Udp Helper Configuration - 3Com 4500 PWR 26-Port Configuration Manual

51

UDP Helper Configuration

When configuring UDP helper, go to these sections for information you are interested in:
Introduction to UDP Helper
Configuring UDP Helper
Displaying and Maintaining UDP Helper
UDP Helper Configuration Examp
Intro duction to UDP Helper
Sometimes, a host needs to forward broadcasts to obtain network configuration information or requ
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, S4500 series Ethernet switches provide the UDP Helper function
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
With UDP Helper enabled, the device decides whether to forward a received UDP broadcast packet
acco rding to the UDP destination port number of the packet.
If the destination port numbe
device modifies the destination IP address in the IP header and then sends the
specified destination server.
Otherwise, the device sends the packet to the upper layer protocol for processing.
By default, to prevent IP broadcast storm, S4500 series Ethernet switches do not forward IP packets
with the TTL field being 0 or 1. In some cases, however, you may need to enable UDP Helper to forward
broadcasts with the TTL field being 1
U DP Helper TTL-keep feature, which allows forwarding of broadcasts with the TTL field being 1 without
decrementing the TTL value by one.
Relay forwarding of BOOTP/DHCP broadcast packets is implemented by the DHCP relay function
u sing UDP ports 67 and 68, so these two ports cannot be configured as UDP Helper relay ports.
By default, with UDP Helper enable
destination port numbers listed in
Table 51-1 List of default UDP ports
DNS (Domain Name System)
NetBIOS-DS (NetBIOS Datagram Service)
NetBIOS-NS (NetBIOS Name Service)
le
r of the packet matches the one pre-configured on the device, the
. To meet this need, S4500 series Ethernet switches provide the
d, the device forwards broadcast packets with the six UDP
Table 51-1.
Protocol
51-1
specified destination server.
UDP port number
53
138
137
est
to relay
packet to the