Cisco Systems 3750E manual Establishing an IP Broadcast Address, Flooding IP Broadcasts, 38-17

Chapter 38 Configuring IP Unicast Routing

Configuring IP Addressing

Establishing an IP Broadcast Address

The most popular IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.

Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	interface interface-id	Enter interface configuration mode, and specify the interface to
		configure.
Step 3
	ip broadcast-address ip-address	Enter a broadcast address different from the default, for example
		128.1.255.255.
Step 4
	end	Return to privileged EXEC mode.
Step 5
	show ip interface [interface-id]	Verify the broadcast address on the interface or all interfaces.
Step 6
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To restore the default IP broadcast address, use the no ip broadcast-addressinterface configuration command.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)

•The packet must be a MAC-level broadcast.

•The packet must be an IP-level broadcast.

•The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.

•The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address specified with the ip broadcast-addressinterface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		38-17