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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 8 Configuring Supervisor Engine Redundancy Using RPR and SSO About Supervisor Engine Redundancy
RPR Operation
RPR is supported in Cisco IOS Release 12.2(12c)EW and later releases. When a redundant supervisor
engine runs in RPR mode, it starts up in a partially-initialized state and is synchronized with the
persistent configuration of the active supervisor engine.
Note Persistent configuration includes the following components: startup-config, boot variables,
config-register, and VLAN database.
The redundant supervisor engine pauses the startup sequence after basic system initialization, and in the
event that the active supervisor engine fails, the redundant supervisor engine becomes the new active
supervisor engine.
In a supervisor engine switchover, traffic is disrupted because in the RPR mode all of the physical ports
restart since there is no state maintained between supervisor engines relating to module types and
statuses. When the redundant supervisor engine completes its initialization, it reads hardware
information directly from the module.
SSO Operation
SSO is supported in Cisco IOS Release 12.2(20)EWA and later releases. When a redundant supervisor
engine runs in SSO mode, the redundant supervisor engine starts up in a fully-initialized state and
synchronizes with the persistent configuration and the running configuration of the active supervisor
engine. It subsequently maintains the state on the protocols listed below, and all changes in hardware
and software states for features that support stateful switchover are kept in synchronization.
Consequently, it offers zero interruption to Layer 2 sessions in a redundant supervisor engine
configuration.
Because the redundant supervisor engine recognizes the hardware link status of every link, ports that
were active before the switchover remain active, including the uplink ports. However, because uplink
ports are physically on the supervisor engine, they will be disconnected if the supervisor engine is
removed.
If the active supervisor engine fails, the redundant supervisor engine become active. This newly active
supervisor engine uses existing Layer 2 switching information to continue forwarding traffic. Layer 3
forwarding is delayed until the routing tables have been repopulated in the newly active supervisor
engine.
SSO supports stateful switchover of the following Layer 2 features. The state of these features is
preserved between both the active and redundant supervisor engines:
• 802.3
• 802.3u
• 802.3x (Flow Control)
• 802.3ab (GE)
• 802.3z (Gigabit Ethernet including CWDM)
• 802.3ad (LACP)
• 802.1p (Layer 2 QoS)
• 802.1q
• 802.1X (Authentication)