1-1
1 UDP Helper Configuration
When configuring UDP helper, go to these sections for information you are interested in:
z Introduction to UDP Helper
z Configuring UDP Helper
z Displaying and Maintaining UDP Helper
z UDP Helper Configuration Example
Introduction to UDP Helper
Sometimes, a host needs to forward broadcasts to obtain network configuration information or request
the names of other devices on the network. However, if the server or the device to be requested is
located in another broadcast domain, the host cannot obtain such information through broadcast.
To solve this problem, S4200G series Ethernet switches provide 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 device decides whether to forward a received UDP broadcast packet
according to the UDP destination port number of the packet.
z If the destination port number of the packet matches the one pre-configured on the device, the
device modifies the destination IP address in the IP header and then sends the packet to the
specified destination server.
z Otherwise, the device sends the packet to the upper layer protocol for processing.
Relay forwarding of BOOTP/DHCP broadcast packets is implemented by the DHCP relay function
using UDP ports 67 and 68, so these two ports cannot be configured as UDP Helper relay ports.
By default, with UDP Helper enabled, the device forwards broadcast packets with the six UDP
destination port numbers listed in
Table 1-1.
Table 1-1 List of default UDP ports
Protocol UDP port number
DNS (Domain Name System) 53
NetBIOS-DS (NetBIOS Datagram Service) 138
NetBIOS-NS (NetBIOS Name Service) 137
TACACS (Terminal Access Controller Access Control System) 49
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