I am trying to set up an ESP tunnel with a Cisco 871 and another device but can't seem to get it working. The peers start negotiating an IKE SA but only the first three or four (depending on which peer is the initiator) messages of ISAKMP are sent. The Key Exchange does not succeed and so no tunnel is set up. If anyone could provide a hint as to what could be the problem then I would greatly appreciate it.
I attach part of the log and the running config from the 871 and a Wireshark capture of the exchanged ISAKMP messages.
Thanks in advance,