3Com Switch 4500 Family Manual page 627

Parameters
port-number: Number of the UDP port with which UDP packets are to be forwarded, in the range 0 to
65535 (except for 67 and 68).
dns: Forwards Domain Name System (DNS) data packets. The corresponding UDP port number is 53.
netbios-ds: Forwards NetBIOS data packets. The corresponding UDP port number is 138.
netbios-ns: Forwards NetBIOS name service data packets. The corresponding UDP port number is
137.
tacacs: Forwards terminal access controller access control system (TACACS) data packet. The
corresponding UDP port number is 49.
tftp: Forwards Trivial File Transfer Protocol (TFTP) data packets. The corresponding UDP port number
is 69.
time: Forwards time service data packets. The corresponding UDP port number is 37.
Description
Use the udp-helper port command to configure the UDP port with which broadcast packets are to be
forwarded.
Use the undo udp-helper port command to remove the configuration.
By default, the UDP Helper enabled device forwards broadcast packets with any of the six UDP port
numbers 53, 138, 137, 49, 69 and 37.
Note that:
You need to enable the UDP Helper function before specifying any UDP port; otherwise, the
system prompts error information. When the UDP helper function is disabled, all configured UDP
ports are disabled, including the default ports.
The relaying of BOOTP/DHCP broadcast packets is implemented through the DHCP relay agent
function using UDP port 67 and 68. That is, the UDP port number cannot be set to 67 or 68 for UDP
Helper.
The dns, netbios-ds, netbios-ns, tacacs, tftp, and time keywords correspond to the six default
ports. You can configure the default ports by specifying port numbers or the corresponding
parameters. For example, udp-helper port 53 and udp-helper port dns specify the same port.
When you view the configuration information by using the display current-configuration
command, information about default UDP ports is not displayed. Such information is displayed
only when a default port is disabled.
Currently, you can configure up to 256 UDP ports on the device.
Examples
# Enable forwarding of UDP broadcasts with a destination UDP port number of 100.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] udp-helper port 100
# Disable forwarding of UDP broadcasts with a destination UDP port number of 53.
[Sysname] undo udp-helper port 53
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