mediumMultiple Choice
300-410 Practice Question: Which statement correctly describes the behavior…
Which statement correctly describes the behavior of IPv6 Unicast Reverse Path Forwarding (uRPF) in strict mode?
⚠ Common exam trap
Cisco often tests the distinction between strict and loose mode, and the trap here is that candidates confuse 'route points back to that interface' with 'route is a connected route,' leading them to incorrectly select Option B.
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
✓
It verifies that the source address is reachable via the same interface and that the route points back to that interface.
IPv6 uRPF strict mode requires that the source address of an incoming packet is reachable via the exact same interface on which the packet arrived, and that the best matching route in the FIB points back to that interface. This prevents spoofed traffic by ensuring the source is topologically correct from the router's perspective, as defined in RFC 3704.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It verifies that the source address is in the routing table, but does not check the incoming interface.
Why it's wrong here
Strict mode checks both the routing table entry and that the entry's outgoing interface matches the packet's arrival interface. Verifying reachability alone describes loose mode, which is chosen when asymmetric routing makes interface matching impractical but source-spoofing protection is still wanted.
- ✗
It checks that the source address is reachable via the same interface and that the route is a connected route.
Why it's wrong here
Strict uRPF requires the source to be reachable via the same interface the packet arrived on, but the matching route need not be connected; any route in the RIB via that interface satisfies the check. Requiring a connected route describes a stricter, non-standard behaviour that no implementation enforces.
- ✓
It verifies that the source address is reachable via the same interface and that the route points back to that interface.
Why this is correct
Strict uRPF checks the source address against the routing table and requires the return path to the source to exit through the same interface the packet arrived on. This reverse-path match on the identical ingress interface is the defining constraint of strict mode, unlike loose mode.
- ✗
It only checks that the source address is not a multicast or link-local address.
Why it's wrong here
Strict uRPF performs a full reverse-path lookup against the routing table and validates the incoming interface; it does not merely filter multicast or link-local sources. That filtering resembles basic source-address validation or ACLs, which would be chosen when only malformed source ranges need discarding, not path verification.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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