05-Layer 3 - IP Services Configuration Guide

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08-UDP Helper Configuration
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Introduction to UDP Helper

Sometimes, a host needs to forward broadcasts to obtain network configuration information or request the names of other routers on the network. However, if the server or the router to be requested is located in another broadcast domain, the host cannot obtain such information through broadcast.

To solve this problem, the router provides the UDP Helper function to relay specified UDP packets. In other words, UDP Helper functions as a relay agent that converts UDP broadcast packets into unicast packets and forwards them to a specified destination server.

With UDP Helper enabled, the router decides whether to forward a received UDP broadcast packet according to the UDP destination port number of the packet.

·           If the destination port number of the packet matches the one pre-configured on the router, the router modifies the destination IP address in the IP header, and then sends the packet to the specified destination server.

·           If not, the router sends the packet to the upper layer protocol for processing.

Configuring UDP Helper

To configure UDP Helper:





1.     Enter system view.



2.     Enable UDP Helper.

udp-helper enable

Disabled by default.

1.     Enable the forwarding of packets with the specified UDP destination port number(s).

udp-helper port { port-number | dns | netbios-ds | netbios-ns | tacacs | tftp | time }

No UDP port number is specified by default.

2.     Enter interface view.

interface interface-type interface-number


3.     Specify the destination server to which UDP packets are to be forwarded.

udp-helper server ip-address

No destination server is specified by default.




·       The UDP Helper enabled router cannot forward DHCP broadcast packets. In other words, the UDP port number cannot be set to 67 or 68.

·       The UDP Helper enabled device must not forward DHCP broadcast packets that use destination port 67 or 68. Therefore, the UDP port numbers set with the udp-helper port command must not include 67 or 68.

·       The configuration of all UDP ports is removed if you disable UDP Helper.

·       You can configure up to 256 UDP port numbers to enable the forwarding of packets with these UDP port numbers.

·       You can configure up to 20 destination servers on an interface.


Displaying and maintaining UDP Helper





Displays the information of forwarded UDP packets.

display udp-helper server [ interface interface-type interface-number ] [ | { begin | exclude | include } regular-expression ]

Available in any view

Clear statistics about packets forwarded.

reset udp-helper packet

Available in user view


UDP Helper configuration example

Network requirements

As shown in Figure 1, the IP address of the interface GigabitEthernet 3/1/1 on Router A is Configure UDP helper on Router A to forward broadcast packets with UDP destination port number 55 and destination IP address or to the destination server

Figure 1 Network diagram


Configuration procedure




Make sure that a route from Router A to the network segment is available.


# Enable UDP Helper.

<RouterA> system-view

[RouterA] udp-helper enable

# Enable the forwarding of broadcast packets with the UDP destination port number 55.

[RouterA] udp-helper port 55

# Specify the server with the IP address of as the destination server to which UDP packets are to be forwarded.

[RouterA] interface GigabitEthernet 3/1/1

[RouterA-GigabitEthernet3/1/1] ip address 16

[RouterA-GigabitEthernet3/1/1] udp-helper server