Cisco Systems 3750E manual Forwarding UDP Broadcast Packets and Protocols, 38-16

Chapter 38 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 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 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-protocolinterface configuration command in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 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:

	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]	Verify the configuration on the interface or all interfaces.
	or
	show running-config
Step 8
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip helper-addressinterface configuration command to disable the forwarding of broadcast packets to specific addresses. Use the no ip forward-protocolglobal configuration command to remove a protocol or port.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
38-16	OL-9775-02