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