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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 28 Configuring IP Unicast Routing
Configuring IP Addressing
Forwarding UDP Broadcast Packets and Protocols
User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a
low-overhead, connectionless session between two end systems and does not provide for
acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to determine
address, configuration, and name information. If such a host is on a network segment that does not
include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by
configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can
use more than one helper address per interface.
You can specify a UDP destination port to control which UDP services are forwarded. You can specify
multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older
diskless Sun workstations and the network security protocol SDNS.
By default, both UDP and ND forwarding are enabled if a helper address has been defined for an
interface. The description for the ip forward-protocol interface configuration command in the Cisco
IOS IP and IP Routing Command Reference for Release 12.1 lists the ports that are forwarded by default
if you do not specify any UDP ports.
If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are
configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.
Beginning in privileged EXEC mode, follow these steps to enable forwarding UDP broadcast packets on
an interface and specify the destination address:
Use the no ip helper-address interface configuration command to disable the forwarding of broadcast
packets to specific addresses. Use the no ip forward-protocol global configuration command to remove
a protocol or port.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter interface configuration mode, and specify the Layer 3 interface
to configure.
Step 3
ip helper-address address Enable forwarding and specify the destination address for forwarding
UDP broadcast packets, including BOOTP.
Step 4
exit Return to global configuration mode.
Step 5
ip forward-protocol {udp [port] | nd | sdns} Specify which protocols the router forwards when forwarding
broadcast packets.
Step 6
end Return to privileged EXEC mode.
Step 7
show ip interface [interface-id]
or
show running-config
Verify the configuration on the interface or all interfaces.
Step 8
copy running-config startup-config (Optional) Save your entries in the configuration file.
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