# Virtual Switching System

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Cisco proprietary network virtualisation technology

**Virtual Switching System** (**VSS**) is a [Cisco](/source/Cisco) [proprietary network](/source/Proprietary_protocol) [virtualisation technology](/source/X86_virtualization) that aggregates two physical [Ethernet switches](/source/Network_switch) into a single logical switch for enhanced [redundancy](/source/Redundancy_(engineering)), [scalability](/source/Scalability), and simplified management in enterprise and [data centre environments](/source/Data_center).[1][2][3] Introduced in 2008, VSS primarily supports [Cisco Catalyst](/source/Cisco_Catalyst) 4500, 6500, and 6800 series switches, allowing them to operate as one virtual entity with a shared [control plane](/source/Control_plane), [data plane](/source/Data_plane), and [management interface](/source/Management_interface).[4][5][6] This [technology](/source/Technology) reduces [protocol overhead](/source/Overhead_(computing)), eliminates [spanning tree](/source/Spanning_tree) loops, and provides sub-second [failover](/source/Failover), making it suitable for high-availability [Layer 2](/source/Data_link_layer) and [Layer 3](/source/Network_layer) networks.[7][8][9]

As of 2025, VSS remains in use for legacy deployments, though [Cisco](/source/Cisco) has transitioned to newer technologies like [StackWise](/source/Cisco_Catalyst) Virtual for Catalyst 9000 series switches.[10][11]

## Background

VSS was developed by [Cisco](/source/Cisco) to address limitations in traditional [multi-chassis](/source/Multi-chassis_link_aggregation_group) setups, such as the need for [Spanning Tree Protocol](/source/Spanning_Tree_Protocol) (STP) to prevent loops and the complexity of managing multiple switches.[12][13][14] It was introduced in 2008 for the [Catalyst 6500 series](/source/Cisco_Catalyst_6500), enabling two chassis to form a [virtual switch](/source/Network_function_virtualization) via a high-speed [Virtual Switch Link](/source/Virtual_routing_and_forwarding) (VSL).[15][16][17] The VSL, typically using 10 [Gigabit](/source/Bit) [Ethernet](/source/Ethernet) or higher, synchronises configuration, state, and forwarding tables between the [switches](/source/Switch).[18][19][20]

VSS operates with one switch as the active [virtual switch](/source/Network_function_virtualization) (handling [control plane](/source/Control_plane) functions like [routing protocols](/source/Routing_protocol)) and the other as standby, though both actively forward [data traffic](/source/Network_traffic).[21][22][23] This design supports Multi-Chassis EtherChannel (MEC), where links from both [chassis](/source/Chassis) connect to downstream devices as a single bundle, eliminating STP blocking ports.[24][25][9]

Support was later extended to the Catalyst 4500 series with [Supervisor Engine](/source/Supervisor_Engine_(Cisco)) 7-E/7-LE (from [Cisco IOS XE](/source/Cisco_IOS_XE) 3.4.0SG) and 6800 series.[26][27] VSS requires identical hardware pairs and compatible software versions, with configuration involving domain IDs and switch priorities.[28][29]

## Components

Key elements of VSS include:

- [Virtual Switch Link](/source/Virtual_routing_and_forwarding) (VSL): A special [EtherChannel](/source/EtherChannel) (up to 8 links) connecting the two chassis, carrying control traffic, [data](/source/Data), and [synchronisation](/source/Synchronization). It uses [Link Aggregation Control Protocol](/source/Link_aggregation) (LACP) for [redundancy](/source/Redundancy_(engineering)).[30][31][20]

- Virtual Switch Domain: A [unique identifier](/source/Unique_identifier) (1–255) grouping the [switches](/source/Switch), preventing misconfigurations.[29][32]

- Switch Priority and Roles: Configured via commands like *switch virtual domain <ID> switch <1|2> priority <value>*, determining active/standby roles.[29][33]

- Multi-Chassis EtherChannel (MEC): Allows [load balancing](/source/Load_balancing_(computing)) across both chassis to connected devices, treating them as one switch.[9][34][35]

VSS uses a single [IP address](/source/IP_address) for management and a virtual [MAC address](/source/MAC_address) derived from the active switch.[36][37]

### Configuration

Basic VSS [configuration](/source/Configuration_management) on Catalyst 6500/4500 series involves the following steps: [29] 1. Enabling VSS mode on both switches with *switch virtual domain <ID> switch <1|2>*. 2. Configuring VSL ports (e.g., TenGigabitEthernet interfaces) and forming the [EtherChannel](/source/EtherChannel). 3. Setting priorities and reloading the [switches](/source/Switch). 4. Verifying with commands like *"show switch virtual*" and "*show switch virtual link*".

For example, on the active switch, the switch converts to virtual mode, followed by domain setup.[29] [Cisco IOS XE](/source/Cisco_IOS_XE) 3.4.0SG or later is required, with [ROMMON](/source/Cisco_ASA) updates for the 4500 series.[38]

## Performance

### Benefits

VSS provides several advantages:

- Simplified Management: A single configuration point reduces operational complexity and errors.[9][39]

- Increased [Bandwidth](/source/Bandwidth_(computing)) and [Scalability](/source/Scalability): Doubles [port density](/source/Computer_port_(hardware)) and forwarding capacity without STP overhead.[40]

- High Availability: Sub-second [failover](/source/Failover) (typically 1–3 seconds) with nonstop forwarding during switch failures or upgrades.[39]

- Loop Prevention: Eliminates STP by treating the pair as one switch, reducing convergence time.[41]

It supports features like In-Service Software Upgrades (ISSU) for minimal [downtime](/source/Downtime).[20]

### Limitations

- Limited to two [chassis](/source/Chassis) (no [multi-chassis](/source/Multi-chassis_link_aggregation_group) stacking beyond pairs).[11]

- Hardware-specific: Requires compatible supervisors (e.g., Sup720 for 6500, Sup7-E for 4500); not supported on Catalyst 9000 without [StackWise](/source/Cisco_Catalyst) Virtual.[42][29]

- Potential single point of failure if VSL fails, though LACP provides [redundancy](/source/Redundancy_(engineering)).[9]

- End-of-life for some supported models (e.g., 6500 series).[43][11]

Common issues include VSL mismatches or licence requirements for advanced features.[9]

### Comparative technologies

- [StackWise](/source/Cisco_Catalyst) Virtual: [Cisco](/source/Cisco)'s successor for the Catalyst 9000 series, supporting similar benefits with modern hardware.[11]

- [Virtual Port Channel](/source/EtherChannel) (vPC): For [Nexus series](/source/Google_Nexus), enabling [multi-chassis](/source/Multi-chassis_link_aggregation_group) LACP without full [virtualisation](/source/Virtualization).[44]

- [Cisco](/source/Cisco) [Virtual Networking](/source/Virtual_network_interface): A broader suite including [Nexus 1000V](/source/Cisco_Nexus_switches) for [hypervisor](/source/Hypervisor) integration.[45]

Comparison with similar technologies Technology Vendor Max Chassis Key Feature STP Required? Successor/Alternative VSF VSX HPE-Aruba virtualization technology to create a cluster of two VSS Cisco 2 Unified control plane, VSL No StackWise Virtual vPC Cisco 2+ (in domains) Peer link for sync Yes (optional) ACI Multi-Site MLAG Multi-vendor (e.g., Arista, Juniper) 2+ Open standard, no proprietary links Yes EVPN-VXLAN StackWise Cisco Up to 9 Stacking via cables, single IP No StackWise Virtual MC-LAG Juniper 2 LACP-based Yes QFX Virtual Chassis

VSS unifies control planes, unlike vPC, which maintains separate control planes.[46]

## See also

- [Spanning Tree Protocol](/source/Spanning_Tree_Protocol)

- [Link aggregation](/source/Link_aggregation)

- [Network switch](/source/Network_switch)

## References

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1. **[^](#cite_ref-30)** ["Link Aggregation Control Protocol (LACP) (802.3ad) for Gigabit Interfaces"](https://www.cisco.com/c/en/us/td/docs/ios/12_2sb/feature/guide/gigeth.html). *Cisco*. Retrieved 2025-09-14.

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1. **[^](#cite_ref-40)** Schule, Mike (2024-08-25). ["Understanding VSS: An Introduction to Cisco's Virtual Switching System"](https://orhanergun.net/understanding-vss-an-introduction-to-cisco-s-virtual-switching-system). *orhanergun.net*. Retrieved 2025-09-14.

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1. **[^](#cite_ref-42)** ["Virtual switching system (VSS) Configuration For Cisco 4500 series switches"](https://community.cisco.com/t5/networking-knowledge-base/virtual-switching-system-vss-configuration-for-cisco-4500-series/ta-p/3147865). *community.cisco.com*. 2013-01-24. Retrieved 2025-09-14.

1. **[^](#cite_ref-43)** ["Cisco Catalyst 6500 Virtual Switching System 1440"](https://www.cisco.com/c/en/us/support/switches/catalyst-6500-virtual-switching-system-1440/series.html). *Cisco*. Retrieved 2025-09-14.

1. **[^](#cite_ref-44)** ["Cisco VSS (virtual switching system) or Cisco vPC (virtual port channels)? | DX NetOps"](https://community.broadcom.com/communities/community-home/digestviewer/viewthread?MID=803973). *community.broadcom.com*. Retrieved 2025-09-14.

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## External links

- [Cisco VSS Configuration Guide](https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vss.html)

- [Cisco StackWise Virtual Overview](https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-9000/nb-06-cat-9k-stack-wp-cte-en.html)

---
Adapted from the Wikipedia article [Virtual Switching System](https://en.wikipedia.org/wiki/Virtual_Switching_System) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Virtual_Switching_System?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.