In Cisco EIGRP, each destination network can have one or more successor routes. These successors are the best next hops used for forwarding. In which EIGRP data structures are successor routes stored?

Introduction / Context:
Enhanced Interior Gateway Routing Protocol (EIGRP) organizes its information in several logical tables, each with a specific role. Understanding where EIGRP stores successor routes is essential for troubleshooting and for interpreting show commands such as show ip route and show ip eigrp topology. This question focuses on the internal data structures used by EIGRP and checks whether you know how the topology table and the IP routing table relate to each other when selecting best paths called successors.

Given Data / Assumptions:

• EIGRP is configured and operating on the router.
• Successor routes are defined as the best paths to each destination network.
• EIGRP maintains at least a topology table, a neighbor table, and the router also maintains an IP routing table.
• The question asks specifically about where successor routes are stored.

Concept / Approach:
EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the best path, known as the successor, and possibly backup paths called feasible successors. All potential routes, both successors and backup paths, are kept in the EIGRP topology table. Among these, DUAL chooses the best successor route for each destination, and that best route is then installed into the IP routing table for actual packet forwarding. The neighbor table only holds information about neighbors, such as IP address and hold time. It does not store per destination routing entries. Therefore, successors must exist in both the EIGRP topology table and the IP routing table, not just one or the other.

Step-by-Step Solution:
1. Recall that EIGRP maintains three primary data structures: the neighbor table, topology table, and IP routing table. 2. Understand that the topology table contains all learned routes, including successors and feasible successors, with their metrics. 3. Recognize that the router installs only the best route, the successor, into the global IP routing table for forwarding. 4. Note that the neighbor table is limited to neighbor specific information, not destination network routes. 5. Conclude that successor routes reside in the EIGRP topology table and are also copied into the IP routing table as active routes.

Verification / Alternative check:
If you run show ip eigrp topology, you see all successors and possible backup routes for each destination. The same routes that are marked as successors there also appear in show ip route as EIGRP learned routes. This confirms that successors exist simultaneously in both tables. On the other hand, show ip eigrp neighbors does not list any per network routes, only neighbors, which verifies that the neighbor table is not where successor routes are stored.

Why Other Options Are Wrong:
Option a: Only in the IP routing table is incorrect because EIGRP uses the topology table internally to track all candidates and compute successors. Option b: Only in the neighbor table is wrong because the neighbor table does not store routes to destination networks. Option c: In both the IP routing table and the neighbor table is incorrect since the neighbor table still does not contain per destination route entries. Option e: In the neighbor table and the EIGRP topology table only is incorrect because successor routes must be installed into the IP routing table for real forwarding decisions.

Common Pitfalls:
A typical misunderstanding is to think that EIGRP stores all routing data solely in the routing table. In reality, the IP routing table is just the final decision table, not the full list of all possible paths. Another common error is confusing the neighbor table with the topology table and assuming neighbor entries are equivalent to route entries. Keeping the roles clear helps: the neighbor table tracks who you talk to, the topology table tracks what routes you know, and the IP routing table tracks which single best route you actually use. Remembering this separation makes questions about EIGRP data structures much easier to answer on exams and in real-world troubleshooting.

Final Answer:
EIGRP successor routes are kept in both the IP routing table and the EIGRP topology table.