Chapter 38 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-protocolglobal 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 34, “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, only IP packets permitted by the

 

 

access list can be translated

 

 

Note The ip directed-broadcastinterface 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]

Verify the configuration on the interface or all interfaces.

 

or

 

 

show running-config

 

Step 8

 

 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

 

 

 

Use the no ip directed-broadcastinterface configuration command to disable translation of directed broadcast to physical broadcasts. Use the no ip forward-protocolglobal configuration command to remove a protocol or port.

 

 

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

 

 

 

 

 

 

OL-9775-02

 

 

38-15

 

 

 

 

 

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Cisco Systems 3750E manual Ip directed-broadcast access-list-number, Ip forward-protocol udp port nd sdns, 38-15