- A
It allows uRPF to use the default route as a valid path for source reachability.
Normally, loose uRPF ignores default routes; 'allow-default' includes them.
- B
It allows the router to accept packets with source addresses from the default prefix.
Why wrong: The default route is ::/0, not a specific prefix; the keyword affects how uRPF treats default routes.
- C
It disables uRPF for packets matching the default route.
Why wrong: It does the opposite: it enables uRPF to consider the default route as valid.
- D
It is used to allow multicast traffic through uRPF.
Why wrong: The 'allow-default' keyword is specific to default route handling, not multicast.
Question 1,802 of 2,152
allow-default Keyword in uRPF Loose Mode — Purpose
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Quick Answer
The answer is that the allow-default keyword permits uRPF loose mode to accept packets whose source address is reachable only via a default route. Without this keyword, loose mode uRPF checks whether the source address of an incoming packet has any matching route in the FIB, but it excludes the default route from that check, meaning packets sourced from networks that only match a default route would be dropped. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how uRPF interacts with default routing, often appearing in a scenario where a router must accept traffic from a downstream network that has no specific route back—only a default. A common trap is assuming loose mode automatically accepts all sources, but the allow-default keyword is required to include the default route as a valid return path. Memory tip: think of “allow-default” as “permit the default” to avoid dropping traffic that relies on a default route for reachability.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
Normally, loose uRPF ignores default routes; 'allow-default' includes them.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
It allows the router to accept packets with source addresses from the default prefix.
Why it's wrong here
The default route is ::/0, not a specific prefix; the keyword affects how uRPF treats default routes.
- ✗
It disables uRPF for packets matching the default route.
Why it's wrong here
It does the opposite: it enables uRPF to consider the default route as valid.
- ✗
It is used to allow multicast traffic through uRPF.
Why it's wrong here
The 'allow-default' keyword is specific to default route handling, not multicast.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Trap categories for this question
Keyword trap
The default route is ::/0, not a specific prefix; the keyword affects how uRPF treats default routes.
Detailed technical explanation
How to think about this question
Under the hood, uRPF performs a FIB lookup on the source IP address of incoming packets. Without 'allow-default', the FIB lookup excludes the default route (::/0), so packets whose source address only matches the default route are considered unreachable and are dropped. With 'allow-default', the default route is included in the lookup, which is critical in scenarios like BGP multihoming where a default route is used for traffic from unknown sources, or when using a route-map with 'set interface null0' to selectively drop traffic based on source prefix matching.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
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IPv6 Traffic Filtering and uRPF — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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