Enabling Directed Broadcast-To-Physical Broadcast Translation - Cisco Catalyst 3750 Software Configuration Manual

Chapter 28 Configuring IP Unicast Routing

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
Network Security with ACLs."
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts
on an interface:
Command
Step 1 configure terminal
Step 2
interface interface-id
Step 3
ip directed-broadcast [access-list-number]
Step 4
exit
Step 5
ip forward-protocol {udp [port] | nd | sdns} Specify which protocols and ports the router forwards when
Step 6 end
Step 7
show ip interface [interface-id]
or
show running-config
Step 8 copy running-config startup-config
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.
78-15870-01
Purpose
Enter global configuration mode.
Enter interface configuration mode, and specify the interface to
configure.
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.
Return to global configuration mode.
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
•
Return to privileged EXEC mode.
Verify the configuration on the interface or all interfaces.
(Optional) Save your entries in the configuration file.
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