Troubleshooting guide

3. Troubleshooting Functional Failures During Operation
156
3.27 DHCP snooping problems
3.27.1 Problems related to DHCP
If DHCP cannot distribute IP addresses in a DHCP snooping configuration, isolate the cause of the
problem according to the failure analysis method described in the following table.
Table 3-84: Failure analysis method when DHCP cannot distribute IP addresses in a DHCP
snooping configuration
No. Items to check Action
1 Execute the
show logging command, and check
whether any hardware failure is recorded in the
operation log.
If any hardware failure is recorded in the operation log,
replace the device.
For other cases, go to No. 2.
2 Check whether IP addresses cannot be newly
distributed or only IP addresses already assigned
cannot be updated.
If IP addresses cannot be newly distributed, go to No. 3.
If assigned IP addresses cannot be updated, go to No. 9.
3 Execute the
show ip dhcp snooping
statistics command to check the operation
status of DHCP snooping.
If a port is displayed as an untrusted port at which DHCP
snooping is enabled and the port is the one connected to the
target device (to which an IP address cannot be distributed),
go to No. 4.
If the target device is connected to another port, DHCP
snooping is not enabled for the device.
Check the network configuration and the settings of the
DHCP server, and if there is no problem, go to No. 10.
4 Check the connection method between the clients
and server.
If the Switch is connected as a Layer 2 switch between the
clients and server, go to No. 8.
If the DHCP server on the Switch is used, go to No. 5.
If the DHCP relay on the Switch is used, go to No. 5.
If there is a DHCP relay between the Switch and clients, go
to No. 6.
If a device that adds Option 82 data is located between the
Switch and clients, go to No. 7.
If multiple conditions described above are met, see each
item in the order above.
5 Make sure that there is no problem with the
operation of the DHCP server and DHCP relay.
See 3.7.2 IP addresses cannot be assigned by the DHCP
functionality and make sure that the DHCP server and DHC
relay can distribute IP addresses.
If there is no problem, go to No. 8.
6 If packets via DHCP relay are forwarded, make
sure that the
no ip dhcp snooping verify
mac-address configuration command is set.
Packets forwarded via DHCP relay are discarded because
the client hardware address and the source MAC address in
the packets are different.
To forward those packets, set the
no ip dhcp snooping
verify mac-address configuration command.
7 If packets that contain the relay agent information
option are forwarded, make sure that the
ip dhcp
snooping information option
allow-untrusted configuration command is
set.
By default, packets that contain the relay agent information
option (Option 82) are discarded.
To forward those packets, set the
ip dhcp snooping
information option allow-untrusted configuration
command.