Cisco IOS DHCP Relay Agent

Hi Diana,

A router becomes a DHCP relay when it “relays” DHCP broadcasts to another interface. We do this with the “IP helper-address” command.

This command, however, does more than just forwarding DHCP broadcasts, here’s a list:

  • UDP 69 - TFTP
  • UDP 67 - BOOTP Client
  • UDP 68 - BOOTP Server
  • UDP 37 - Time Protocol
  • UDP 49 - TACACS
  • UDP 53 - DNS
  • UDP 137 - NetBios
  • UDP 138 - NetBios Datagram

So by default, it will relay all broadcasts to these UDP destination ports. If you want you can also enable relaying for other destination ports. For example:

ip forward-protocol udp 3000

This would relay broadcast packets with destination UDP port 3000. You can also disable some of the default ports:

no ip forward-protocol udp 67
no ip forward-protocol udp 68
no ip forward-protocol udp 137
no ip forward-protocol udp 138

Hope this helps!

Rene