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The Cisco IOS can be set to provide DHCP services for a network

Theory

Cisco routers can be configured as DHCP servers. Three steps:

1. Configure a DHCP pool - This includes the subnet, the default gateway, the lease, DNS servers, ect...
2. Configure IP address exclusions - Be sure to make the routers own address unavailable for allocation
3. Either configure a DHCP database agent OR disable DHCP conflict logging - DHCP address conflicts can either be set not to log (disable DHCP conflict logging) or be logged to a central server (dhcp database agent)

Commands

• ip dhcp pool name (global) - creates a dhcp pool
  • network network [mask|prefix-length] (dhcp subcommand) - defines the IP addresses that can be assigned by DHCP
  • default-router address [address2...address3...] (dhcp subcommand) - assigns default route(s) to the pool
  • lease {days[hours][minutes]|infinite} (dhcp subcommand) - defines the lease length
  • ip dhcp excluded-address [low-address high-address] (dhcp subcommand) - defines the IPs excluded from the dhcp pool
  • host address [mask|prefix-length] (dhcp subcommand) - used with the hardware address command below to predefine a host's IP address
  • hardware-address hardware-address type (dhcp subcommand) - works with the command above, used to define MAC addresses

Default Settings

• DHCP server or DHCP forwarding is not enabled by default.

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• Fantastic Cisco technote on understanding and troubleshooting DHCP.

• In order to forward DHCP requests off of the subnet, use the ip helper-address command.

Online Resources

• Explanation of the DHCP "import all" command from Internetwork Expert.

• Be sure to check out this video on configuring the IOS DHCP server from the guys at blindhog.net.

• slaptijack shows how to store DHCP binding information in NVRAM so it is preserved in the event of a reboot.

• Cisco Notepad takes a look at disabling DHCP and BOOTP services.

• Cisco configuration guide on configuring the IOS DHCP Relay Agent.

• Ciscozine explores DHCP and how to configure it on your router.

• Networkers Online configures a router to act as a DHCP server.

• A nice post on configuring the IOS DHCP server from Evilrouters.net.

• Understand DHCP option 82 from Internetwork Expert.

• Chris from Layer 3 has a multipart article on the inner workings of DHCP:
  • DHCP part 1
  • DHCP part 2
  • DHCP part 3

• How to forward DHCP requests with the IP helper command from Cisco Tips.

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