The EIGRP Loop-Free Alternate Fast Reroute feature allows the Enhanced Interior Gateway Routing Protocol (EIGRP) to reduce the routing transition time to less than 50 ms by precomputing repair paths or backup routes and installing these paths or routes in the Routing Information Base (RIB). Fast Reroute (FRR) is the mechanism that enables traffic that traverses a failed link to be rerouted around the failure.
In EIGRP networks, precomputed backup routes or repair paths are known as
loop-free alternates (LFAs). This module describes how to configure the EIGRP Loop-Free Alternate Fast Reroute feature and enable load-sharing and tie-breaking configurations for the feasible successors or LFAs that are identified by EIGRP.Protected Path
The "protected path" refers to the primary path that is typically used for forwarding network traffic. In a routing scenario, there is a primary path that is the preferred route for sending data. The "protected path" is the route that is being safeguarded, and the goal is to determine loop-free alternate (LFA) paths in case there is a failure or disruption in the primary path. These LFAs are the alternative paths that can be used to maintain connectivity in the event of a failure on the primary path.
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.
LFA Tie-Breaking Rules
When there are multiple candidate LFAs for a given primary path, EIGRP uses a tie-breaking rule to select one LFA per primary path per prefix. The LFA tie-breaking rules define the criteria for selecting a specific Loop-Free Alternate (LFA) path among multiple candidates. The purpose of these rules is to determine the most suitable LFA when there are several options available. The four rules mentioned (Interface-disjoint, Linecard-disjoint, Lowest-repair-path-metric, and Shared Risk Link Group (SRLG)-disjoint) are used to prioritize and eliminate certain LFAs based on specific conditions.
- Interface-disjoint: Eliminates LFAs that share the outgoing interface with the protected path.
- Linecard-disjoint: Eliminates LFAs that share the line card with the protected path.
- Lowest-repair-path-metric: Eliminates LFAs whose metric to the protected prefix is high. Multiple LFAs with the same lowest path metric may remain in the routing table after this tie-breaker is applied.
- Shared Risk Link Group (SRLG)-disjoint: Eliminates LFAs that belong to any of the protected path SRLGs. SRLGs refer to situations where links in a network share a common fiber (or a common physical attribute). If one link fails, other links in the group may also fail. Therefore, links in a group share risks.