Quick Answer
The answer is an IPv6 access-list configured with the flow-label keyword. This is correct because the IPv6 Flow Label field is a 20-bit value in the IPv6 header designed for per-flow packet classification, and Cisco IOS supports matching this field directly within an extended IPv6 ACL using the `flow-label` parameter. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of advanced IPv6 ACL features beyond basic source/destination matching; a common trap is assuming only QoS policies can inspect the flow label, but the ACL match is a distinct filtering capability. Remember that the flow-label match is optional and must be explicitly specified in the access-list entry, often combined with other match criteria like protocol or port. A useful memory tip: think of the flow label as a “traffic fingerprint” — you match it with the `flow-label` keyword in the ACL, not with a class-map or route-map.
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which IPv6 traffic filter can be used to match traffic based on the Flow Label field?
IPv6 access-list with the 'flow-label' option
The 'flow-label' keyword allows matching the 20-bit Flow Label field in the IPv6 header.
IPv6 prefix-list
Why wrong: Prefix-lists match on prefixes, not flow labels.
IPv6 route-map with match ipv6 address
Why wrong: Route-maps can reference access-lists but do not directly match flow labels.
IPv6 uRPF
Why wrong: uRPF checks routing table reachability, not flow labels.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IPv6 access-list with the 'flow-label' option
IPv6 access-lists support matching on the flow label using the 'flow-label' keyword.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
IPv6 access-list with the 'flow-label' option
Why this is correct
The 'flow-label' keyword allows matching the 20-bit Flow Label field in the IPv6 header.
Related concept
Standard ACLs match source addresses.
IPv6 prefix-list
Why it's wrong here
Prefix-lists match on prefixes, not flow labels.
IPv6 route-map with match ipv6 address
Why it's wrong here
Route-maps can reference access-lists but do not directly match flow labels.
IPv6 uRPF
Why it's wrong here
uRPF checks routing table reachability, not flow labels.
Common exam traps
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
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
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
IPv6 Traffic Filtering and uRPF — study guide chapter
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FAQ
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Standard ACLs match source addresses..
The correct answer is: IPv6 access-list with the 'flow-label' option — IPv6 access-lists support matching on the flow label using the 'flow-label' keyword.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
Standard ACLs match source addresses.
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Last reviewed: Jun 18, 2026
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