This 300-410 practice question tests your understanding of ipv6 first hop security. 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 IPv6 DMVPN phase 2 spoke-to-spoke tunnel failures. Spoke routers are able to communicate with the hub, but direct spoke-to-spoke traffic is not working. Router R1 (spoke) has the following relevant configuration:
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
In DMVPN phase 2, spoke-to-spoke tunnels require that NHRP redirect and shortcut are enabled. Without these, spokes send traffic through the hub. The correct answer identifies that the hub is not configured with 'ipv6 nhrp redirect' and spokes are not configured with 'ipv6 nhrp shortcut', preventing dynamic spoke-to-spoke tunnel establishment.
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 tunnel mode is multipoint, but the spokes need to be configured with 'tunnel mode gre ip' for direct communication.
Why it's wrong here
DMVPN phase 2 uses multipoint GRE tunnels on spokes as well.
✓
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
Why this is correct
Without redirect and shortcut, spokes do not learn each other's NHRP mappings and send traffic through the hub.
The spokes have different NHRP network IDs, preventing registration.
Why it's wrong here
The show output indicates both spokes are registered, so network IDs match.
✗
The IPv6 addresses on the tunnel interfaces are in different subnets.
Why it's wrong here
All tunnel interfaces should be in the same subnet for DMVPN to work.
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 show output indicates both spokes are registered, so network IDs match.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'. — In DMVPN phase 2, spoke-to-spoke tunnels require that NHRP redirect and shortcut are enabled. Without these, spokes send traffic through the hub. The correct answer identifies that the hub is not configured with 'ipv6 nhrp redirect' and spokes are not configured with 'ipv6 nhrp shortcut', preventing dynamic spoke-to-spoke tunnel establishment.
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.
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