If you have an existing BOOTP environment (for example, a set of old Unix workstations and X-terminals) and want to deploy DHCP on the same LAN segment, you could run into interesting compatibility issues, as the DHCP servers by default responds to BOOTP requests.

However, IOS has an interesting feature when you use a router as a DHCP server: you can tell it to ignore the BOOTP requests with the ip dhcp bootp ignore global configuration command (introduced in 12.2T and 12.3). Even more, the router can respond to DHCP requests and forward BOOTP requests to a non-local BOOTP server configured with the ip helper-address interface configuration command.