Troubleshooting guide

3. Troubleshooting Functional Failures During Operation
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3.8 Communication failures in the high-reliability functionality
based on a redundant configuration
3.8.1 Communication failures occurring when uplink redundancy is used
If switching cannot be performed as expected when uplink redundancy is used, isolate the
cause of the problem according to the failure analysis method described in the following
table.
Table 3-32 Failure analysis method for uplink redundancy
No. Items to check and commands Action
1 Use the show switchport
backup operation command to
check the primary and secondary
pair information.
 Pair information is not displayed: Continue with No. 2.
 Pair information is displayed:
- If the Status information for the port displayed using the
show switchport backup operation command does not
change immediately after the physical port enters the
link-down status, continue with No. 3.
- If automatic preemption or timer preemption is not possible
after the primary port enters the link-up status, continue with
No. 4.
2 Use the show running-config
operation command to check the
uplink redundancy configuration.
The port channel interface is specified for the secondary port:
The configuration for the target port channel interface might have
not been set.
Check the configuration of the target port channel interface. If the
configuration has not been set, set it.
3 Check the link debounce setting for
the target port.
If the link debounce configuration command has not been set
(that is, the default of 2000 milliseconds is used for operation) or if
a value greater than 2000 (milliseconds) is set, reduce the set
value.
4 If automatic preemption or timer
preemption to the primary port is not
possible, use the show
switchport backup operation
command to check the information
displayed for Status for the
primary port.
 Blocking displayed:
- If a hyphen (-) is displayed for Delay under Preemption,
neither automatic preemption nor timer preemption has been
set. Use the switchport backup interface
configuration command to set preemption.
- If a value other than 0 is displayed for Limit (time) under
Preemption, the preemption time has not been reached.
Wait a while.
Alternatively, execute the select switchport backup
interface operation command.
 Down displayed:
The status of the port is link down. Check the status of the
upstream switch and the cable connection.
 For other cases, go to No. 5.
5 Check whether Spanning Tree is
running on the upstream switch of
the primary port.
When Spanning Tree is running, the port enters the Listening
or Learning status after recovery from a link-down condition,
and therefore communication is disabled for a while. If Spanning
Tree is running on the upstream switch, set the timer preemption
to 30 seconds or longer.
For other cases, go to No. 6.
6 Check whether the upstream switch
can receive flush control frames.
Reception is possible: Continue with No. 7.
Reception is not possible: Continue with No. 8.