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300-410 Practice Question: An engineer configures uRPF strict mode on an…
An engineer configures uRPF strict mode on an interface that is part of an IPv6 BGP multihoming setup with two upstream providers. The router receives BGP routes from both providers and has equal-cost paths to some destinations. Traffic from the router to those destinations is intermittently dropped. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the subtle interaction between uRPF strict mode and ECMP, where candidates mistakenly think uRPF works with any valid route, but they overlook the requirement that the incoming interface must match the FIB's outgoing interface for the source prefix.
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
✓
ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check.
With uRPF strict mode enabled, the router checks that the source address of an incoming packet matches a route in the FIB and that the incoming interface is the same as the interface used to reach that source. In an ECMP scenario with two upstream providers, traffic from the router to a destination may be load-balanced across both links, but return traffic from that destination may arrive on a different interface than the one uRPF expects. This asymmetric routing causes uRPF strict mode to drop the packets, as they fail the single-interface check.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check.
Why this is correct
Correct. uRPF strict mode expects symmetric routing; ECMP breaks that assumption.
- ✗
BGP routes are not installed in the routing table, so uRPF has no entry to check.
Why it's wrong here
Incorrect. BGP routes are installed in the routing table if they are best paths.
- ✗
The router has 'ipv6 cef' disabled, causing uRPF to fail.
Why it's wrong here
Incorrect. uRPF requires CEF, but the symptom is intermittent drops, not complete failure.
- ✗
The upstream providers are using different AS paths, causing BGP to not install equal-cost paths.
Why it's wrong here
Incorrect. Equal-cost paths can exist even with different AS paths if the IGP metric is equal.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
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.