Introduction To Udp Helper - 3Com MSR 50 Series Configuration Manual

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Introduction to UDP
Helper
UDP H
When configuring UDP Helper, go to these sections for information you are
interested in:
"Introduction to UDP Helper" on page 647
■
"Configuring UDP Helper" on page 648
■
"Displaying and Maintaining UDP Helper" on page 648
■
"UDP Helper Configuration Example" on page 649
■
n
UDP Helper can be currently configured on VLAN interfaces and Layer 3 Ethernet
interfaces (including subinterfaces) only.
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, the device 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 device decides whether to forward a received UDP
broadcast according to the UDP destination port number of the packet.
If the destination port number of the packet matches the one pre-configured
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on the device, the device modifies the destination IP address in the IP header,
and then sends the packet to the specified destination server.
If not, the device sends the packet to the upper layer protocol for processing.
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By default, with UDP Helper enabled, the device forwards broadcast packets with
the six UDP destination port numbers listed in
Table 35 List of default UDP ports
Protocol
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)
C
ELPER ONFIGURATION
Table 35.
UDP port number
69
53
37
137
138
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