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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 statefulprotocol redundancy and imageversioning.
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 threeseconds. Thisreduced downtimeis achievedby synchronizingmany ofthe Layer2, Layer3, andLayer 4protocols
1
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 forall protocolsand features requiringhigh-availability support.Most of theseprotocols areonly running onthe active
supervisor engine. In theevent ofa high-availability switchover, thenew active supervisor enginecan start theprotocols 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 AvailabilitySupported Feature—High availability iffully supported.The state ofthe featureis preservedbetween
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 supervisorengine failovertook place, theGMRP protocolwould be restartedfrom the initializationstate
(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 isnot supported. The protocol databasefor 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.
