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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter32 Configuring IP Unicast Routing Configuring IP Addressing
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 st orm problem
is to use a single broadcast address scheme on a network. In most mod ern IP implementations, you can
set the address to be used as the broadcast address. Many implementations, including the one in the
switch, support several addressing schemes for forwarding broadcast messages.
Perform the tasks in these sections to enable these schemes:
•Enabling Directed Broadcast-to-Physical Broadcast Translation, page 32-13
•Forwarding UDP Broadcast Packets and Protocols, page 32-14
•Establishing an IP Broadcast Address, page 32-15
•Flooding IP Broadcasts, page 32-15
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 acc ess lis ts, see C hapter 28, “Configuri ng
Network Security with ACLs.”
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts
on an interface:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter interface configuration mode, and specify the interface to
configure.
Step3 no shutdown Enable the interface if necessary. By default, UNIs are disabled and
NNIs are enabled.
Step4 ip directed-broadcast [access-list-number] Enable directed broadcast-to-physical broadcast translation o n t h e
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.
Step5 exit Return to global configuration mode.
Step6 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