Which of the following commands should you issue when troubleshooting basic IKE peering to determine
whether PSKs are present and matching on both peers? (Select the best answer.)

A.
ping

B.
traceroute

C.
show crypto isakmp policy

D.
debug crypto isakmp

Explanation:
You should issue the debug crypto isakmp command to determine whether preshared keys (PSKs) are present
and matching on both peers. If there is a PSK mismatch between the peers, you will see the 1d00h:%
CRYPTO4IKMP_BAD_MESSAGE: IKE message from 10.11.12.13 failed its sanity check or is malformed
debug error message. If a PSK is missing on one of the peers, you will see the
1d00h:#CRYPTO4IKMP_NO_PRESHARED_KEY: Preshared key for remote peer at 10.11.12.13 is missing
debug error message. To create a PSK, issue the crypto isakmp key key {address | ipaddress [mask] |
hostname name} [noxauth] command.
When troubleshooting basic Internet Key Exchange (IKE) peering, you should perform the following steps:
1. Verify that the peers can reach each other.
2. Verify that the IKE policies match on both peers.
3. Verify that the peers successfully authenticate each other.
To verify that the peers can reach each other, you can issue the ping command. A successful ping indicates
that connectivity between the peers exists. If the ping is not successful, you can issue the traceroute command
to see where the fault is occurring along the path between the two peers.
To verify that the IKE policies match on both peers, you can issue the show crypto isakmp policy command to
display the IKE phase 1 policy settings that are configured on the router, including the encryption algorithm,
hash algorithm, authentication method, DiffieHellman (DH) key exchange mechanism, and security association
(SA) lifetime. The following displays sample output from the show crypto isakmp policy command:
RouterA#show crypto isakmp policy
Global IKE policy
Protection suite of priority 20
encryption algorithm: AES – Advanced Encryption Standard (128 bit keys) hash algorithm:
Secure Hash Standard authentication method: PreShared Key DiffieHellman group: #14
(2048 bit) lifetime: 3600 seconds, no volume limit
To configure IKE phase 1 policy parameters, issue the crypto isakmp policy prioritycommand to enter ISAKMP
policy configuration mode, where you can issue the following commands:
– authentication – encryption
– group- hash
– lifetime
You can issue the debug crypto isakmp command to determine whether an IKE phase 1 policy mismatch is
occurring. The debug error message 1d00h: ISAKMP (0:1): atts are not acceptable. Next payload is 0 will
appear when there is a phase 1 policy mismatch between the peers.
To verify that the peers successfully authenticate each other, you should issue the debug crypto isakmp
command. If the PSKs are present and matching on both peers, the IKE SA should establish successfully and
communication between the sites should occur.

Cisco: IPsec Troubleshooting: Understanding and Using debug Commands: debug crypto isakmp
Cisco: Configuring Internet Key Exchange Version 2 (IKEv2): Example How a Policy Is Matched