Cisco Catalyst 4500 series Administration Manual page 223

Chapter 5 Configuring Virtual Switching Systems
The Catalyst 4500 series switches support dual supervisors in a redundant chassis, which can be
configured for SSO or RPR mode. However, when a chassis is running in VSS mode, it supports a second
supervisor engine, but only in rommon mode. In-Chassis-Standby (ICS) can not participate in control,
management, or forwarding plane functioning. This makes ports on the supervisor engine in rommon
mode available for forwarding although it neither participates in any switchover nor provides protection
against any failure. In VSS mode, an In-Chassis-Active (ICA) supervisor engine participates in VSS
control/ management operation and manages ports on the supervisor engine in rommon mode.
If the second supervisor engine is inserted in a redundant chassis, the following information applies:
It must also be manually configured for VSS mode, i.e., it must have been converted from standalone
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to VSS mode previously. If you insert a supervisor engine that was not configured for VSS mode, it
will disrupt the operation of the ICA supervisor engine. If it was previously configured, automatic
boot must be disabled (i.e., to boot only to ROM Monitor) with the confreg command in rommon.
The supervisor engine does not takeover or boot automatically when the ICA supervisor engine fails.
A manual boot up is required to make it participate in VSS; it then functions as an ICA supervisor
engine.
More details on rommon commands are found at this URL:
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/15.1.2/XE_340/configuration/guide/r
ommon.html#wp1013959
A supervisor engine's conversion from standalone to VSS occurs per engine. If two supervisor
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engines exist in a chassis, one should be retained in rommon or removed, before conversion occurs.
You can convert the second supervisor to VSS mode while the first supervisor is removed or in
rommon, with the additional step of setting it to "boot only to ROM Monitor." When both engines
are converted to VSS, they can be inserted into the chassis together and re-booted.
Booting a chassis with two supervisor engines configured for VSS causes one of the engines to
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become the ICA and participate in VSS. The other engine, which becomes the ICS, will
continuously reload. The secondary supervisor (the ICS) must be configured to "boot only to ROM
Monitor" with automatic boot disabled.
When the ICA fails, the ICS doesn't take over because ICS support of SSO or RPR mode is
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unavailable. ICS (the secondary supervisor) must be booted manually to become the ICA and
manage the VSS operations. For this to happen, the former active supervisor engine must remain in
rommon mode.
ISSU support requires ICA supervisor engines on both chassis. The ICS supervisor engine does not
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participate in upgrade or any forwarding operations.
Because ICS supervisor engines do not communicate with ICA supervisors, VSS and other
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configurations must be done at conversion time on the ICS. If not done or the configurations do not
match the necessary VSS parameters (like, SwitchId, Domain, and VSL configurations), it cannot
form a VSS when ICA goes down and ICS is booted manually. You can, however, enter these
"bootup" commands to make it join an existing VSS domain.
When a supervisor engine in VSS mode is booting in a chassis, where an ICA supervisor engine already
Note
exists, the ICS supervisor engine (the one that is booting) is continuously reset. It must be manually put
into rommon by disabling auto-boot. Simultaneously, the ICS may display a message that it has crashed
and might generate a crashdump. Because the supervisor engine is going down, this message is harmless;
it does not affect the functionality of VSS. Instead of resetting itself gracefully, the engine might crash
while attempting a reset.
OL-30933-01
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
Understanding Virtual Switching Systems
5-7