CCNA EIGRP questions and answers help you review how Enhanced Interior Gateway Routing Protocol discovers neighbors, calculates routes, and reacts to network changes. Although the current CCNA places greater emphasis on broader routing concepts, EIGRP terminology remains useful for understanding Cisco routing behavior and troubleshooting.
1. What Is an EIGRP Successor?
A successor is the best loop-free route to a destination. It is selected by the Diffusing Update Algorithm (DUAL), installed in the routing table when appropriate, and used to forward traffic. The successor and other learned paths are recorded in the topology table, not the neighbor table.
2. What Is a Feasible Successor?
A feasible successor is a backup route that satisfies the feasibility condition. Its reported distance must be lower than the current successor’s feasible distance. This rule gives EIGRP confidence that the backup path is loop-free. If the successor fails and a feasible successor exists, the router can use it without performing a new network-wide calculation.
3. What Do Feasible Distance and Reported Distance Mean?
- Feasible distance: the local router’s best calculated metric to reach the destination.
- Reported distance: the metric a neighbor reports from itself to that destination.
Comparing these values is central to the feasibility condition and feasible-successor selection.
4. Which EIGRP Tables Should You Know?
- Neighbor table: directly connected routers that formed an EIGRP adjacency.
- Topology table: routes learned from EIGRP neighbors, including successors and possible backup paths.
- Routing table: the best routes currently installed for packet forwarding.
5. How Does EIGRP Form Neighbor Relationships?
Routers exchange hello packets on enabled interfaces. Important settings must be compatible, including the autonomous system number, address-family configuration, and authentication when used. Interfaces must also have working Layer 3 connectivity. Passive interfaces advertise connected networks but do not send hellos or form neighbors.
6. What Is the EIGRP Composite Metric?
By default, classic EIGRP bases its metric mainly on bandwidth and delay. Reliability and load can be included through K values, but changing K values is uncommon and mismatched values can prevent neighbor relationships. Understand the concept and verify the actual configuration rather than memorizing an isolated number.
7. What Do Passive and Active Routes Mean?
In the topology table, passive is the normal stable state. Active means EIGRP is searching for a route after losing the successor without having a suitable feasible successor. A route that remains active too long can lead to a stuck-in-active condition and neighbor resets.
EIGRP Troubleshooting Workflow
- Check interface status and IP addressing.
- Confirm EIGRP is enabled on the correct interfaces.
- Verify autonomous system and authentication settings.
- Review neighbor relationships and hold timers.
- Inspect the topology and routing tables.
- Check passive-interface configuration and route filtering.
Useful commands include show ip eigrp neighbors, show ip eigrp topology, show ip protocols, and show ip route eigrp.
Frequently Asked Questions
Is a successor stored in the neighbor table?
No. The neighbor table contains adjacent EIGRP routers. Route information is stored in the topology table.
Does every alternate path become a feasible successor?
No. It must satisfy the feasibility condition.
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