System information

Troubleshooting TCP/IP 7-113
TCP/IP: Problems Forwarding BOOTP and Other UDP Broadcasts
Possible Problem Solution
Missing or misconfigured ip
helper-address specification
Step 1 Use the debug ip udp privileged exec command on the router that should
be receiving packets from the host. Check the output of the command to
see whether packets are being received from the host.
Caution: This debug command can use considerable CPU cycles on the
router. Do not enable it if your network is heavily congested. You can
attach a protocol analyzer to see whether UDP broadcasts are being
received from the host if your network is congested.
Step 2 If the router receives packets from the host, there is a problem with the
host or the application. Consult the documentation for the host or
application.
If the router does receive packets from the host, use the show
running-config privileged exec command to check the configuration of
the router interface that first receives the packet from the host.
Step 3 Look for an ip helper-address address interface configuration command
entry for that interface. Make sure that the specified address is correct (it
should be the IP address of a server application such as a BOOTP server).
If there is no command entry, then no helper address is configured.
Step 4 If there is no IP helper address configured, or if the wrong address is
specified, add or change the helper address using the ip helper-address
address interface configuration command.
For example, to configure the IP address 192.168.192.6 as the helper
address on router Ethernet interface 0, enter the following commands:
C4500(config)#interface e0
C4500(config-if)#ip helper-address 192.168.192.6
UDP broadcasts being
forwarded out nondefault
ports
Specifying an IP helper address ensures that broadcasts from only a certain default
set of UDP ports are forwarded. UDP broadcasts forwarded out other ports require
further configuration.
Enter an ip forward-protocol udp port global configuration command on the
router for each applicable port. For example, to forward UDP broadcasts from
port 200, enter the following command:
C4500(config)#ip forward-protocol udp 200
To allow forwarding of all UDP broadcasts, enter the following command:
C4500(config)#ip forward-protocol udp