I have an IPSec tunnel (not working among others doing well) and I can't find out where the problem could be. The configuration is without errors (double-checked and nearly the same as for other tunnels). Here's the part of debug output:
crypto_isakmp_process_block:src:126.96.36.199, dest:188.8.131.52 spt:500 dpt:500 ISAKMP (0): processing NOTIFY payload 36136 protocol 1 spi 0, message ID = 2671659589 ISAMKP (0): received DPD_R_U_THERE from peer 184.108.40.206 ISAMKP (0): DPD_R_U_THERE: received seq_no 11020532 out of range, expected 4793190 return status is IKMP_NO_ERR_NO_TRANS
And it's repeating from time to time in bursts of 4, 5. Any ideas? :-)
PS. My access to the device on the other side (D-Link DFL-200, iirc) is very limited.