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Configuration Guide - IP Service
S1720, S2700, S5700, and S6720 V200R011C10

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DHCP policy VLAN, DNS, mDNS gateway, mDNS relay, UDP Helper, IP performance optimization, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel, and IPv4 over IPv6 tunnel.

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Overview of UDP Helper

Overview 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 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 9-1, 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 9-1  Networking diagram of UDP helper relaying broadcast 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) value 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, the switch relays the UDP packets with six specified destination ports by default. Manual configuration is required if the switch needs to relay the UDP packets with other destination ports. The switch supports a maximum of 16 UDP ports including the 6 port numbers supported by default after UDP helper is enabled.

Table 9-1 lists the default UDP ports.
Table 9-1  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. For details about DHCP relay, see Configuring a DHCP Relay Agent.
Updated: 2020-02-26

Document ID: EDOC1000178170

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