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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 42 Configuring IP Unicast Routing Configuring IP Addressing
Enabling Directed Broadcast-to-Physical Broadcast Translation
By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts
makes 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. When an access list is
specified, 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 37, “Configuring
Network Security with ACLs.”
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts
on an interface:
Use the no ip directed-broadcast interface configuration command to disable translation of directed
broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to
remove a protocol or port.
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, only IP packets permitted by the
access list can be translated
Note The ip directed-broadcast interface configuration command
can be configured on a VPN routing/forwarding(VRF)
interface and is VRF aware. Directed broadcast traffic is
routed only within the VRF.
Step 4 exit Return to global configuration mode.
Step 5 ip forward-protocol {udp [port] | nd | sdns}Specify which protocols and ports the router forwards when
forwarding broadcast packets.
udp—Forward UPD datagrams.
port: (Optional) Destination port that controls which UDP
services are forwarded.
nd—Forward ND datagrams.
sdns—Forward SDNS datagrams
Step 6 end Return to privileged EXEC mode.
Step 7 show ip interface [interface-id]
or
show running-config
Verify the configuration on the interface or all interfaces.
Step 8 copy running-config startup-config (Optional) Save your entries in the configuration file.