Troubleshooting guide

3. Troubleshooting Functional Failures During Operation
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4. Check the settings of the device used as the DHCP relay agent. If the Switch is used as the
relay agent, see (1) Communication problems on DHCP and BOOTP relays.
5. If DHCP snooping is used, packets might have been discarded by DHCP snooping. Check
whether the setting conditions for DHCP snooping in the configuration are correct. For details
about the procedure, see 3.27 DHCP snooping problems.
(b) Checking the log messages and interface
One probable cause of disabled assignment of IP addresses to clients is that communication
between the client and the server has been disabled. Check the log messages displayed by the
Switch or use the
show ip interface command to check whether the interface status is Up or Down.
For details about the procedure, see 3.7.1 Communication is not possible or is disconnected.
(c) Identifying the range for a failure (from the Switch)
If a failure has not occurred on the Switch, a failure might have occurred somewhere on the route
between the Switch and the remote device. To identify the range for a failure in order to determine
the fault location on the route, do the following:
1. Log in to the Switch.
2. If there are devices such as a router between the client and the server, use the
ping command
to check the communication between the router and the remote device (DHCP client). If the
communication with the remote device cannot be verified by using the
ping command,
execute the
ping command again to check communication with each of the devices up to the
client, beginning with the device closest to the Switch. For details about examples of using
the
ping command and how to interpret the execution result, see the Configuration Guide.
3. If the server and the client are directly connected, check the hub and cable connections.
4. Select a suitable next step in the failure analysis flowchart depending on whether the range of
the failure determined by the
ping command is in the neighboring device or remote device.
(d) Checking the routing information
You need to check the routing information obtained by the Switch if (a) communication is still
disabled after address resolution with the neighboring device is completed, (b) communication is
disabled on the route to the remote device, or (c) the route to the remote device has a problem. To
carry out the check, do the following:
1. Log in to the Switch.
2. Use the
show ip route command to check the routing information obtained by the Switch.
(e) Checking the filters and QoS configuration information
Only certain packets might have been discarded by filtering, or packets might have been discarded
by bandwidth monitoring, drop control, or the QoS control shaper.
On the Switch and the relay device between the client and server, make sure that the setting
conditions for filters and QoS control in the configuration are correct, and that bandwidth
monitoring, drop control, or the shaper is used appropriately in the system configuration. For
details about the procedure, see 3.25.1 Checking the filters and QoS configuration information.
In addition, if DHCP snooping is used, packets might have been discarded by a terminal filter.
Check whether the setting conditions for DHCP snooping in the configuration are correct. For
details about the procedure, see 3.27 DHCP snooping problems.
(f) Checking the Layer 2 network
If you do not find any incorrect settings or a failure in the steps (a) to (e), there might be a problem
with the Layer 2 network. Check the Layer 2 network according to 3.6 Layer 2 network
communication failures.