Cisco Systems 78-11741-02 Forwarding UDP Broadcast Packets and Protocols, On an interface, IPC-32

Configuring IP Addressing

Configuring Broadcast Packet Handling

To enable forwarding of IP directed broadcasts, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip directed-broadcast	Enables directed broadcast-to-physical broadcast translation
[access-list-number]	on an interface.

Forwarding UDP Broadcast Packets and Protocols

Network hosts occasionally use User Datagram Protocol (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 normally are not forwarded. You can remedy this situation by configuring the interface of your 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 you can specify the network security protocol, Software Defined Network Service (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-protocolglobal configuration command in the Cisco IOS IPCommand Reference, Volume 1 of 3: Addressing and Services publication 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 Dynamic Host Configuration Protocol (DHCP) information, which means that the Cisco IOS software is compatible with DHCP clients. (DHCP is defined in RFC 1531.)

To enable forwarding and to specify the destination address, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip helper-addressaddress	Enables forwarding and specifies the destination address for
	forwarding UDP broadcast packets, such as BOOTP and
	DHCP.

To specify which protocols will be forwarded, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip forward-protocol {udp [port] nd	Specifies which protocols will be forwarded over which ports.
sdns}

See the “Helper Addresses Example” section at the end of this chapter for an example of how to configure helper addresses.

Cisco IOS IP Configuration Guide

IPC-32