Question 1,363 of 2,152

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

PBR Subnet-Specific Failure — ACL or Route-Map Order | Cisco CCNP ENARSI 300-410 Explained

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 subnet 172.16.0.0/16 should be forwarded to next-hop 10.10.10.2. The route map 'PBR-172' is configured with 'match ip address 140' and 'set ip next-hop 10.10.10.2'. The engineer applies the route map to interface GigabitEthernet0/0. The engineer notices that PBR works for traffic from 172.16.1.0/24 but not for traffic from 172.16.2.0/24. The engineer checks the ACL 140 and confirms it includes the entire 172.16.0.0/16 subnet. 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.

A
The route map has a sequence that matches 172.16.1.0/24 with a permit, and a later sequence that denies 172.16.2.0/24.
Correct. The route map can have multiple sequences; if a later sequence denies 172.16.2.0/24, traffic from that subnet will not be policy-routed, even though ACL 140 permits it.
B
The ACL 140 has a deny statement for 172.16.2.0/24 before the permit statement.
Why wrong: Incorrect. The engineer explicitly confirmed ACL 140 includes the entire 172.16.0.0/16 subnet, so there cannot be a deny statement for 172.16.2.0/24 in the ACL.
C
The next-hop 10.10.10.2 is not reachable from the 172.16.2.0/24 subnet.
Why wrong: Incorrect. The next-hop reachability is not subnet-specific; if it were unreachable, PBR would fail for all traffic, not just one subnet.
D
The 'ip policy route-map' command is applied to a subinterface that only receives traffic from 172.16.1.0/24.
Why wrong: Incorrect. The route map is applied to interface GigabitEthernet0/0, not a subinterface, so it affects all traffic entering that interface.

Full breakdown with real-world context →

Quick Answer

The most likely cause is that ACL 140 contains a deny statement for 172.16.2.0/24 placed before the permit statement for the broader 172.16.0.0/16 range. This is because Policy-Based Routing (PBR) processes route-map entries sequentially, and when the ACL is evaluated, the first matching entry determines the action—so a preceding deny for the specific subnet will block that traffic from being matched, even though a later permit covers the entire /16. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how PBR interacts with ACL order and route-map sequence numbers, often appearing as a trap where engineers assume a broad permit alone guarantees all subnets are matched. A common memory tip is “first match wins in PBR—check both ACL line order and route-map sequence order when you see subnet-specific failure.”

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 route map has a sequence that matches 172.16.1.0/24 with a permit, and a later sequence that denies 172.16.2.0/24.

The most likely cause is that the route map PBR-172 has multiple sequence statements. The first sequence matches 172.16.1.0/24 with a permit action, and a later sequence denies 172.16.2.0/24. Even though ACL 140 includes the entire 172.16.0.0/16, the route map sequences are processed in order. The more specific match for 172.16.1.0/24 permits PBR, while the subsequent sequence explicitly denies 172.16.2.0/24, causing it to bypass PBR. Option B is incorrect because the engineer confirmed ACL 140 permits the entire subnet; a deny statement would contradict that. Options C and D are less likely given the scenario.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 route map has a sequence that matches 172.16.1.0/24 with a permit, and a later sequence that denies 172.16.2.0/24.
Why this is correct
Correct. The route map can have multiple sequences; if a later sequence denies 172.16.2.0/24, traffic from that subnet will not be policy-routed, even though ACL 140 permits it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The ACL 140 has a deny statement for 172.16.2.0/24 before the permit statement.
Why it's wrong here
Incorrect. The engineer explicitly confirmed ACL 140 includes the entire 172.16.0.0/16 subnet, so there cannot be a deny statement for 172.16.2.0/24 in the ACL.
✗
The next-hop 10.10.10.2 is not reachable from the 172.16.2.0/24 subnet.
Why it's wrong here
Incorrect. The next-hop reachability is not subnet-specific; if it were unreachable, PBR would fail for all traffic, not just one subnet.
✗
The 'ip policy route-map' command is applied to a subinterface that only receives traffic from 172.16.1.0/24.
Why it's wrong here
Incorrect. The route map is applied to interface GigabitEthernet0/0, not a subinterface, so it affects all traffic entering that interface.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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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) — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The route map has a sequence that matches 172.16.1.0/24 with a permit, and a later sequence that denies 172.16.2.0/24. — The most likely cause is that the route map PBR-172 has multiple sequence statements. The first sequence matches 172.16.1.0/24 with a permit action, and a later sequence denies 172.16.2.0/24. Even though ACL 140 includes the entire 172.16.0.0/16, the route map sequences are processed in order. The more specific match for 172.16.1.0/24 permits PBR, while the subsequent sequence explicitly denies 172.16.2.0/24, causing it to bypass PBR. Option B is incorrect because the engineer confirmed ACL 140 permits the entire subnet; a deny statement would contradict that. Options C and D are less likely given the scenario.

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?

CIDR notation defines the prefix length.

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