mediumMultiple Choice
300-410 Feasible Successor Practice Question
A network engineer runs the following command to troubleshoot an EIGRP issue:
R1# show ip eigrp topology 10.1.1.0/24
IP-EIGRP (AS 100): Topology entry for 10.1.1.0/24 State: Passive, Query origin flag: 1, 1 Successor(s), FD is 131072 Routing Descriptor Blocks:
10.1.2.2 (GigabitEthernet0/0), from 10.1.2.2, Send flag: 0x0
Composite metric: (131072/130816), Route is Internal Vector metric: Minimum bandwidth is 10000 Kbit Total delay is 100 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1
10.1.3.3 (GigabitEthernet0/1), from 10.1.3.3, Send flag: 0x0
Composite metric: (131328/131072), Route is Internal Vector metric: Minimum bandwidth is 10000 Kbit Total delay is 200 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 2
What does this output indicate?
⚠ Common exam trap
Candidates often misread the topology table and assume that any path with a reported distance less than or equal to the FD is a feasible successor. However, the feasibility condition requires a strict inequality (RD < FD). Here, the second path's RD equals the FD, so it is not a feasible successor.
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
✓
Only one path is available; the second path is a backup that is not used.
The output shows two paths for 10.1.1.0/24. The first path has a composite metric of (131072/130816), making the feasible distance (FD) 131072. The second path has a composite metric of (131328/131072), where the reported distance (131072) equals the FD. For a path to be a feasible successor, the reported distance must be strictly less than the FD (RD < FD). Since the second path's reported distance equals the FD, it does not satisfy the feasibility condition and is not a feasible successor. Therefore, only the first path is a successor and is installed in the routing table. The second path remains in the topology table but is not a feasible successor and will not be used for forwarding or as a backup unless a topology change causes it to satisfy the feasibility condition. Thus, option A is the best answer, though the term 'backup' in option A is imprecise because the second path is not a feasible 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.
- ✓
Only one path is available; the second path is a backup that is not used.
Why this is correct
Correct. Only the first path is a successor and is installed. The second path is not a feasible successor (RD == FD), so it is not immediately usable; it is only a backup that requires a query to become active.
- ✗
Both paths are feasible successors, but only the first is installed in the routing table.
Why it's wrong here
Incorrect. The second path's reported distance (131072) equals the feasible distance (131072), so it does not satisfy the feasibility condition (RD < FD). Hence, it is not a feasible successor. Only the first path is a feasible successor.
- ✗
Both paths are installed in the routing table for load balancing.
Why it's wrong here
Incorrect. Only the successor is installed. EIGRP does not load balance over this second path because it is not a feasible successor, and variance is not configured.
- ✗
The route is in active state, indicating a query is in progress.
Why it's wrong here
Incorrect. The route is in Passive state, as indicated by 'State: Passive'. An active state would show 'State: Active'.
Visual reference
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
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