udp-helper enable
Function
The udp-helper enable command enables the UDP Helper function.
The undo udp-helper enable command disables the UDP Helper function.
By default, the UDP Helper function is disabled.
Usage Guidelines
Usage Scenario
Hosts on an enterprise intranet may need to obtain the network configuration or resolve host names by sending UDP broadcast packets to the server. If the host and the server are located in different broadcast domains, broadcast packets cannot reach the server and the host cannot obtain the configuration from the server. The switch provides the UDP Helper function to solve this problem. UDP Helper can relay the UDP broadcast packets with specified destination ports. It converts the broadcast packets into unicast packets and sends the unicast packets to the specified destination servers.
The packets that can be forwarded by UDP Helper must meet the following requirements:
- The destination MAC address is the broadcast MAC address (ffff-ffff-ffff).
- The Time-to-Live (TTL) is larger than 1.
- The protocol type is UDP.
- The destination port is a specified UDP port.
Precautions
UDP helper enables a device to relay DHCP packets destined for UDP port 67 (that is, DHCP packets sent to the DHCP server).
However, a UDP helper-enabled device cannot relay DHCP packets destined for UDP port 68 (that is, DHCP packets sent to the DHCP client). To relay this type of DHCP packets, you must enable the DHCP relay function.
After the UDP Helper function is enabled, the switch relays broadcast packets destined for the following UDP ports by default.
Protocol |
UDP Port Number |
|---|---|
Trivial File Transfer Protocol (TFTP) |
69 |
Domain Name System (DNS) |
53 |
Time Service |
37 |
NetBIOS Name Service (NetBIOS-NS) |
137 |
NetBIOS Datagram Service (NetBIOS-DS) |
138 |
Terminal Access Controller Access Control System (TACACS) |
49 |
After UDP Helper is disabled, all specified UDP ports and default ports are canceled.
After the UDP Helper function is enabled, the switch automatically relays the UDP broadcast packets on six default ports. If you want to relay the packets on only one default port, run the undo udp-helper port { dns | netbios-ds | netbios-ns | tacacs | tftp | time } command to disable packet relay on other five ports.