You would think that an expired DHCP lease is not a big deal for a DHCP client. Although the interface IP address is lost, you can always try to get a new address from the DHCP server.
IOS has a different opinion: when the DHCP lease expires on a router configured with ip address dhcp interface configuration command, the interface is administratively shut down and re-enabled. Here’s a sample printout taken from a router running 12.4(24)T software:
%DHCP-5-RESTART: Interface FastEthernet0/1 is being restarted by DHCP
%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to administratively down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
%DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/1 assigned DHCP address 10.17.1.15, mask 255.255.255.0, hostname Client
You might wonder how you could ever end up with an expired lease when there’s a working DHCP server on the network. It’s simple if your DHCP server runs on IOS: when you clear the DHCP bindings on a router running a DHCP server, it stops responding to lease extension requests (DHCPREQUEST packets) from unknown clients.