EIGRP implements DUAL to select paths and guarantee freedom from routing loops. J. J. Garcia Luna-Aceves developed DUAL, which is mathematically proven to result in a loop-free topology, providing no need for periodic updates or route hold-down mechanisms that make convergence slower.
DUAL selects a best path and a second-best path to reach a destination. The successor route is the route with the lowest path metric to reach a destination. The successor is the first next-hop router for the successor route. The best path selected by DUAL is the successor, and the second-best path (if available) is the feasible successor. The feasible successor is a route that satisfies the feasibility condition and is maintained as a backup route.
The reported distance (RD) is the distance reported by a route to reach a prefix. The feasible distance (FD) is the lowest calculated metric of a path to reach the destination. A feasibility condition exists for a route to be considered a backup route where the RD received for that route must be less than the FD calculated on the local router.
Table 3-7 summarizes EIGRP terminology.
Table 3-7 Default EIGRP Values for Bandwidth and Delay
| EIGRP Term | Definition |
| Successor route | The route with the lowest metric to reach a destination. |
| Successor | The first next-hop router for the successor route. |
| Feasible distance (FD) | The best metric along a path to a destination network, including the metric to the neighbor advertising that path. |
| Reported distance (RD) | The total metric along a path to a destination network, as advertised by an upstream neighbor. |
| Feasibility condition | A condition in which the reported distance received for a route is less than the feasible distance calculated locally, thus making it a backup route. |
| Feasible successor | A route that satisfies the feasibility condition and is maintained as a backup route. |
The topology table in Example 3-6 shows the feasible distance. The example also shows two paths (Ethernet 0 and Ethernet 1) to reach 172.16.4.0/30. Because the paths have different metrics, DUAL chooses only one successor.
Example 3-6 Feasible Distance, as Shown in the EIGRP Topology Table
Router8#
show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(172.16.3.1)
Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,
r – reply Status, s – sia Status
P 172.16.4.0/30, 1 successors, FD is 2195456
via 172.16.1.1 (2195456/2169856), Ethernet0
via 172.16.5.1 (2376193/2348271), Ethernet1
P 172.16.1.0/24, 1 successors, FD is 281600
via Connected, Ethernet0
The route entries in Example 3-6 are marked with a P for the passive state. A destination is in passive state when the router is not performing recomputations for the entry. If the successor goes down and the route entry has feasible successors, the router does not need to perform recomputations and does not go into active state.
DUAL places the route entry for a destination into active state if the successor goes down and there are no feasible successors. EIGRP routers send query packets to neighboring routers to find a feasible successor to the destination. A neighboring router can send a reply packet that indicates it has a feasible successor or a query packet. The query packet indicates that the neighboring router does not have a feasible successor and will participate in the recomputation. A route does not return to passive state until it has received a reply packet from each neighboring router. If the router does not receive all the replies before the “active-time” timer expires, DUAL declares the route as stuck in active (SIA). The default active timer is 3 minutes.
Table 3-8 summarizes EIGRP route states.
Table 3-8 EIGRP Route States
| EIGRP Route State | Definition |
| Active | The current successor no longer satisfies the feasibility condition, and no feasible successors are identified for that destination. The router is in the query process to find a loop-free alternative route. |
| Passive | The router has identified successors to a destination. The router is not performing a recomputation. |
| Stuck in active | The router that issued the query gives up and clears its connection to the router that is not answering, effectively restarting the neighbor session. |