The SSO HSRP feature enables the Cisco IOS HSRP subsystem software to detect that a standby RP is installed and the system is configured in SSO redundancy mode. Further, if the active RP fails, no change occurs to the HSRP group itself and traffic continues to be forwarded through the current active gateway device.
Prior to introduction of the SSO HSRP feature, when the primary RP of the active device failed, it would stop participating in the HSRP group and trigger another switch in the group to take over as the active HSRP switch.
SSO HSRP is required to preserve the forwarding path for traffic destined to the HSRP virtual IP address through an RP switchover.
Configuring SSO on the edge device enables the traffic on the Ethernet links to continue during an RP failover without the Ethernet traffic switching over to an HSRP standby device (and then back, if preemption is enabled).
You may want to disable SSO HSRP by using the
sso command if you have LAN segments that should switch HSRP traffic to a redundant device while SSO maintains traffic flow for other connections.