Question 1,055 of 2,152

Policy-Based Routing (PBR) →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that CEF switching bypasses policy-based routing unless the `ip route-cache policy` command is configured. This occurs because Cisco Express Forwarding (CEF) processes packets in hardware or fast-switching paths, and by default, the `ip policy route-map` command only applies to process-switched traffic. When a specific host like 10.1.2.100 is not being policy-routed while other traffic works, it often indicates that host’s traffic is being CEF-switched—perhaps due to a different traffic pattern or route entry—and thus skips the route-map lookup entirely. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: candidates assume a correctly applied route-map and ACL guarantee PBR operation, but the interaction with CEF is a frequent troubleshooting pitfall. The key is remembering that PBR and CEF do not play well together without explicit configuration. Memory tip: “CEF skips PBR—add `ip route-cache policy` to make them stick.”

300-410 Policy-Based Routing (PBR) Practice Question

This 300-410 practice question tests your understanding of policy-based routing (pbr). The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 network engineer is troubleshooting PBR on a Cisco router where traffic from source 10.1.2.0/24 should be forwarded to next-hop 192.168.1.2. The route map 'PBR-TEST' is configured with 'match ip address 101' and 'set ip next-hop 192.168.1.2'. The engineer applies the route map to interface GigabitEthernet0/0. The engineer notices that PBR works for most traffic, but traffic from a specific host (10.1.2.100) is not being policy-routed. The engineer checks the ACL 101 and confirms it includes 10.1.2.0/24. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Study the full ACL explanation →

A
The router is using CEF switching, and PBR is not applied to CEF-switched traffic without the 'ip route-cache policy' command.
Correct because by default, PBR only affects process-switched packets; CEF-switched packets ignore PBR unless 'ip route-cache policy' is enabled.
B
The host 10.1.2.100 is sending traffic with a different source IP than expected.
Why wrong: Incorrect because the ACL matches the subnet, so any host in that subnet should be matched; the symptom is specific to one host, but the ACL is correct.
C
The 'set ip next-hop' command requires the next-hop to be directly connected, and 192.168.1.2 is not reachable.
Why wrong: Incorrect because if the next-hop were unreachable, PBR would fall back to routing table for all traffic, not just one host.
D
The route map is missing a 'sequence 10' statement; PBR requires explicit sequence numbers.
Why wrong: Incorrect because route maps can have implicit sequence numbers; missing explicit numbers do not cause selective failure.

Full breakdown with real-world context →

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 router is using CEF switching, and PBR is not applied to CEF-switched traffic without the 'ip route-cache policy' command.

If PBR is working for most traffic but not for a specific host, it could be due to the route map being applied to a subinterface while the host traffic arrives on a different subinterface, or the host traffic is being fast-switched and bypassing PBR. However, a common cause is that the host's traffic is being processed by CEF and the 'ip policy route-map' command does not affect CEF-switched packets unless 'ip route-cache policy' is enabled. In modern IOS, PBR by default only applies to process-switched packets unless 'ip route-cache policy' is configured.

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 router is using CEF switching, and PBR is not applied to CEF-switched traffic without the 'ip route-cache policy' command.
Why this is correct
Correct because by default, PBR only affects process-switched packets; CEF-switched packets ignore PBR unless 'ip route-cache policy' is enabled.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The host 10.1.2.100 is sending traffic with a different source IP than expected.
Why it's wrong here
Incorrect because the ACL matches the subnet, so any host in that subnet should be matched; the symptom is specific to one host, but the ACL is correct.
✗
The 'set ip next-hop' command requires the next-hop to be directly connected, and 192.168.1.2 is not reachable.
Why it's wrong here
Incorrect because if the next-hop were unreachable, PBR would fall back to routing table for all traffic, not just one host.
✗
The route map is missing a 'sequence 10' statement; PBR requires explicit sequence numbers.
Why it's wrong here
Incorrect because route maps can have implicit sequence numbers; missing explicit numbers do not cause selective failure.

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?

Policy-Based Routing (PBR) — This question tests Policy-Based Routing (PBR) — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The router is using CEF switching, and PBR is not applied to CEF-switched traffic without the 'ip route-cache policy' command. — If PBR is working for most traffic but not for a specific host, it could be due to the route map being applied to a subinterface while the host traffic arrives on a different subinterface, or the host traffic is being fast-switched and bypassing PBR. However, a common cause is that the host's traffic is being processed by CEF and the 'ip policy route-map' command does not affect CEF-switched packets unless 'ip route-cache policy' is enabled. In modern IOS, PBR by default only applies to process-switched packets unless 'ip route-cache policy' is configured.

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

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Standard ACLs match source addresses.

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