Cisco WS-C6506 Software Manual page 701

Chapter 23 Configuring Redundancy
Configuring Redundant Supervisor Engines on the Switch
High-Availability Overview
For high availability, a system database is maintained on the active supervisor engine and the updates
are sent to the standby supervisor engine for any change of data in the system database. The active
supervisor engine communicates and updates the standby supervisor engine when any state changes
occur, ensuring that the standby supervisor engine knows the current protocol state of the supported
features. The standby supervisor engine knows the current protocol states for all modules, ports, and
VLANs; the protocols can initialize with this state information and start running immediately.
The active supervisor engine controls the system bus (backplane), sends and receives the packets to and
from the network, and controls all modules. The protocols run on the active supervisor engine only.
The standby supervisor engine is isolated from the system bus and does not switch packets, but it does
receive the packets from the switching bus to learn and populate its Layer 2 forwarding table for
Layer 2-switched flows. In addition, the standby supervisor engine receives the packets from the
switching bus to learn and populate tables for the Layer 3-switched flows. The standby supervisor engine
does not participate in forwarding any packets and does not communicate with any modules.
If you enable high availability when the standby supervisor engine is running, the image version
compatibility is checked and if found compatible, the database synchronization is started.
High-availability compatible features continue from the saved states on the standby supervisor engine
after a switchover.
When you disable high availability, the database synchronization is not done and all features must restart
on the standby supervisor engine after a switchover.
If you change high availability from enabled to disabled, synchronization from the active supervisor
engine is stopped and the standby supervisor engine discards all the current synchronization data.
If you change high availability from disabled to enabled, synchronization from the active to the standby
supervisor engine is started (if the standby supervisor engine is present and its image version is
compatible).
NVRAM synchronization occurs regardless of high availability being enabled or disabled (if there are
compatible NVRAM versions on the two supervisor engines).
If you do not install a standby supervisor engine during the system bootup, the active supervisor engine
detects this and the database updates are not queued for synchronization. Similarly, when you reset or
remove the standby supervisor engine, the synchronization updates are not queued and any pending
updates in the synchronization queue are discarded. When you hot insert or restart a second supervisor
engine that becomes the standby supervisor engine, the active supervisor engine downloads the entire
system database to the standby supervisor engine. Only after this global synchronization is completed,
the active supervisor engine queues and synchronizes the individual updates to the standby supervisor
engine.
When you hot insert or restart a second supervisor engine, it might take a few minutes for the global
Note
synchronization to complete.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
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OL-8978-04