3Com 7757 Configuration Manual page 851

Fabric software and I/O Module (line processing unit) software must be
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identical. Otherwise the Switch 7750 cannot work normally.
To keep the software of Fabric and I/O Module identical, you need to restart
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the I/O Module after you upgrade the host software of the Fabric of the Switch
7750 Ethernet switches.
The Switch 7758 features the double Fabrics and active-standby switchover
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function. If a switch possesses two Fabrics, with the active-standby switchover
function enabled, you can in turn upgrade and restart the two Fabrics with one
Fabric being active. Although Fabric can be upgraded through hot backup,
because the I/O Module must be restarted to keep identical with the Fabric's
software, your services will still be interrupted during the I/O Module restart
period. Therefore, you are recommended to restart the whole switch straight
after you upgrade the host software of the Fabric of the Switch 7758.
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