IP FRR
Definition
When a router detects a fault at the physical or data link layer, IP fast reroute (FRR) enables the router to report the fault to the upper-layer routing system, and to immediately use a backup link to forward packets. IP FRR is a method that implements fast route backup.
Purpose
On traditional IP networks, when a fault occurs at the lower layer of the forwarding link, the physical interface on the router becomes Down. After the router detects the fault, it informs the upper-layer routing system to recalculate routes and then update routing information. Usually, it takes the routing system several seconds to re-select an available route.
Second-level convergence is intolerable to services that are sensitive to delay and packet loss because it may lead to service interruption. For example, Voice over Internet Protocol (VoIP) services are only tolerant of millisecond-level interruption.
IP FRR resolves this by ensuring that the forwarding system rapidly detects a link fault and then uses a backup route to restore services as soon as possible.
IP FRR Classification and Implementation
IP FRR, which is designed for routes on IP networks, is classified into IP FRR on public networks and IP FRR on private networks.
IP FRR on public networks protects routers on public networks.
IP FRR on private networks protects Customer Edges (CEs).
IP FRR is implemented as follows:
If the primary link is available, you can configure an IP FRR policy to provide the forwarding information of the backup route to the forwarding engine.
If the forwarding engine detects a link fault, the engine uses the backup link to forward traffic before the routes on the control plane converge.
IP FRR Typical Applications
In the example shown in Figure 1-5, IP FRR is configured to improve network reliability. CE1 is dual-homed to PE1 and PE2 and has two outbound interfaces and two next hops configured. That is, link B functions as the backup of link A. When link A fails, traffic can be rapidly switched to link B.
