disabling port channels (PagP), and turning trunking off for ports to which workstations are directly attached, the fast
switchover time can be reduced to approximately 10 seconds. In a live network environment, these switchover times present
a major disruption to network operations.
Supervisor High Availability Feature
The High Availability software feature of Cisco Catalyst OS further enhances the Cisco Catalyst 6500 Series hardware
redundancy by also providing protocol redundancy. This feature includes stateful protocol redundancy and image versioning.
The High Availability feature must be enabled via the CLI for these features to operate.
Sup-A> (enable) set system highavailability enable
System high availability enabled.
As a general practice with redundant supervisors, it is recommended that the High Availability feature be enabled for normal
operation.
Supervisor Stateful Protocol Redundancy
The stateful supervisor switchover is when the switchover time from the active to the standby supervisor is reduced to less
than three seconds. This reduced downtime is achieved by synchronizing many of the Layer 2, Layer 3, and Layer 4 protocols
between the active and standby supervisor engines and is called maintaining protocol state.
For stateful protocol redundancy between dual supervisor engines, a protocol state database is maintained on each supervisor
engine for all protocols and features requiring high-availability support. Most of these protocols are only running on the active
supervisor engine. In the event of a high-availability switchover, the new active supervisor engine can start the protocols from
the updated database state, rather than the initialization state. This is how a redundant system can maintain stateful protocol
redundancy and minimal network downtime when the active supervisor engine goes offline.
• High Availability Supported Feature—High availability if fully supported. The state of the feature is preserved between
the active and standby supervisor engines in the protocol database.
• High Availability Compatible Feature—High availability is not supported for these features. The protocol database for
these features is not synchronized between supervisor engines. The feature can be used if the High Availability feature is
enabled. For example, if GARP Multicast Registration Protocol (GMRP) and high availability were both enabled and a
high-availability supervisor engine failover took place, the GMRP protocol would be restarted from the initialization state
(non-stateful). The stateful protocol redundancy is still in place for the supported features if a compatible feature is
enabled.
• High Availability Incompatible Feature—High availability is not supported. The protocol database for these features is
not synchronized between supervisor engines. The feature should not be enabled if the High Availability feature is
enabled. These features are not supported with high availability enabled because incorrect behavior may result.
Important: Do not use these features if a high-availability system is required.
1. Layer 4 protocols include the Layer 4 information in extended IP access lists.
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