Cisco IE 4000, Software Configuration Manual

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810
Configuring IP Unicast Routing
Configuring IP Addressing
Configuring Broadcast Packet Handling
After configuring an IP interface address, you can enable routing and configure one or more routing protocols, or you can
configure the way the switch responds to network broadcasts. A broadcast is a data packet destined for all hosts on a
physical network. The switch supports two kinds of broadcasting:

A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast address
includes the network or subnet fields.

A flooded broadcast packet is sent to every network.
Note: You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the storm-control
interface configuration command to set traffic suppression levels.
Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including
intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating
broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a
network. In most modern IP implementations, you can set the address to be used as the broadcast address. The switch
supports several addressing schemes for forwarding broadcast messages.

Enabling Directed Broadcast-to-Physical Broadcast Translation, page 810

Forwarding UDP Broadcast Packets and Protocols, page 812

Establishing an IP Broadcast Address, page 813

Flooding IP Broadcasts, page 813
Enabling Directed Broadcast-to-Physical Broadcast Translation
By default, IP-directed broadcasts are not forwarded; they are dropped to make routers less susceptible to
denial-of-service attacks. You can enable forwarding of IP-directed broadcasts on an interface where the broadcast
becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the
ip forward-protocol global
configuration command are forwarded.
BEFORE YOU BEGIN
You can specify an access list to control which broadcasts are forwarded. Only those IP packets permitted by the access
list are eligible to be translated from directed broadcasts to physical broadcasts.