mediumMultiple Choice
300-410 Practice Question: Review this configuration: route-map RMAP permit…
Review this configuration:
route-map RMAP permit 10 match ipv6 address prefix-list PREFIX set interface null0 ! ipv6 prefix-list PREFIX seq 5 permit 2001:db8:5::/48 !
interface GigabitEthernet0/6
ipv6 verify unicast source reachable-via any allow-default
What is the purpose of the 'allow-default' keyword?
⚠ Common exam trap
A common mix-up: candidates confuse 'allow-default' with allowing default source addresses or disabling uRPF, when in fact it simply includes the default route in the uRPF source reachability check.
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 allows uRPF to use the default route as a valid path for source reachability.
The 'allow-default' keyword in the 'ipv6 verify unicast source reachable-via any' command modifies Unicast Reverse Path Forwarding (uRPF) behavior. By default, uRPF checks the FIB for a matching route to the source address, but it excludes the default route. Adding 'allow-default' permits uRPF to consider the default route (::/0) as a valid path for source reachability, ensuring that traffic with source addresses that only match the default route is not dropped.
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 allows uRPF to use the default route as a valid path for source reachability.
Why this is correct
Unicast RPF normally discards packets whose source has no specific route in the RIB. The allow-default keyword permits the default route to satisfy the reachability check, so sources reachable only via the default route pass verification rather than being dropped.
- ✗
It allows the router to accept packets with source addresses from the default prefix.
Why it's wrong here
The allow-default keyword permits a default route to satisfy the reachability check, not a default prefix source. It is tempting because it references defaults, but it applies to the FIB lookup for the source, allowing the packet when only a default route matches rather than a specific route to the source subnet.
- ✗
It disables uRPF for packets matching the default route.
Why it's wrong here
allow-default does not disable uRPF; it accepts the default route as a legitimate return path so packets lacking a specific route still pass the check. Disabling is tempting when uRPF drops traffic, but the keyword relaxes the reachability test rather than removing verification.
- ✗
It is used to allow multicast traffic through uRPF.
Why it's wrong here
allow-default permits the default route as a valid return path during uRPF checks; it has no multicast role, since multicast uses separate RPF logic. It is tempting because uRPF and multicast RPF both verify sources, but allow-default targets unicast reachability via the default route.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
BGP Prefix Filtering
BGP Prefix Filtering is the practice of controlling which network routes (prefixes) a router accepts or advertises to its BGP neighbors, preventing unwanted or harmful routes from spreading across the internet.
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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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.