In Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP), what is a successor route?

Introduction / Context:
EIGRP is a Cisco proprietary advanced distance-vector routing protocol that uses concepts like successor and feasible successor routes to ensure loop-free and efficient path selection. Understanding what a successor route is helps you interpret EIGRP topology and routing tables and troubleshoot how the protocol chooses paths for forwarding user traffic.

Given Data / Assumptions:

• EIGRP maintains a neighbor table, a topology table, and a routing table.
• For each destination network, EIGRP selects one or more routes based on metrics like bandwidth and delay.
• The term successor is specific to EIGRP and relates to the best route to a destination.
• EIGRP may also keep backup routes known as feasible successors.

Concept / Approach:
A successor route in EIGRP is the route with the lowest metric, called the feasible distance, to a particular destination network. This route is considered loop-free and is placed directly into the EIGRP routing table, where it is used to forward packets. A feasible successor, if present, is a backup route that meets the feasibility condition. It is stored in the topology table and can be promoted to successor if the primary path fails. Therefore, the correct definition emphasizes that a successor route is the active, primary forwarding route for a destination.

Step-by-Step Solution:
1. Recall that EIGRP calculates a composite metric for each path to a destination.2. Among all the candidate paths, EIGRP chooses the path with the lowest feasible distance as the primary route.3. This chosen primary route is called the successor route and is installed in the IP routing table.4. EIGRP may also identify other loop-free backup paths that meet the feasibility condition, called feasible successors.5. Thus, a correct statement must describe the successor as the primary forwarding route, possibly backed up by a feasible successor.

Verification / Alternative check:
You can confirm this by examining EIGRP show commands on a Cisco router. The show ip eigrp topology command lists successor and feasible successor routes. The successor appears as the primary entry, and the same network appears in the routing table via show ip route eigrp, confirming that the successor is used for actual data forwarding.

Why Other Options Are Wrong:
Option A describes a feasible successor, not the successor route. Option C refers to an active route, which indicates that EIGRP is currently performing a diffusing computation and is not the stable primary route. Option D confuses the concept with external routes; external routes can still have successors, but being external does not define the term successor.

Common Pitfalls:
Learners often confuse successor and feasible successor, assuming both are used for load sharing by default. Another pitfall is assuming that any route in the topology table is automatically in the routing table, which is incorrect. Only successor routes are installed in the routing table for forwarding traffic, while feasible successors stay as backups until needed.

Final Answer:
The primary route selected by EIGRP to forward traffic to a destination network, possibly backed up by a feasible successor.