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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing
Configuring IP Addressing
Beginning in privileged EXEC mode, follow these steps to enable forw arding 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.
Forwarding UDP Broadcast Packets and Protocols
User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a
low-overhead, connectionless session between two end systems and does not provide for
acknowledgment of received datagrams. Network hosts occasio na lly use U DP b roa dca sts to d ete rmi ne
address, configuration, and name information. If such a host is on a network segment that doe s not
include a server, UDP broadcasts are normally not forwarded. You can remedy this situ ation by
configuring an interface on a router to forward certain cl asses of b roa dc asts t o a h elpe r ad dress . You can
use more than one helper address per interface.
You can specify a UDP destination port to control which UDP services are forwarded. You can specify
multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older
diskless Sun workstations and the network security protocol SDNS.
By default, both UDP and ND forwarding are enabled if a h elper a ddress ha s b een defi ned for an
interface. The description for the ip forward-protocol interface configuration command in the Cisco
IOS IP and IP Routing Command Reference for Release 12.1 lists the ports that are forwarded by default
if you do not specify any UDP ports.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter interface configuration mode, and specify the interface to
configure.
Step3 ip directed-broadcast [access-list-number] Enable directed broadcast-to-physical broadcast translati on on t he
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.
Step4 exit Return to global configuration mode.
Step5 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
Step6 end Return to privileged EXEC mode.
Step7 show ip interface [interface-id]
or
show running-config
Verify the configuration on the interface or all interfaces.
Step8 copy running-config startup-config (Optional) Save your entries in the configuration file.