Quick Answer
The answer is packet loss on the IPsec tunnel. This is identified by comparing the encaps and decaps counters in the show crypto ipsec sa output: 150 packets were encapsulated and encrypted outbound, but only 145 were decapsulated and decrypted inbound, meaning five packets never reached the remote peer. On the Cisco CCNP ENARSI 300-410 exam, this output tests your ability to interpret IPsec SA statistics for DMVPN tunnels, where mismatched packet counts—especially when the protected identity uses GRE protocol 47—indicate loss rather than a configuration error. A common trap is to focus on the zero send/recv errors and assume the tunnel is healthy, but the encaps/decaps disparity reveals the true issue, often caused by MTU fragmentation or routing black holes. Memory tip: think of encaps as “sent” and decaps as “received”—if they don’t match, packets are disappearing somewhere along the path.
This 300-410 practice question tests your understanding of dmvpn. 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: DMVPN, local addr 10.1.1.1protected vrf: (none) local ident (addr/mask/prot/port): (172.16.0.0/255.255.255.0/47/0) remote ident (addr/mask/prot/port): (172.16.0.0/255.255.255.0/47/0) current_peer 10.1.1.2 port 500 PERMIT, flags={origin_is_acl,} #pkts encaps: 150, #pkts encrypt: 150, #pkts digest: 150 #pkts decaps: 145, #pkts decrypt: 145, #pkts verify: 145 #send errors 0, #recv errors 0
local crypto endpt.: 10.1.1.1, remote crypto endpt.: 10.1.1.2 path mtu 1500, ip mtu 1500, ip mtu idb Tunnel0 current outbound spi: 0x12345678(305419896) PFS (Y/N): N, DH group: none
Based on this output, what is the problem?
There is packet loss on the IPsec tunnel.
Encaps count (150) is higher than decaps count (145), indicating loss.
The IPsec SA is using PFS.
Why wrong: PFS (Y/N): N indicates PFS is not used.
The tunnel is not encrypting traffic.
Why wrong: Encrypt/decrypt counts are non-zero, so encryption is happening.
The remote peer is 10.1.1.1.
Why wrong: Remote peer is 10.1.1.2, not 10.1.1.1.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
There is packet loss on the IPsec tunnel.
The output shows IPsec SA details for a DMVPN tunnel. The protect identity uses GRE protocol (47) and the SA is between two spoke routers (172.16.0.0/24). The packet counts show 150 encapsulated but only 145 decapsulated, indicating packet loss on the tunnel. This could be due to MTU issues or routing problems.
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
For each option: why learners choose it and why it is or isn't the right answer here.
There is packet loss on the IPsec tunnel.
Why this is correct
Encaps count (150) is higher than decaps count (145), indicating loss.
Related concept
CIDR notation defines the prefix length.
The IPsec SA is using PFS.
Why it's wrong here
PFS (Y/N): N indicates PFS is not used.
The tunnel is not encrypting traffic.
Why it's wrong here
Encrypt/decrypt counts are non-zero, so encryption is happening.
The remote peer is 10.1.1.1.
Why it's wrong here
Remote peer is 10.1.1.2, not 10.1.1.1.
Common exam traps
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
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.
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
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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
DMVPN — This question tests DMVPN — CIDR notation defines the prefix length..
The correct answer is: There is packet loss on the IPsec tunnel. — The output shows IPsec SA details for a DMVPN tunnel. The protect identity uses GRE protocol (47) and the SA is between two spoke routers (172.16.0.0/24). The packet counts show 150 encapsulated but only 145 decapsulated, indicating packet loss on the tunnel. This could be due to MTU issues or routing problems.
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.
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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