1.4.2 Broadcast packet forwarding method

In the Switch, you can use configuration commands to set whether to forward broadcast packets to a network or subnetwork that is directly connected to the Switch via IP forwarding (referred to as directed broadcast hereafter). To set the operation of the receiving interface, use the ip subnet-broadcast command. To set the operation of the sending interface for each subnet, use the directed-broadcast parameter in the ip address command.

Directed broadcasts are not forwarded by default when the above configuration commands are not executed. If you set forwarding of directed broadcasts, you need to be aware that attacks might be directed against terminals as shown in the following figure.

Figure 1-5: Example of an attack using broadcast packets to a subnetwork

The switch forwards directed broadcast packets when the ip subnet-broadcast command has been set and the directed-broadcast parameter has been specified in the ip address command. The following table describes the relationship between these commands and the parameter. The figure after the table shows an example of executing these commands.

Table 1-6: Command settings and operations
ip subnet-broadcast command	ip address command
ip subnet-broadcast command	directed-broadcast specified	directed-broadcast not specified
When default settings are used and ip subnet-broadcast is set	OK	NG
When no ip subnet-broadcast is set	NG	NG

Legend: OK: Directed broadcast packets are forwarded. NG: Directed broadcast packets are not forwarded.

Figure 1-6: Command setting example

<Structure of this section>

(1) Network broadcast

A network broadcast is a broadcast to a network that is not divided into subnetworks. Suppose a network broadcast IP packet for destination 100.1.255.255 is sent to network 100.1.0.0/16. When the Switch is directly connected to network 100.1.0.0/16, the switch follows the switch settings for forwarding broadcasts in the configuration to determine whether to forward the network broadcast IP packet to the devices connected to the switch. The following figure shows how a network broadcast works.

Figure 1-7: Network broadcast

(2) Subnetwork broadcast

A subnetwork broadcast is a broadcast to a single subnetwork.

For example, consider a case in which network 100.1.0.0/16 is divided into two subnetworks, 100.1.1.0/24 and 100.1.2.0/24, and a subnet broadcast IP packet (broadcast to subnetwork 100.1.1.0/24) is sent to destination 100.1.1.255. If the Switch is directly connected to subnetwork 100.1.1.0/24, the switch follows the switch settings for forwarding broadcasts in the configuration to determine whether to forward the subnetwork broadcast IP packet to the devices connected to the switch. The following figure shows how a subnetwork broadcast works.

Figure 1-8: Subnetwork broadcast

(3) All-subnetwork broadcast

An all-subnetwork broadcast refers to a broadcast to all subnetworks. The Switch uses an all-subnetwork broadcast as a normal path.

If, for example, network 100.1.0.0/16 is divided into two subnetworks, 100.1.1.0/24 and 100.1.2.0/24, and an all-subnetwork broadcast IP packet is sent to destination 100.1.255.255, the packet reaches the Switch directly connected to subnetworks 100.1.1.0/24 and 100.1.2.0/24. However, the packet is not forwarded to subnetworks 100.1.1.0/24 and 100.1.2.0/24, and the switch discards the packet. If there is another corresponding path other than the default path, IP packets are sent via that path. The following figure shows how an all-subnetwork broadcast works.

Figure 1-9: All-subnetwork broadcasts