The answer is that the remote peer is not sending traffic back, as indicated by the packet counters showing 100 packets encapsulated and encrypted outbound, but zero packets decapsulated or decrypted inbound. This asymmetry in IPsec traffic reveals a one-way tunnel: Router R1 is successfully sending encrypted data to peer 10.1.1.2, but no return traffic is being received, pointing to a routing issue on the remote router or a firewall blocking the return path. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to troubleshoot asymmetric IPsec traffic by interpreting the show crypto ipsec sa output, where mismatched encaps and decaps counts are a classic indicator of a broken return route. A common trap is assuming the problem is with the local router’s encryption configuration, but the zero inbound counters clearly shift focus to the remote side. Memory tip: “Encaps out, decaps zero—check the remote hero.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 runs the following command on Router R1:
R1# show crypto ipsec sa peer 10.1.1.2
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 remote peer is not sending traffic back; check routing on the remote router.
The outbound packet count (encaps) is 100, but inbound (decaps) is 0. This suggests that traffic is being sent through the tunnel but no responses are being received, possibly due to a routing issue on the remote side or a firewall blocking return traffic.
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 is working correctly; all packets are being encrypted and decrypted.
Why it's wrong here
Inbound counters are zero, indicating no decryption is happening.
✓
The remote peer is not sending traffic back; check routing on the remote router.
Why this is correct
Outbound packets are being sent but none received; the remote side may not have a route back or the crypto ACL is misconfigured.
The local crypto ACL is misconfigured; it is not matching traffic.
Why it's wrong here
Outbound packets are being encapsulated, so the ACL is matching.
✗
The IPsec SA is not established; the tunnel is down.
Why it's wrong here
The SA is present and active; packets are being sent.
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.
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 remote peer is not sending traffic back; check routing on the remote router. — The outbound packet count (encaps) is 100, but inbound (decaps) is 0. This suggests that traffic is being sent through the tunnel but no responses are being received, possibly due to a routing issue on the remote side or a firewall blocking return traffic.
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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Question Discussion
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