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Catalyst 3560 Switch Software Configuration Guide
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Chapter 36 Configuring IP Unicast Routing
Configuring IP Addressing
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 occasionally use UDP broadcasts to find
address, configuration, and name information. If such a host is on a network segment that does not
include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by
configuring an interface on a router to forward certain classes of broadcasts to a helper address. 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 helper address has been defined for an
interface. The description for the ip forward-protocol interface configuration command in the Cisco
IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 lists the ports that
are forwarded by default if you do not specify any UDP ports.
If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are
configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.
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.
Command Purpose