3Com 4007, Implementation Manual, Page: 60 / 684

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Thus, the system generally treats both EMEs as a single logical device.
(However, for some management activities, such as image download and
Telnet connections, you must treat the two modules as separate devices.)
The primary EME and the secondary EME become synchronized after
redundancy is established. When any configuration or non-volatile data is
modified on the primary EME, the data is automatically modified on the
secondary EME. New events are also stored on each EME, and the event
log files are synchronized. All flash file system activity (copying files to,
deleting files from) on the primary EME is mirrored on the secondary
EME. Thus, if the primary EME fails for any reason, the secondary EME
immediately takes over all primary functions.
Events that occurred before EME redundancy was established are not
synchronized.
There are times when you may not want to automatically copy a file from
the primary EME to the secondary EME (for example, temporary files).
You can configure the system software to set these parameters during
initialization. All files that are designated not to be automatically copied
to the secondary EME are lost after a fail-over.
The Failover Process If you remove (deinstall) the primary EME or if the module fails in some
way, the following process occurs automatically:
1 The system initiates the fail-over mechanism (after, for example, the
primary module fails or is removed).
2 The secondary EME becomes the primary EME.
Because it learned all the configuration settings from the primary EME, it
continues to provide all the management functions with no interruption
to the system.
3 If you remove a failed EME that used to be primary and install a new EME
in that slot, the new module remains secondary and automatically powers
on in standby mode without modifying its configuration.
If, after the failover occurs, the failed EME recovers to a normal operating
condition, it will remain in the secondary state. The failover mechanism is
non-revertive. Even if the problem that caused the failover is resolved, the
the failover process does not switch the primary state back to the original
primary EME.