Can somebody explain these log messages for me? They seem to be killing my router.
Sorry should have put this in networking section.
If this is some type of DOS attack, could it make the router become completely unresponsive, even to the serial port?
Try this search “what is isakmp” from http://search.netscape.com
You will find it related to: ISAKMP—The Internet Security Association and Key Management Protocol
Its a beginning to finding out what is trying to access your network and which port the offending source is coming from then you can work on blocking it entirely OR staisfying its request 
It’s some kind of IPsec message, if you don’t use IpSec, you can block everything that it uses to communicate. I think the messages appear due to a problem that was fixed a while ago. Upgrade to latest ROS please