Principles of IP FRR

Overview

Fast Reroute (FRR) functions when the lower layer (physical layer or data link layer) detects a fault. The lower layer reports the fault to the upper layer routing system and immediately forwards packets through a backup link.

If a link fails, FRR helps reduce the impact of the link failure on services transmitted on the link.

Background

On traditional IP networks, when a fault occurs at the lower layer of the forwarding link, the physical interface on the router goes Down. After the router detects the fault, it instructs the upper layer routing system to recalculate routes and then update routing information. The routing system takes several seconds to reselect an available route.

For services that are sensitive to packet loss and delay, a convergence time of several seconds is intolerable because it may lead to service interruptions. For example, the maximum convergence time tolerable for Voice over IP (VoIP) services is within milliseconds. IP FRR enables the forwarding system to detect a fault and then to take measures to restore services as soon as possible.

Classification and Implementation

IP FRR, which is designed for routes on IP networks, consists of public network IP FRR and VPN IP FRR.

• Public network IP FRR: protects routers on the public network.

• VPN IP FRR: protects Customer Edges (CEs).

The static routes that are imported between public and private networks do not support IP FRR.

IP FRR is implemented as follows:

• IP FRR can be enabled or disabled using commands.

• When optimal routes are selected from the routes discovered by routing protocols, a backup link is selected for each preferred primary link based on the protocol priority, and then the forwarding information of primary and backup links is provided for the forwarding engine.

Implementation of IP FRR Between Different Protocols

When IP FRR between different protocols is enabled, and optimal routes are selected from protocol routes, a backup link is selected for each preferred primary link based on the protocol priority, and then the forwarding information of primary and backup links is provided for the forwarding engine.

If the forwarding engine detects that the primary link is unavailable after IP FRR between different protocols is enabled, the system can use the backup link to forward traffic before the routes converge on the control plane.

Comparison Between IP FRR and Load Balancing