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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 that 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 find 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 configure 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.
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.
BEFORE YOU BEGIN
See the description for the ip forward-protocol interface configuration command in the Cisco IOS IP Application
Services Command Reference for the list of ports that are forwarded by default if you do not specify any UDP ports.
DETAILED STEPS
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.
EXAMPLE
The following example defines a helper address and uses the ip forward-protocol command. Using the udp keyword
without specifying any port numbers will allow forwarding of UDP packets on the default ports.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface ethernet 0
Switch(config-if)# ip helper-address 10.24.42.2
Switch(config-if)# exit
Command Purpose
1. configure terminal Enter global configuration mode.
2. interface interface-id Enter interface configuration mode, and specify the Layer 3
interface to configure.
3. no shutdown Enable the interface if necessary. By default, UNIs and ENIs
are disabled and NNIs are enabled.
4. ip helper-address address Enable forwarding and specify the destination address for
forwarding UDP broadcast packets, including BOOTP.
5. exit Return to global configuration mode.
6. ip forward-protocol {udp [port] | nd |
sdns}
Specify which protocols the router forwards when
forwarding broadcast packets.
7. end Return to privileged EXEC mode.
8. show ip interface [interface-id]
or
show running-config
Verify the configuration on the interface or all interfaces.
9. copy running-config startup-config (Optional) Save your entries in the configuration file.
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