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 of the Layer 2
protocols, and all changes in hardware and software states for features that
support stateful switchover are kept in sync. Consequently, it offers zero
interruption to Layer 2 sessions in a redundant supervisor engine
configuration. SSO is supported in 12.2(20)EWA and later releases.
Because the redundant supervisor engine recognizes the hardware link status
of every link, ports that were active before the switchover will remain active,
including the uplink ports. However, because uplink ports are physically on the
supervisor engine, they will be disconnected ONLY if the supervisor engine is
removed.
If the active supervisor engine fails, the redundant supervisor
engine becomes active. This newly active supervisor engine uses existing Layer
2 switching information to continue forwarding traffic. Layer 3 forwarding will
be delayed until the routing tables have been repopulated in the newly active
supervisor engine.