Have a look at the config guide:
Quad-Supervisor (In-chassis Standby Supervisor Engine) Support
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:
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It must also be manually configured for VSS mode, i.e., it must have been converted from standalone 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:
•A supervisor engine's conversion from standalone to VSS occurs per engine. If two supervisor 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 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 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 participate in upgrade or any forwarding operations.
•Because ICS supervisor engines do not communicate with ICA supervisors, VSS and other 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.
HTH
