If the client is not configured with
the address of the DHCP server, it will
broadcast a DHCPDISCOVER.
You may want to pay attention to what
the client is doing during an auto
assignment versus an ipconfig/release.
Plenty of data on how the cisco
operates DHCP (client, server, relay)
can be found here:
http://www/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt1/1cfdhcp.htm