hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An enterprise uses EIGRP for IPv6 with route…
An enterprise uses EIGRP for IPv6 with route summarization. Router R1 has a summary route 2001:db8:1::/48 via Null0 redistributed into EIGRP. Router R2 receives this summary and has a more specific route 2001:db8:1:1::/64 learned via a different interface. R2's IPv6 uRPF is configured in strict mode on the interface facing R1. Traffic from a host behind R2 destined to 2001:db8:1:2::1 is being dropped. R2 shows 'ipv6 cef' indicates the summary route points to R1, but uRPF checks fail. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates overlook how a less specific summary route can satisfy the FIB lookup but still fail strict mode's interface check, leading them to incorrectly blame routing loops or source address issues.
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 summary route 2001:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface.
R2's strict mode uRPF checks that the source address of incoming traffic is reachable via the exact interface on which the packet arrived. When R2 receives a packet from a host behind it destined to 2001:db8:1:2::1, the source address is from the host's subnet (e.g., 2001:db8:1:2::/64). R2's FIB has a less specific summary route 2001:db8:1::/48 pointing to R1 (via the interface facing R1), but the more specific /64 route is learned via a different interface. Strict uRPF requires a matching route in the FIB that points back out the same interface the packet arrived on; since the summary route points to R1's interface, uRPF fails, dropping the traffic.
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 summary route 2001:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface.
Why this is correct
uRPF strict mode requires the source address to be reachable via the same interface. The summary route points to R1, but the source is directly connected, causing a mismatch.
- ✗
R2's uRPF is configured in loose mode, which requires a matching route in the FIB, but the summary route is not installed.
Why it's wrong here
Loose mode only requires a route in the FIB, not necessarily via the same interface. The issue is strict mode.
- ✗
EIGRP redistribution of the summary route creates a routing loop, causing uRPF to fail.
Why it's wrong here
There is no loop; the summary is via Null0 on R1, not a loop.
- ✗
The host behind R2 has an incorrect source address, causing uRPF to drop all traffic.
Why it's wrong here
The source address is correct for the subnet, but uRPF checks routing, not host configuration.
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.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
BGP Route Aggregation
BGP Route Aggregation is a technique that combines multiple specific network routes into a single summary route to reduce the size of routing tables and improve network efficiency.
Key term
uRPF
Unicast Reverse Path Forwarding is a network security feature that verifies the source address of incoming packets to prevent IP spoofing attacks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 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 300-410 exam.