Question 723 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

Flexible NetFlow Causing Route Leaking Failure in VRF

This 300-410 practice question tests your understanding of netflow and flexible netflow. 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.

A VRF-aware network uses route leaking between VRF A and VRF B. After configuring Flexible NetFlow to monitor traffic in VRF A, some routes that were previously leaked to VRF B disappear. Router R1 has: ip route vrf A 10.0.0.0 255.0.0.0 Null0. route-map LEAK permit 10 match ip address prefix-list GLOBAL. The prefix-list GLOBAL permits 10.0.0.0/8. The flow monitor is applied to the VRF A interface. What is the root cause?

A
The flow monitor is configured with a match ip address prefix-list that references the same prefix-list as the route-map, but the flow monitor's action is to drop packets matching that prefix, causing the route to be withdrawn.
This is the root cause. The flow monitor's drop action on the matched prefix causes the route to be withdrawn, either by triggering routing protocol reactions or by interfering with the leak process.
B
The route-map LEAK is also applied to the flow monitor as a filter, and the flow monitor's match statement is misconfigured to deny the route, preventing the route from being leaked.
Why wrong: Incorrect. A flow monitor does not use the route-map as a filter, and even if it did, it would not prevent route leaking. Route leaking is controlled by the route-map applied to the VRF, not by the flow monitor.
C
The VRF A interface has a flow monitor applied that uses a flow record with the 'vrf' field, causing the router to ignore the route leaking configuration.
Why wrong: Incorrect. The presence of the 'vrf' field in a flow record does not cause the router to ignore route leaking. The flow record only affects how traffic is monitored, not routing table operations.
D
The route leaking configuration requires a next-hop that is not reachable due to the flow monitor consuming bandwidth.
Why wrong: Incorrect. Bandwidth consumption from the flow monitor is not a realistic root cause for route disappearance. Route leaking does not depend on bandwidth, and even if the interface were congested, routes would not be withdrawn.

Full breakdown with real-world context →

Quick Answer

The answer is that the route-map LEAK is also applied to the flow monitor as a filter, and the flow monitor’s match statement is misconfigured to deny the route, preventing the route from being leaked. This occurs because Flexible NetFlow can use a route-map as a traffic filter for the flow monitor, and if that same route-map is used for VRF route leaking, the router applies the match logic in both contexts. When the flow monitor’s match ip address prefix-list statement includes a deny for the 10.0.0.0/8 prefix, it blocks the route from being considered for the leak, even though the prefix-list permits it in the route-map. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how Flexible NetFlow interacts with VRF-aware features and the subtle conflict that arises when shared route-maps are used for both monitoring and route leaking. A common trap is assuming the flow monitor only collects statistics without affecting routing behavior. Memory tip: “A shared map can trap a leak—if the monitor denies, the leak will sleep.”

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

✓

The flow monitor is configured with a match ip address prefix-list that references the same prefix-list as the route-map, but the flow monitor's action is to drop packets matching that prefix, causing the route to be withdrawn.

The flow monitor is configured with a match ip address prefix-list statement referencing the same prefix-list used in the route-map LEAK. However, the flow monitor's action is set to drop packets matching that prefix. When the flow monitor is applied in the input direction on the VRF A interface, it evaluates packets before they are processed for routing. If the flow monitor matches a packet with the 10.0.0.0/8 destination, it drops the packet. This can cause the router to withdraw the route because the router may interpret the sustained packet drops for that prefix as an indication that the next hop is unreachable, or because the flow monitor's drop action interferes with the route-leaking process. Although Flexible NetFlow typically does not affect routing, a misconfigured flow monitor with a drop action can inadvertently influence route availability, leading to the disappearance of leaked routes.

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

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
The flow monitor is configured with a match ip address prefix-list that references the same prefix-list as the route-map, but the flow monitor's action is to drop packets matching that prefix, causing the route to be withdrawn.
Why this is correct
This is the root cause. The flow monitor's drop action on the matched prefix causes the route to be withdrawn, either by triggering routing protocol reactions or by interfering with the leak process.
Related concept
Standard ACLs match source addresses.
✗
The route-map LEAK is also applied to the flow monitor as a filter, and the flow monitor's match statement is misconfigured to deny the route, preventing the route from being leaked.
Why it's wrong here
Incorrect. A flow monitor does not use the route-map as a filter, and even if it did, it would not prevent route leaking. Route leaking is controlled by the route-map applied to the VRF, not by the flow monitor.
✗
The VRF A interface has a flow monitor applied that uses a flow record with the 'vrf' field, causing the router to ignore the route leaking configuration.
Why it's wrong here
Incorrect. The presence of the 'vrf' field in a flow record does not cause the router to ignore route leaking. The flow record only affects how traffic is monitored, not routing table operations.
✗
The route leaking configuration requires a next-hop that is not reachable due to the flow monitor consuming bandwidth.
Why it's wrong here
Incorrect. Bandwidth consumption from the flow monitor is not a realistic root cause for route disappearance. Route leaking does not depend on bandwidth, and even if the interface were congested, routes would not be withdrawn.

Common exam traps

Common exam trap: ACLs stop at the first match

ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.

Detailed technical explanation

How to think about this question

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.

KKey Concepts to Remember

Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.

TExam Day Tips

Check inbound versus outbound direction.
Read the ACL from top to bottom.
Look for a broader permit or deny above the intended line.

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

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.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The flow monitor is configured with a match ip address prefix-list that references the same prefix-list as the route-map, but the flow monitor's action is to drop packets matching that prefix, causing the route to be withdrawn. — The flow monitor is configured with a match ip address prefix-list statement referencing the same prefix-list used in the route-map LEAK. However, the flow monitor's action is set to drop packets matching that prefix. When the flow monitor is applied in the input direction on the VRF A interface, it evaluates packets before they are processed for routing. If the flow monitor matches a packet with the 10.0.0.0/8 destination, it drops the packet. This can cause the router to withdraw the route because the router may interpret the sustained packet drops for that prefix as an indication that the next hop is unreachable, or because the flow monitor's drop action interferes with the route-leaking process. Although Flexible NetFlow typically does not affect routing, a misconfigured flow monitor with a drop action can inadvertently influence route availability, leading to the disappearance of leaked routes.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Standard ACLs match source addresses.

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