What UDP (User Datagram Protocol) services does the ip helper-address command forward besides DHCP (Dynamic Host Configuration Protocol)?
By default, it forwards 8 UDP services including DHCP, DNS (Domain Name System), TFTP (Trivial File Transfer Protocol), TACACS, and NetBIOS.
* ip helper-address is a general broadcast relay — these eight UDP services ride along by default. *
| Port | Service |
|---|---|
| 37 | Time |
| 49 | TACACS |
| 53 | DNS |
| 67 | DHCP/BOOTP server |
| 68 | DHCP/BOOTP client |
| 69 | TFTP |
| 137 | NetBIOS name service |
| 138 | NetBIOS datagram service |
Key insight: The ip helper-address command is not DHCP-specific — it's a general broadcast relay mechanism for common UDP services. This is why a single helper-address command can solve DHCP, DNS, and TFTP broadcast forwarding all at once.
Tip: If you only want to relay specific services, you can use ip forward-protocol udp <port> and no ip forward-protocol udp <port> to selectively enable/disable which UDP broadcasts get forwarded.
Go deeper:
Bootstrap Protocol / BOOTP (Wikipedia) — BOOTP/DHCP ports 67/68 and the broadcast-forwarding model the helper-address generalizes.