Cisco ASA Troubleshoot IKE Policy

Notes for troubleshooting Cisco ASA IKE Policy – there must be a match between site-to-site / lan-to-lan (L2L) endpoint IPSec Peers for the ISAKMP (IKE) session to be established. Traditionally this was known of IPSec Phase One, but as of IKEv2 there may be new terminology to describe establishing an IKE Session between peers? This type of troubleshooting is especially important when attempting to connect to non-Cisco or 3rd-party endpoints like AWS, Azure, etc.

It appears that the Cisco ASA debug and log messages do not provide enough detail to identify specific IKE policies presented by peers when attempting to establish an ISAKMP session. In order to capture these proposed IKE policies, use the ASA capture ability. Substitute your own capture name in place of CAP_NAME.

  • Define an access-list to identify interesting traffic by IP or protocol – usually bi-directional between the Cisco ASA outside IP and other IKE endpoint.
  • Create a capture using the capture command and previously defined access-list on the ASA IKE interface (usually “outside”).
  • Use the “show capture CAP_NAME” command to view a list of packets in the capture buffer. If needed, make adjustments to the access-list and use “clear capture CAP_NAME” command to remove the erroneous packets from the capture buffer.
  • Use “capture CAP_NAME stop” and “no capture CAP_NAME stop” to stop and re-start the capture if needed.
  • Use the “no capture CAP_NAME” command to remove the capture when no longer needed – you may need to stop and/or clear it to fully remove it. Use “show capture” command to verify that the capture is no longer present.
  • Copy a pcap file to the ASA flash using “copy /pcap capture:CAP_NAME flash:/CAP_NAME.pcap” command.
  • Use “scp” or similar to transfer your pcap file to a computer for review with Wireshark. The standard Packet Detail view for ISAKMP (IKE) in Wireshark will have the details of IKE policies proposed by both peers when attempting to establish an ISAKMP/IKE Session.

Once the ASA and peer endpoint agree upon a compatible IKE/ISAKMP policy, the phase one session will be established. This will be visible with the “show crypto ikev2 sa” command (assuming IKEv2). You can filter with “| begin 1.2.3.4” to look for a specific peer IP of 1.2.3.4 (substitute your peer IP).

Troubleshooting Phase Two IPSec ESP Data Sessions is a little easier with the ASA (ESP = Encapsulating Security Payload IPSec Security Associations). The standard warning and error logs are usually sufficient to determine what the problem might be (traffic list doesn’t match, proposal doesn’t match, etc). Use commands like “logging enable” “logging asdm warnings” and “show log asdm” commands to view log messages or manage log settings. There are other ways to interact with logs on the ASA, but the built-in asdm log buffer is one of the easiest to work with.

Good luck with your Cisco ASA IKE troubleshooting sessions!

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About notesbytom

Keeping technology notes on WordPress.com to free up my mind to solve new problems rather than figuring out the same ones repeatedly :-).
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