Fast Reroute Overview

 

Fast reroute provides redundancy for an LSP path. When you enable fast reroute, detours are precomputed and preestablished along the LSP. In case of a network failure on the current LSP path, traffic is quickly routed to one of the detours. Figure 1 illustrates an LSP from Router A to Router F, showing the established detours. Each detour is established by an upstream node to avoid the link toward the immediate downstream node and the immediate downstream node itself. Each detour might traverse through one or more label-switched routers (or switches) that are not shown in the figure.

Fast reroute protects traffic against any single point of failure between the ingress and egress routers (or switches). If there is a failure in a scaled fast reroute scenario, the devices lose reachability to all the peers that were connected through the failed link. This leads to traffic interruption, as the BGP session among the devices goes down. If there are multiple failures along an LSP, fast reroute itself might fail. Also, fast reroute does not protect against failure of the ingress or egress routers.

Figure 1: Detours Established for an LSP Using Fast Reroute
Detours Established for an LSP Using
Fast Reroute

If a node detects that a downstream link has failed (using a link-layer-specific liveness detection mechanism) or that a downstream node has failed (for example, using the RSVP neighbor hello protocol), the node quickly switches the traffic to the detour and, at the same time, signals the ingress router about the link or node failure. Figure 2 illustrates the detour taken when the link between Router B and Router C fails.

Figure 2: Detour After the Link from Router B to Router C Fails
Detour After the Link from Router B to
Router C Fails

If the network topology is not rich enough (there are not enough routers with sufficient links to other routers), some of the detours might not succeed. For example, the detour from Router A to Router C in Figure 1 cannot traverse link A-B and Router B. If such a path is not possible, the detour does not occur.

Note that after the node switches traffic to the detour, it might switch the traffic again to a newly calculated detour soon after. This is because the initial detour route might not be the best route. To make rerouting as fast as possible, the node switches traffic onto the initial detour without first verifying that the detour is valid. Once the switch is made, the node recomputes the detour. If the node determines that the initial detour is still valid, traffic continues to flow over this detour. If the node determines that the initial detour is no longer valid, it again switches the traffic to a newly computed detour.

Note

If you issue show commands after the node has switched traffic to the initial detour, the node might indicate that the traffic is still flowing over the original LSP. This situation is temporary and should correct itself quickly.

The time required for a fast-rerouting detour to take effect depends on two independent time intervals:

  • Amount of time to detect that there is a link or node failure—This interval depends greatly on the link layer in use and the nature of the failure. For example, failure detection on an SONET/SDH link typically is much faster than on a Gigabit Ethernet link, and both are much faster than detection of a router failure.

  • Amount of time required to splice the traffic onto the detour—This operation is performed by the Packet Forwarding Engine, which requires little time to splice traffic onto the detour. The time needed can vary depending on the number of LSPs being switched to detours.

Fast reroute is a short-term patch to reduce packet loss. Because detour computation might not reserve adequate bandwidth, the detours might introduce congestion on the alternate links. The ingress router is the only router that is fully aware of LSP policy constraints and, therefore, is the only router able to come up with adequate long-term alternate paths.

Detours are created by use of RSVP and, like all RSVP sessions, they require extra state and overhead in the network. For this reason, each node establishes at most one detour for each LSP that has fast reroute enabled. Creating more than one detour for each LSP increases the overhead, but serves no practical purpose.

To reduce network overhead further, each detour attempts to merge back into the LSP as soon as possible after the failed node or link. If you can consider an LSP that travels through n router nodes, it is possible to create n – 1 detours. For instance, in Figure 3, the detour tries to merge back into the LSP at Router D instead of at Router E or Router F. Merging back into the LSP makes the detour scalability problem more manageable. If topology limitations prevent the detour from quickly merging back into the LSP, detours merge with other detours automatically.

Figure 3: Detours Merging into Other Detours
Detours Merging into Other Detours