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 sync. 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 will 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 will be 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)
• 802.1D (Spanning Tree Protocol)
• 802.3af (Inline power)
• PAgP
• VTP
• Dynamic ARP Inspection
• DHCP snooping
• IP source guard
• IGMP snooping (versions 1 and 2)
• DTP (802.1q and ISL)
• MST
• PVST+
• Rapid-PVST
• PortFast/UplinkFast/BackboneFast
• BPDU guard and filtering
• Voice VLAN
• Port security
• Unicast MAC filtering
• ACL (VACLS, PACLS, RACLS)
• QOS (DBL)
• Multicast storm control/broadcast storm control
SSO is compatible with the following list of features. However, the protocol database for these features is not synchronized between the redundant and active supervisor engines:
• 802.1Q tunneling with Layer 2 Protocol Tunneling (L2PT)
• Baby giants
• Jumbo frame support
• CDP
• Flood blocking
• UDLD
• SPAN/RSPAN
• NetFlow
The following features are learned on the redundant supervisor engine if the SSO feature is enabled:
• All Layer 3 protocols on Catalyst 4500 series switches (Switch Virtual Interfaces)