Question 522 of 2,152

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

PBR Asymmetric Routing in DMVPN: Ensuring Return Routes on Backup Hub

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?

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.

Full breakdown with real-world context →

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.

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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

Visual reference

Quick reference

Asymmetric Encryption Algorithm Comparison

Algorithm	Key Exchange	Signatures	Equivalent Security Key	Notes
RSA-3072	Yes	Yes	128-bit	Widely deployed; slow for bulk data
ECDSA P-256	No	Yes	128-bit	Fast signatures; standard TLS certs
ECDH / ECDHE	Yes	No	128-bit	Perfect forward secrecy in TLS 1.3
DH / DHE	Yes	No	128-bit (3072-bit key)	Replaced by ECDHE in modern TLS
Ed25519	No	Yes	~128-bit	SSH keys, modern PKI

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 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?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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