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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 29 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. For more information,
see
Chapter 19, “Configuring Traffic Control.”
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 29-12
•
Forwarding UDP Broadcast Packets and Protocols, page 29-13
•
Establishing an IP Broadcast Address, page 29-14
•
Flooding IP Broadcasts, page 29-14
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.
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.
For more information on access lists, see
Chapter 26, “Configuring Network Security with ACLs.”
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts
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 directed-broadcast [access-list-number]
Enable directed broadcast-to-physical broadcast translation on the
interface. You can include an access list to control which broadcasts
are forwarded. When an access list is specified, only IP packets
permitted by the access list are eligible to be translated.
Step 4
exit
Return to global configuration mode.
