Dell PowerConnect B-RX Multi-Service IronWare Software R02.7.02a Release Notes - Page 30

Hitless Layer 2 OS upgrade Hitless O/S upgrades are supported for Layer 2 switching and Layer 2 protocols only. See the Upgrading Software Images and Configuration Files chapter in the BigIron RX Installation Guide for additional information. The hitless O/S Layer 2 upgrade allows for upgrading the software in a system between two releases of the O/S, which support this functionality and have compatible data structures. A hitless O/S Layer 2 downgrade may also be supported if the current and target code releases have compatible data structures. From time to time, it may become necessary when enhancing the software or adding new features to change or add data structures making releases incompatible. In these cases, an upgrade or downgrade will not be hitless, and will fall back to using the regular Brocade upgrade process - relying on fast reboot. When performing a hitless O/S Layer 2 upgrade or downgrade, use the following guidance:  Hitless O/S Layer 2 upgrade or downgrade is not supported in 2.3.00 or earlier.  Hitless O/S Layer 2 upgrade support is in 2.3.00a and higher patch releases (unless indicated otherwise in the target software's release notes).  Hitless upgrade maintains forwarding states and control plane functionality of the active software from which you are upgrading (i.e. 2.3.00a to 2.3.00d would only maintain the functionality supported by 2.3.00a).  Hitless downgrades are recommended only between compatible patch releases of a feature release such as between 2.3.00b to 2.3.00a (to ensure the higher release has no conflicting functional enhancements that do not exist in the lower release).  Due to the PBIF change introduced in patch release version 02.3.00e and version 02.3.00f, users cannot perform hitless upgrades from earlier 02.3.00 versions or 02.3.00g or higher versions. For information on Hitless Upgrades refer to the "Hitless OS Upgrade" section in the BigIron RX Installation Guide.  Due to Internal changes introduced in patch release version 02.4.00d ,hitless upgrade is not supported onto 02.4.00b and hitless downgrade is not supported from 02.4.00d.  The Hitless upgrade feature is not supported on release 02.5.00b to 02.5.00c for the BigIron RX-32 only. It is supported on all other BigIron RX devices. Using this feature, you can upgrade the Multi-Service IronWare software without a loss of service or disruption in the following supported functions and protocols:  All ports and links remain operational  TOS-based QoS  Layer-2 Switching  Layer-2 Protocols:  MRP  STP  VSRP  RSTP  MSTP IronWare Software Release 02.7.02a for Brocade BigIron RX Series Swtches Release Notes v 1.0 Page 30 of 46