path between node X and node Y.

Link protection: indicates that the object to be protected is the traffic passing through an OSPF IP FRR-enabled link. The link cost must satisfy the inequality Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D). S indicates the source node of traffic; N indicates the node on the backup link; D indicates the destination node of traffic.

As shown in Figure 5-15, traffic is transmitted from RouterS to RouterD. The link cost satisfies the link protection inequality. When the primary link fails, RouterS switches the traffic to the backup link RouterS -> RouterN so that the traffic can be further transmitted along downstream paths. This ensures that traffic interruption is less than 50 ms.

Figure 5-15 OSPF IP FRR link protection

Link-node dual protection: Figure 5-16 shows link-node dual protection of OSPF IP FRR. Node protection takes precedence over link protection.

Link-node dual protection must satisfy the following situations:

The link cost must satisfy the inequality Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D).

The interface cost of the router must satisfy the inequality Distance_opt(N, D) < Distance_opt(N, E) + Distance_opt(E, D).

S indicates the source node of traffic; E indicates the faulty node; N indicates the node on the backup link; D indicates the destination node of traffic.

Figure 5-16 OSPF IP FRR link-node dual protection

Updated: 2019-05-17

Document ID: EDOC1000174069

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