The answer is that no interesting traffic is being sent through the tunnel, so you must check the crypto ACL and routing. This is correct because the `show crypto ipsec sa` output displays zero packet counters for both encapsulation and decapsulation, meaning the IPsec security associations are established but have never processed a single packet. In IPsec operation, the crypto ACL defines which traffic is “interesting” and triggers encryption; if the counters remain at zero, the router is not matching any traffic to that ACL, or the traffic is not reaching the tunnel interface due to a missing or incorrect route. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between a control-plane issue (like mismatched ISAKMP or IPsec parameters) and a data-plane issue, where the tunnel is up but idle. A common trap is assuming the problem is with Phase 1 or Phase 2 negotiation when the SAs are actually active—the zero counters specifically point to a traffic-matching failure. Memory tip: “Zero packets? Check the ACL and the path—if the tunnel is up but empty, the traffic isn’t interesting.”
300-410 IPsec Site-to-Site VPN Practice Question
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:
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No interesting traffic is being sent through the tunnel; check the crypto ACL and routing.
All packet counters are zero, indicating no traffic has been encrypted or decrypted. This could be due to a misconfigured crypto ACL that does not match the actual traffic, or routing issues preventing traffic from reaching the tunnel.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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; the tunnel is down.
Why it's wrong here
The SA may be established but no traffic is flowing; the output does not show SA state.
✓
No interesting traffic is being sent through the tunnel; check the crypto ACL and routing.
Why this is correct
Zero packet counts indicate no traffic matches the crypto ACL or routing is not directing traffic to the tunnel.
Related concept
Standard ACLs match source addresses.
✗
The tunnel is working correctly; all packets are being encrypted.
Why it's wrong here
Zero packets means nothing is being encrypted.
✗
The remote peer is not responding; the SA is in MM_NO_STATE.
Why it's wrong here
The SA might be up; zero packets do not indicate peer unresponsiveness.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Trap categories for this question
Command / output trap
The SA may be established but no traffic is flowing; the output does not show SA state.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: No interesting traffic is being sent through the tunnel; check the crypto ACL and routing. — All packet counters are zero, indicating no traffic has been encrypted or decrypted. This could be due to a misconfigured crypto ACL that does not match the actual traffic, or routing issues preventing traffic from reaching the tunnel.
What should I do if I get this 300-410 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command to troubleshoot an IPsec Site-to-Site VPN issue:
R1# show crypto engine connections active
Crypto Engine Connections
ID Type Algorithm Encrypt Decrypt LastSeqNo
1 IPsec ESP-3DES+SHA 0 0 0
2 IPsec ESP-3DES+SHA 0 0 0
3 IPsec ESP-AES+SHA 0 0 0
What does this output indicate?
easy
A.The IPsec VPN is actively encrypting and decrypting traffic.
✓ B.The IPsec SAs are established but no traffic is flowing, possibly due to routing or ACL issues.
C.The crypto engine is overloaded and cannot process packets.
D.The IPsec SAs are using the wrong transform set.
Why B: The output shows three IPsec Security Associations (SAs) with zero encrypted and zero decrypted packets, and a LastSeqNo of 0. This indicates the SAs are established (the tunnels are up) but no traffic is being processed through them. The most common causes are routing issues preventing traffic from reaching the tunnel interface, or ACLs that do not match the interesting traffic for encryption.
Variation 2. A network engineer runs the following command to troubleshoot an IPsec Site-to-Site VPN issue:
R1# show crypto ipsec sa detail
interface: Tunnel0
Crypto map tag: CMAP, local addr 192.168.1.1
protected vrf: (none)
local ident (addr/mask/prot/port): (192.168.1.0/255.255.255.0/0/0)
remote ident (addr/mask/prot/port): (192.168.2.0/255.255.255.0/0/0)
current_peer 192.168.2.2 port 500
PERMIT, flags={origin_is_acl,}
#pkts encaps: 0, #pkts encrypt: 0, #pkts digest: 0
#pkts decaps: 0, #pkts decrypt: 0, #pkts verify: 0
#pkts compressed: 0, #pkts decompressed: 0
#pkts not compressed: 0, #pkts compr. failed: 0
#pkts not decompressed: 0, #pkts decompress failed: 0
#send errors 0, #recv errors 0
local crypto endpt.: 192.168.1.1, remote crypto endpt.: 192.168.2.2
path mtu 1500, ip mtu 1500, ip mtu idb Serial0/0/0
current outbound spi: 0x0(0)
PFS (Y/N): N, DH group: none
inbound esp sas:
spi: 0x0(0)
transform: esp-3des esp-sha-hmac ,
in use settings ={Tunnel, }
conn id: 0, flow_id: 0, sibling_flags 80000000, crypto map: CMAP
sa timing: remaining key lifetime (k/sec): (0/0)
IV size: 8 bytes
replay detection support: N
outbound esp sas:
spi: 0x0(0)
transform: esp-3des esp-sha-hmac ,
in use settings ={Tunnel, }
conn id: 0, flow_id: 0, sibling_flags 80000000, crypto map: CMAP
sa timing: remaining key lifetime (k/sec): (0/0)
IV size: 8 bytes
replay detection support: N
What does this output indicate?
medium
A.The IPsec SA is fully established and encrypting traffic.
✓ B.The IPsec SA is in a pending state; the SPI is 0, meaning the SA negotiation is incomplete or the SA has been deleted.
C.The IPsec SA is using PFS, which is causing the SA to be rekeyed frequently.
D.The crypto map is not applied to the interface, so the SA is not used.
Why B: The output shows SPI values of 0x0 for both inbound and outbound ESP SAs, with zero packet encapsulation and encryption counts. This indicates that the IPsec Security Association (SA) negotiation is incomplete or the SA has been deleted, as a valid SA would have a non-zero SPI and active packet counters. The 'current_peer' and crypto map are present, but the SA is not operational.
Last reviewed: Jun 18, 2026
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