I have a Squid proxy server on my network that is connected to the DHCPd server (commodity router) through a commodity switch. If there is a break in the connection (eg router needs to be rebooted) I always have to reboot the proxy server as well as the device seems to lose its IP address and will not get a new one until it is rebooted. Not (necessarily) a function of the proxy server as a previous machine that fulfilled this function (on the same wired connection) had the same problem. So, is this sort of behaviour common? Would the switch filter packets in this way? Any clues as to what the real problem is?
Usually when an IP address changes you need to restart your Squid.
If your Squid server does not survive a reboot of the dhcp server, you really need to extend your lease time.