If you're interested in DHCP Option 82 in your Cisco Catalyst-based network, you'll be happy to know that the configuration can be summed up in 4 easy steps.
- Enable IP DHCP Snooping. This is an easy global configuration command:
ip dhcp snooping. Although we're only turning this on to get our Catalyst switch to start inserting Option 82 information in the DHCP requests, DHCP Snooping is great for monitoring which IP addresses have been assigned to devices attached to the switch.
- Enable IP DHCP Snooping on your VLANs. More than likely, you want this
information for all VLANs in your network. If so, the command you need is
ip dhcp snooping vlan 1 4094. If you only want to enable this on VLAN 7 (for example),
ip dhcp snooping vlan 7will do the trick.
- Configure "trusted" DHCP ports. In relation to DHCP Snooping, "trusted"
ports are ports that DHCP assignments come in on or that will have DHCP
requests with Option 82 information already inserted. In the case of Cisco
Catalyst switches, ports that represent inter-switch links or connect directly
to the DHCP server should be set to "trusted". This is done with the interface
level configuration command
ip dhcp snooping trust. NOTE: If there are no trusted ports on your network, DHCP will stop functioning.
Putting it all together
Here's a sample configuration for those that like to see it being done.
SLAP#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SLAP(config)#ip dhcp snooping SLAP(config)#ip dhcp snooping vlan 1 4094 SLAP(config)#interface GigabitEthernet 0/1 SLAP(config-if)#description Inter-switch Link SLAP(config-if)#ip dhcp snooping trust SLAP(config-if)#interface GigabitEthernet 0/2 SLAP(config-if)#description DCHP Server SLAP(config-if)#ip dhcp snooping trust SLAP(config-if)#^Z SLAP#