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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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UDP Helper Overview

UDP Helper Overview

This section describes the background and functions of UDP Helper.


Hosts on a network may need to obtain the network configuration or resolve host names by sending UDP broadcast packets to the server. If the hosts and server are located in different broadcast domains, broadcast packets cannot reach the server and the hosts cannot obtain the required information from the server.

The switch modules 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.

As shown in Figure 6-39, HostA uses a host name to access HostB, and the NetBIOS Name Service (NetBIOS-NS) server resolves the host name of HostB. The NetBIOS-NS server and HostA are in different broadcast domains, so the UDP broadcast packet with destination port UDP 137 sent by HostA cannot reach the NetBIOS-NS server. After UDP helper is enabled on the Switch, the Switch can forward the packet with destination port UDP 137 to the NetBIOS-NS server through unicast so that the NetBIOS-NS server can resolve the host name of HostB.

Figure 6-39 UDP helper relays multicast packets

Packets Forwarded by UDP Helper

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.

UDP Helper Ports

After UDP helper is enabled on the switch modules, the switch modules relays the UDP packets with six specified destination ports by default. Manual configuration is required if the switch modules needs to relay the UDP packets with other destination ports.

Table 6-13 lists the default UDP ports.
Table 6-13 Default UDP ports supported by UDP helper


UDP Port Number

Trivial File Transfer Protocol (TFTP)


Domain Name System (DNS)


Time Service


NetBIOS Name Service (NetBIOS-NS)


NetBIOS Datagram Service (NetBIOS-DS)


Terminal Access Controller Access Control System (TACACS)


UDP helper does not relay DHCP packets. That is, the destination port number cannot be 67 or 68. To relay DHCP packets, enable the DHCP relay function on the switch modules. For details about DHCP relay, see Configuring DHCP Relay Agent.
Updated: 2019-08-09

Document ID: EDOC1000041694

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