Campus lan and Wireless lan solution Design Guide

Stateful Switchover (SSO) 
Stateful switchover (SSO) synchronizes active process information, as well as configuration information, 
between active and standby supervisors of a modular chassis / StackWise Virtual pair, or between the active 
and standby switches in a switch stack. SSO allows the standby supervisor / switch to immediately take over in 
sub-second time if the active supervisor / switch fails. 
Non-stop Forwarding (NSF) 
Non-stop forwarding (NSF) helps to suppress routing flaps in SSO enabled devices. NSF allows for graceful 
restart of L3 routing protocols, in the event of the failure of the active supervisor of a modular chassis / 
StackWise Virtual pair, or the failure of the active switch of a switch stack. When the supervisor or switch 
switches over from the active to the hot-standby, it will continue switching IP data traffic flows in hardware.
However, the device in the active role requires time to re-establish control plane peering with IP routing 
neighbors. NSF allows for the forwarding of data packets to continue along known routes while the routing 
protocol information is being restored following a switchover. 
Supervisor Redundancy 
Cisco Catalyst C9404R, C9407R, C9410R, and C9606R chassis models support 1+1 supervisor redundancy 
(Sup-1, Sup1XL, or Sup-1XL-Y on Catalyst 9400 Series, and Sup-1 on Catalyst 9600 Series). The primary 
supervisor is active and is responsible for normal system operation. The secondary supervisor serves as a 
standby, monitoring the operation of the primary. Information is synchronized between supervisors to allow the 
standby supervisor engine to immediately take over in sub-second time if the primary engine fails. Non-stop 
forwarding / stateful switchover (NSF / SSO) offers continuous packet forwarding during supervisor engine 
switchover.
Note: 
When implementing 1+1 supervisor engines on Catalyst 9400 Series switches, the active uplink ports 
are automatically spread across the two supervisors for link-level resiliency. The Catalyst 9600 Series 
supervisor engine does not support uplinks on the module itself. For link-level resiliency on the Catalyst 9600 
Series, spread uplinks ports across the switch linecards.