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.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
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.
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 permits packets whose source address is reachable via a default route. Without it, loose uRPF would drop packets with sources matching only a default route.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
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
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Static NAT maps one inside address to one outside address..
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 permits packets whose source address is reachable via a default route. Without it, loose uRPF would drop packets with sources matching only a default route.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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