350-401 Architecture Practice Question
A network engineer is troubleshooting a routing loop in an EIGRP network. Which mechanism is designed to prevent routing loops by causing a router to reject routes that are learned from a neighbor that is not the successor?
⚠ Common exam trap
Cisco often tests the distinction between EIGRP's feasibility condition and other distance-vector loop-prevention mechanisms like split horizon or hold-down timers, expecting candidates to confuse these concepts because they all prevent loops but operate at different stages of the routing process.
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
✓
Feasibility condition
The feasibility condition is a loop-prevention mechanism unique to EIGRP. It ensures that a router only accepts a route from a neighbor if that neighbor's reported distance (RD) to the destination is less than the router's own feasible distance (FD). This guarantees that the path through that neighbor is loop-free, effectively rejecting routes learned from any neighbor that is not the successor.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Split horizon
Why it's wrong here
Split horizon prevents routing updates from being sent back out the same interface on which they were received. While EIGRP does honor split horizon on many interfaces, it only blocks immediate re-origination of a learned route and does not guarantee a loop-free alternate path in a more complex topology, so it is not the key mechanism preventing the routing loop in question.
- ✗
Route poisoning
Why it's wrong here
Route poisoning marks a route as unreachable by setting its metric to infinity, which in EIGRP is 4,294,967,295 (compared to RIP's 16). This tells neighboring routers to invalidate the route, but it does not itself verify whether an alternative path is loop-free; DUAL's feasibility condition performs that verification, making route poisoning insufficient on its own and not the correct answer.
- ✗
Hold-down timers
Why it's wrong here
Hold-down timers are a RIP-specific convergence feature that forces routers to ignore new route information for a period after a route is withdrawn. EIGRP does not implement hold-down timers; instead, DUAL relies on a query/reply process and the feasibility condition to recompute paths, so hold-down timers would be irrelevant when troubleshooting an EIGRP routing loop.
- ✓
Feasibility condition
Why this is correct
The feasibility condition is EIGRP's loop-free guarantee: a neighbor advertises a reported distance (RD) that is strictly lower than the current feasible distance (FD) to a destination. This proves the neighbor's path does not pass back through the local router, so the path can safely be used as a feasible successor. If no such neighbor exists, DUAL goes active and queries neighbors, but the feasibility condition remains the core mechanism ensuring that any selected path is genuinely loop-free.
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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