Virtual Switching System (VSS) combines two physical Cisco Catalyst switches into a single virtual switch, which can result in greater network efficiency and bandwidth capacity. One switch chassis becomes the active virtual switch, and the other switch becomes the standby virtual switch. The switch chassis are connected together by a virtual switch link (VSL), which is implemented as an EtherChannel of up to eight physical interfaces.

Configuration, monitoring, and troubleshooting must be performed on the active virtualswitch; console access is disabled on the standby virtual switch. The active virtual switch is responsible for all control plane functions, such as Simple Network Management Protocol (SNMP), Telnet, Secure Shell (SSH), Spanning Tree Protocol (STP), Link Aggregation Control Protocol (LACP), and Layer 3 routing. The data plane is active on both switches.

Virtual Switch Link Protocol (VSLP) is responsible for establishing the VSS. VSLP has two component protocols: Link Management Protocol (LMP) and Role Resolution Protocol (RRP). The VSS initialization process consists of the following steps:
1. The configuration file is pre-parsed for VSL configuration commands.
2. The VSL member interfaces are brought online.
3. LMP verifies link integrity, rejects unidirectional links, and establishes bidirectional communication between switch chassis.
4. LMP exchanges switch IDs in order to detect duplicate IDs.
5. RRP checks hardware versions, software versions, and VSL configurations for compatibility.
6. RRP assigns the active virtual and standby virtual switch roles.
7. Switches come up in Nonstop Forwarding/Stateful Switchover (NSF/SSO) mode or route-processor redundancy (RPR) mode.
8. Switches continue the normal boot process.

If RRP determines that both switches are compatible, both chassis will come up in NSF/SSO mode, in which all modules are powered up and can forward traffic. If RRP determines that an incompatibility exists, the standby virtual switch will come up in RPR mode, in which all modules are powered down. The switch chassis that is started first will always become the active virtual switch unless preemption is configured. If both chassis are started simultaneously, the switch with the highest priority will become the active virtual switch. By default, the priority is set to a value of 100. If priorities are equal, the switch with the lower switch ID will become the active virtual switch.