mediumMultiple Choice
EIGRP Route Selection: Understanding Successor and Metric Comparison
A network engineer is troubleshooting an EIGRP issue in a large enterprise network. Two routers, R1 and R2, are connected via a T1 link. R1 is learning a route to 10.0.0.0/8 from R2 with a metric of 28160, but the same route is also learned from another neighbor with a metric of 26880. The engineer notices that the route from R2 is not being installed in the routing table. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that all learned EIGRP routes are installed in the routing table, but in reality only the successor (lowest metric) is installed unless unequal-cost load balancing is explicitly configured.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route with metric 28160 is not installed because EIGRP selects the route with the lowest metric.
C is correct because EIGRP installs only the route with the best (lowest) metric into the routing table. The route from R2 has a metric of 28160, while the other neighbor advertises the same route with a metric of 26880. Since 26880 is lower, R1 selects that route as the successor and does not install the higher-metric route from R2.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The route from R2 is a feasible successor, so it is not installed in the routing table.
Why it's wrong here
A feasible successor is a backup route stored in the EIGRP topology table, not the routing table, and it must satisfy the feasibility condition where its reported distance is lower than the successor's feasible distance. R2's route is not a feasible successor; it is merely a higher-metric path to the same destination. The route is not installed because EIGRP always prefers the lowest metric route, not due to any feasible successor relationship.
- ✗
EIGRP is using unequal-cost load balancing, so the higher metric route is not used.
Why it's wrong here
Unequal-cost load balancing in EIGRP requires explicit configuration of the 'variance' command, which allows multiple routes with different metrics to be installed when their metric falls within the variance multiplier relative to the best route. Without such configuration, EIGRP defaults to variance 1, meaning only the single best-metric route is used. The scenario does not mention any variance setting, so the higher metric route is excluded due to its higher cost, not because of load-balancing logic.
- ✓
The route with metric 28160 is not installed because EIGRP selects the route with the lowest metric.
Why this is correct
EIGRP computes a composite metric from bandwidth, delay, load, and reliability, then selects the path with the lowest metric as the successor to install in the routing table. Any other paths, even if they are feasible successors or simply higher-metric routes, remain in the topology table. Since 26880 is lower than 28160, the route from R2 is not installed, directly illustrating EIGRP's best-path selection mechanism.
- ✗
The route from R2 is a summary route, so it is not installed in the routing table.
Why it's wrong here
A summary route is an aggregate network address, typically created via manual summarization or auto-summary, and it is installed in the routing table when it represents the most specific or best matching path. The question presents two distinct routes to the same destination prefix with different metrics, not a summary route competing with a more specific route. Therefore, R2's route is not installed because its metric is higher, not because it is a summary route.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.