This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:
R1# show crypto ipsec sa
interface: Tunnel0
Crypto map tag: VPN-MAP, local addr 10.1.1.1
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The tunnel is up but no traffic is being encrypted; the crypto ACL may not match the traffic.
The IPsec SA is present but the packet counters are all zero. This indicates that the tunnel is established but no interesting traffic is being sent through it. The likely cause is that the crypto ACL does not match the actual traffic or routing is not directing traffic into the tunnel.
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 IPsec SA is not established; phase 1 is still pending.
Why it's wrong here
The SA is present with a current peer; phase 1 and 2 are complete.
✓
The tunnel is up but no traffic is being encrypted; the crypto ACL may not match the traffic.
Why this is correct
Zero packet counts indicate no traffic has been encapsulated; check the crypto ACL and routing.
The remote peer is unreachable; the SA is in a dead state.
Why it's wrong here
The SA is still present; if the peer were unreachable, the SA would be deleted or show errors.
✗
The tunnel interface is down; the SA cannot be used.
Why it's wrong here
The output does not show interface state; SA presence suggests tunnel is up.
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 SA is still present; if the peer were unreachable, the SA would be deleted or show errors.
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.
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.
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The tunnel is up but no traffic is being encrypted; the crypto ACL may not match the traffic. — The IPsec SA is present but the packet counters are all zero. This indicates that the tunnel is established but no interesting traffic is being sent through it. The likely cause is that the crypto ACL does not match the actual traffic or routing is not directing traffic into the tunnel.
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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