When a link or a device fails, distributed routing algorithms compute new routes or repair paths. The time taken for this computation is called routing transition. Until the transition is complete and all devices are converged on a common view of the network, the connectivity between the source and destination pairs of devices is interrupted. Repair paths forward traffic during a routing transition.

When a link or a device fails, initially only the neighboring devices are aware of the failure. All other devices in the network are unaware of the nature and location of this failure until information about this failure is propagated through the routing protocol. The propagation of this information may take several hundred milliseconds. Meanwhile, packets affected by the network failure need to be steered to their destinations. A device adjacent to the failed link employs a set of repair paths for packets that would have used the failed link. These repair paths are used from the time the router detects the failure until the routing transition is complete. By the time the routing transition is complete, all devices in the network revise their forwarding data and the failed link is eliminated from the routing computation. Routing protocols precompute repair paths in anticipation of failures so that the repair paths can be activated the moment a failure is detected. In Enhanced Interior Gateway Routing Protocol (EIGRP) networks, precomputed repair paths or backup routes are known as feasible successors or loop-free alternates (LFAs).

A loop-free alternate (LFA) is a precomputed next-hop route that delivers a packet to its destination without looping back. Traffic is redirected to an LFA after a network failure and the LFA makes the forwarding decision without any knowledge of the failure.

Interior Gateway Protocols (IGPs) compute LFAs in the following two ways:

• Per-link (link-based) computation: In link-based LFAs, all prefixes (networks) that are reachable through the primary (protected) link share the same backup information. This means that the whole set of prefixes sharing the primary link also share the repair or the Fast Reroute (FRR) ability. The per-link approach protects only the next-hop address. It need not necessarily protect the destination node. Therefore, the per-link approach is suboptimal and not the best approach for capacity planning because all traffic from the primary link is redirected to the next hop instead of being spread over multiple paths. Redirecting all traffic to the next hop may lead to congestion on the link to the next hop

• Per-prefix (prefix-based) computation: Prefix-based LFAs allow computing backup information per prefix (network) and protect the destination address. The per-prefix approach is preferred over the per-link approach because of its greater applicability and better bandwidth utilization. Per-prefix computations provide better load sharing and better protection coverage than per-link computations because per-prefix computations evaluate all possible LFAs and use tie-breakers to select the best LFA from among the available LFAs.

Note	The repair or backup information computed for a primary path by using prefix-based LFAs may be different from that computed by using link-based LFAs.

EIGRP always computes prefix-based LFAs. EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the successor and feasible successors. EIGRP uses the successor as the primary path and feasible successors as repair paths or LFAs.

1.    enable


2.    configure terminal


3.    router eigrp virtual-name


4.    address-family ipv4 autonomous-system autonomous-system-number


5.    topology base


6.    fast-reroute per-prefix {all | route-map route-map-name}


7.    end


8.    show ip eigrp topology frr

Device> enable

Device# configure terminal

Device(config)# router eigrp name

Device(config-router)# address-family ipv4 autonomous-system 1

Device(config-router-af)# topology base

Device(config-router-af-topology)# fast-reroute per-prefix all

Device(config-router-af-topology)# end

Device# show ip eigrp topology frr

1.    enable


2.    configure terminal


3.    router eigrp virtual-name


4.    address-family ipv4 autonomous-system autonomous-system-number


5.    topology base


6.    fast-reroute load-sharing disable


7.    end


8.    show ip eigrp topology frr

Device> enable

Device# configure terminal

Device(config)# router eigrp name

Device(config-router)# address-family ipv4 autonomous-system 1

Device(config-router-af)# topology base

Device(config-router-af-topology)# fast-reroute load-sharing disable

Device(config-router-af-topology)# end

Device# show ip eigrp topology frr

1.    enable


2.    configure terminal


3.    router eigrp virtual-name


4.    address-family ipv4 autonomous-system autonomous-system-number


5.    topology base


6.    fast-reroute tie-break {interface-disjoint | linecard-disjoint | lowest-backup-path-metric | srlg-disjoint} priority-number


7.    end


8.    show ip eigrp topology frr

Device> enable

Device# configure terminal

Device(config)# router eigrp name

Device(config-router)# address-family ipv4 autonomous-system 1

Device(config-router-af)# topology base

Device(config-router-af-topology)# fast-reroute tie-break lowest-backup-path-metric 2

Device(config-router-af-topology)# end

Device# show ip eigrp topology frr