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Monday, January 25, 2010

Day 15: DHCP Operation

A client on a DHCP (Dynamic Host Configuration Protocol) network will follow these steps to obtain an IP address:
  1. the client sends a DHCP Discover message with a destination IP address of and a destination MAC address of FF-FF-FF-FF-FF-FF.
  2. This DHCP Discover message broadcasts over the network, and the DHCP server replies with a DHCP Offer, including initial IP configuration for the client such as IP address, subnet mask and default gateway.
  3. The requesting client sends a DHCP Request to use the IP address suggested in the DHCP offer.
  4. The DHCP server responds with a DHCP Acknowledgment.
A DHCP server can provide addresses to a host on a different network if the routers on those networks are configured to forward DHCP requests with the ip helper-address command.

The configuration can be done by SDM or CLI.
Use these commands in global configuration mode:
ip dhcp pool pool-name
network network-address subnet-mask
domain-name domain-name
dns-server dns-server-address
default-router default-router-address
lease {days [hours] [minutes] | infinite}

Use these commands to exclude a range of addresses or a single address from the DHCP pool that you want to reserve and assign to specific hosts from global configuration mode:
ip dhcp excluded-address start-address end-address
ip dhcp excluded-address single-address

DHCP service can be started with service dhcp and stopped with no service dhcp.

Verification is done with these show commands:
show running-config
show ip dhcp binding
show ip dhcp server statistics
debug ip dhcp server events

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