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Supervisor engines configured for redundancy appear as "STANDBY HOT" and "STANDBY COLD" in the output of the show redundancy states command
Route Processor Redundancy (RPR) refers to the provision of support for the redundancy feature. In the RPR mode, one of the supervisor engines is active and operational, while the second supervisor engine is in the standby mode. If the active supervisor engine fails, the standby supervisor engine takes over and maintains the operation of the switch.
The adjectives Cold, Warm, and Hot denote the redundancy state. This value indicates whether the system and its components are ready to forward packets to their destination, and to assume the network services functionality. These terms appear along with Cisco Nonstop Forwarding (NSF) with Stateful Switchover (SSO), in the output of show commands issued in Cisco IOS Software, and with many high availability feature descriptions. For example, the show redundancy states command output indicates the redundancy state. Each term conveys the technical underpinnings that surround the amount of internal state information saved to allow increasingly faster switchover or even continuous packet forwarding.
These are the term definitions:
Cold: Cold redundancy refers to the degree of resiliency that a redundant system traditionally provides. A redundant system is cold when no state information is maintained between the backup or standby system and the system it protects.
Warm: Warm redundancy refers to a degree of resiliency beyond the cold standby system. In this case, the redundant system is partially prepared. However, the system does not have all the state information that the primary system knows, for an immediate take-over. Some additional information must be determined or gleaned from the traffic flow or the peer network devices to handle packet forwarding.
Hot: Hot redundancy refers to a degree of resiliency where the redundant system is fully prepared to handle the traffic of the primary system. Substantial state information is saved, so the network service is continuous, and the effect on traffic flow is minimal or nil in the case of a failover.
In the case of a single-processor, non-redundant system, the term warm refers to the fact that Cisco IOS Software is preloaded and partially prepared to resume packet forwarding. However, the system is reinitialized, so all software states that pertain to network services must be acquired again and prepared as with a cold reboot.
There are three modes in redundancy for the standby supervisor when native Cisco IOS Software is used:
RPR The show module command displays Cold
RPR+ The show module command displays Warm
SSO The show module command displays Hot
For any other states, the standby supervisor displays Other in the show module command output.
RPR needs two minutes or more to switch over from active to standby, while RPR+ needs 30 seconds or more. On the other hand, SSO needs 0-3 seconds for the switchover on Layer 2. SSO works with Cisco NSF, which helps reduce the switchover downtime on Layer 3.