- A
The backup hub does not have a route to the source subnet, causing it to drop traffic. Add a static route or enable routing protocol on the backup hub for the spoke subnet.
PBR forwards traffic to the backup hub, but without a return route, the hub cannot respond. This is a common issue in DMVPN with PBR when hubs are not fully meshed.
- B
The PBR route-map on the spoke is missing a 'set ip next-hop verify-availability' command, causing it to use the backup hub even when it is not fully reachable.
Why wrong: The backup hub is reachable (traffic is being forwarded), but it lacks a return route. The 'verify-availability' command checks next-hop reachability, not routing on the remote side.
- C
The spoke's routing table has a better route to the destination via the backup hub, overriding PBR.
Why wrong: PBR overrides the routing table for matched traffic. The debug output confirms PBR is forwarding traffic to the backup hub, so the routing table is not the issue.
- D
The backup hub has a route to the source subnet but with a higher administrative distance, causing it to be ignored.
Why wrong: If the backup hub had a route, it would use it. The 'show ip route' output shows no route at all, indicating the route is missing entirely.
Quick Answer
The answer is that the backup hub lacks a route to the spoke’s source subnet, causing it to drop traffic due to asymmetric routing. When PBR on the spoke forwards traffic to the backup hub after a primary hub failure, the backup hub receives the packets but cannot find a return path to 192.168.1.0/24 in its routing table, so it silently discards them. This scenario tests your understanding of how PBR can create asymmetric routing in a DMVPN backup hub design—a common trap on the Cisco CCNP ENARSI 300-410 exam. The key is that PBR controls only the forward path; the return path depends on the hub’s routing table. To fix it, you must add a static route or enable a dynamic routing protocol on the backup hub for the spoke subnet. Memory tip: “PBR sends it, but routing returns it”—if the hub can’t route back, the packet burns.
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 DMVPN network uses PBR to route traffic from spoke routers to specific hubs based on source IP. After a hub failure, traffic from spoke 1 (source 192.168.1.0/24) is being sent to a backup hub, but the backup hub drops the traffic. Router R1 (spoke) shows: 'show ip policy' shows PBR applied, 'debug ip policy' shows traffic being forwarded to next-hop 10.1.1.2 (backup hub). Router R2 (backup hub) shows: 'show ip route 192.168.1.0' returns no route. What is the root cause?
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 backup hub does not have a route to the source subnet, causing it to drop traffic. Add a static route or enable routing protocol on the backup hub for the spoke subnet.
PBR on the spoke forwards traffic to the backup hub, but the backup hub does not have a route back to the source subnet (192.168.1.0/24). This causes asymmetric routing, where the backup hub drops the traffic because it cannot find a return route. The solution is to ensure the backup hub has a route to the spoke subnet, either via dynamic routing or a static route.
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 backup hub does not have a route to the source subnet, causing it to drop traffic. Add a static route or enable routing protocol on the backup hub for the spoke subnet.
Why this is correct
PBR forwards traffic to the backup hub, but without a return route, the hub cannot respond. This is a common issue in DMVPN with PBR when hubs are not fully meshed.
Related concept
CIDR notation defines the prefix length.
- ✗
The PBR route-map on the spoke is missing a 'set ip next-hop verify-availability' command, causing it to use the backup hub even when it is not fully reachable.
Why it's wrong here
The backup hub is reachable (traffic is being forwarded), but it lacks a return route. The 'verify-availability' command checks next-hop reachability, not routing on the remote side.
- ✗
The spoke's routing table has a better route to the destination via the backup hub, overriding PBR.
Why it's wrong here
PBR overrides the routing table for matched traffic. The debug output confirms PBR is forwarding traffic to the backup hub, so the routing table is not the issue.
- ✗
The backup hub has a route to the source subnet but with a higher administrative distance, causing it to be ignored.
Why it's wrong here
If the backup hub had a route, it would use it. The 'show ip route' output shows no route at all, indicating the route is missing entirely.
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.
Trap categories for this question
Command / output trap
The backup hub is reachable (traffic is being forwarded), but it lacks a return route. The 'verify-availability' command checks next-hop reachability, not routing on the remote side.
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 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.
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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Policy-Based Routing (PBR) — study guide chapter
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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 backup hub does not have a route to the source subnet, causing it to drop traffic. Add a static route or enable routing protocol on the backup hub for the spoke subnet. — PBR on the spoke forwards traffic to the backup hub, but the backup hub does not have a route back to the source subnet (192.168.1.0/24). This causes asymmetric routing, where the backup hub drops the traffic because it cannot find a return route. The solution is to ensure the backup hub has a route to the spoke subnet, either via dynamic routing or a static route.
What should I do if I get this 300-410 question wrong?
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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Last reviewed: Jun 18, 2026
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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