Frequently Asked Questions; Can Dual Supervisors Be Used In Each Chassis With Vss - Cisco Catalyst 6500 User Manual

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Use the erase nvram command instead of the write erase command in order to reset the VSS
configuration. The write erase command erases the startup−config and ROMMon variables. VSS
requires switch−id ROMMon variable in order to boot in VSS mode.
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Do not use preemption. Refer to Cisco recommends that you do not configure switch preemption for
more information.
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Do not use the shutdown command for VSL failure simulation, as it creates a configuration
mismatch. If you disconnect a cable, it provides more realistic failure scenario.
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Do not change the VSL hashing algorithm while the system is in production. Change of the algorithm
requires the port channel to be disabled and re−enabled, with the shutdown and no shutdown
commands. If you shut down a VSL, it causes traffic disruption and can end up in dual−active
scenario.
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Configure the MAC aging timer to three times the MAC synchronization timer value.
The default MAC synchronization and MAC aging timers can cause unknown unicast flooding. VSS
can cause traffic to flow asymmetrically such that the source MAC address is only learned on one
chassis. The MAC aging timer of 300 seconds and MAC synchronization timer of 160 seconds allows
for up to 20 seconds of unknown unicast flooding for any given MAC address in a 320 second
interval. In order to resolve this, change the timers such that the aging timer is three times as long as
synchronization timer, for example, mac−address−table aging−time 480 .
The sample output of the show mac−address−table aging−time is shown here:
switch#sh mac−address−table aging−time
Vlan Aging Time
−−−− −−−−−−−−−−
Global 480
no vlan age other than global age configured
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For VSS to operate with stateful switchover (SSO), both the supervisor engines must run the same
software version.
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If you migrate back to a standalone switch from VSS mode through the switch convert mode
stand−alone command, it completes these tasks:
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Converts interface name with switch/slot/port name to slot/port.
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Removes non−local interfaces from running−config.
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Removes VSL port channels and ports configuration.
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Saves Running−config to Startup−config
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Sets SP rommon variable SWITCH_NUMBER to 0.
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Reloads the switch.
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Reboot of the switch is required when they are strictly necessary; for example, an IOS upgrade or as a
troubleshooting step. A switch being up for more than two years means that it is a stable switch and
the configuration is stable as well.

Frequently Asked Questions

Can dual supervisors be used in each chassis with VSS?

Yes. Dual supervisors in each VSS chassis configured for VSS−mode are supported beginning with SXI4 and
later.
When removing the preempt commands in Catalyst 6500 Series
Switches in VSS Mode, will it reload the switches?
The switch preemption is not recommended. Therefore, removing the commands is a good practice and does
not cause a reload. For more information on the Preemption feature on VSS, refer to Switch Preemption.