Virtual Switching System

Virtual Switching System (VSS) is a Cisco proprietary network virtualisation technology that aggregates two physical Ethernet switches into a single logical switch for enhanced redundancy, scalability, and simplified management in enterprise and data centre environments. ^{[1] [2] [3]} Introduced in 2008, VSS primarily supports Cisco Catalyst 4500, 6500, and 6800 series switches, allowing them to operate as one virtual entity with a shared control plane, data plane, and management interface. ^{[4] [5] [6]} This technology reduces protocol overhead, eliminates spanning tree loops, and provides sub-second failover, making it suitable for high-availability Layer 2 and Layer 3 networks. ^{[7] [8] [9] [10]}

As of 2025, VSS remains in use for legacy deployments, though Cisco has transitioned to newer technologies like StackWise Virtual for Catalyst 9000 series switches. ^{[11] [12]}

Background

VSS was developed by Cisco to address limitations in traditional multi-chassis setups, such as the need for Spanning Tree Protocol (STP) to prevent loops and the complexity of managing multiple switches. ^{[13] [14] [15]} It was introduced in 2008 for the Catalyst 6500 series, enabling two chassis to form a virtual switch via a high-speed Virtual Switch Link (VSL). ^{[16] [17] [18]} The VSL, typically using 10 Gigabit Ethernet or higher, synchronises configuration, state, and forwarding tables between the switches. ^{[19] [20] [21]}

VSS operates with one switch as the active virtual switch (handling control plane functions like routing protocols) and the other as standby, though both actively forward data traffic. ^{[22] [23] [24]} This design supports Multi-Chassis EtherChannel (MEC), where links from both chassis connect to downstream devices as a single bundle, eliminating STP blocking ports. ^{[25] [26] [9]}

Support was later extended to the Catalyst 4500 series with Supervisor Engine 7-E/7-LE (from Cisco IOS XE 3.4.0SG) and 6800 series. ^{[27] [28]} VSS requires identical hardware pairs and compatible software versions, with configuration involving domain IDs and switch priorities. ^{[29] [30]}

Components

Key elements of VSS include:

• Virtual Switch Link (VSL): A special EtherChannel (up to 8 links) connecting the two chassis, carrying control traffic, data, and synchronisation. It uses Link Aggregation Control Protocol (LACP) for redundancy. ^{[31] [32] [21]}
• Virtual Switch Domain: A unique identifier (1–255) grouping the switches, preventing misconfigurations. ^{[30] [33]}
• Switch Priority and Roles: Configured via commands like switch virtual domain <ID> switch <1|2> priority <value>, determining active/standby roles. ^{[30] [34]}
• Multi-Chassis EtherChannel (MEC): Allows load balancing across both chassis to connected devices, treating them as one switch. ^{[9] [35] [36]}

VSS uses a single IP address for management and a virtual MAC address derived from the active switch. ^{[37] [38]}

Configuration

Basic VSS configuration on Catalyst 6500/4500 series involves the following steps: ^[30] 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. 3. Setting priorities and reloading the switches. 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. ^[30] Cisco IOS XE 3.4.0SG or later is required, with ROMMON updates for the 4500 series. ^[39]

Performance

Benefits

VSS provides several advantages:

• Simplified Management: A single configuration point reduces operational complexity and errors. ^{[9] [40]}
• Increased Bandwidth and Scalability: Doubles port density and forwarding capacity without STP overhead. ^[41]
• High Availability: Sub-second failover (typically 1–3 seconds) with nonstop forwarding during switch failures or upgrades. ^[40]
• Loop Prevention: Eliminates STP by treating the pair as one switch, reducing convergence time. ^[42]

It supports features like In-Service Software Upgrades (ISSU) for minimal downtime. ^[21]

Limitations

• Limited to two chassis (no multi-chassis stacking beyond pairs). ^[12]
• Hardware-specific: Requires compatible supervisors (e.g., Sup720 for 6500, Sup7-E for 4500); not supported on Catalyst 9000 without StackWise Virtual. ^{[43] [30]}
• Potential single point of failure if VSL fails, though LACP provides redundancy. ^[9]
• End-of-life for some supported models (e.g., 6500 series). ^{[44] [12]}

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

Comparative technologies

• StackWise Virtual: Cisco’s successor for the Catalyst 9000 series, supporting similar benefits with modern hardware. ^[12]
• Virtual Port Channel (vPC): For Nexus series, enabling multi-chassis LACP without full virtualisation. ^[45]
• Cisco Virtual Networking: A broader suite including Nexus 1000V for hypervisor integration. ^[46]

Comparison with similar technologies

Technology	Vendor	Max Chassis	Key Feature	STP Required?	Successor/Alternative
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. ^[47]

See Also

• Spanning Tree Protocol
• Link aggregation
• Network switch

References

1. "Best Practices for Catalyst 6500 VSS Deployment". Cisco. Retrieved 2025-09-13.
2. "What Is Virtual Switching System (VSS)? - ITU Online IT Training". 2024-06-11. Retrieved 2025-09-13.
3. netseccloud.com. "Understanding VSS: An Introduction to Cisco's Virtual Switching System". netseccloud.com. Retrieved 2025-09-13.
4. "Cisco Expo 2008 - Presentations". www.cisco.com. Retrieved 2025-09-13.
5. "Configure the Catalyst 4500 Series Switch VSS Member Replacement". Cisco. Retrieved 2025-09-13.
6. Molenaar, Rene (2013-08-14). "Cisco 6500 VSS Configuration Example". NetworkLessons.com. Retrieved 2025-09-13.
7. "Campus 3.0 Virtual Switching System Design Guide - VSS Enabled Campus Design [Design Zone]". Cisco. Retrieved 2025-09-13.
8. Schule, Mike (2024-08-25). "Understanding VSS: An Introduction to Cisco's Virtual Switching System". orhanergun.net. Retrieved 2025-09-13.
9. Edgeworth, Brad (2020). CCIE Enterprise Infrastructure Foundation. Cisco Press. pp. 245–260. ISBN   978-0136418337.
10. Schule, Mike (2024-08-25). "Understanding VSS: An Introduction to Cisco's Virtual Switching System". orhanergun.net. Retrieved 2025-09-13.
11. "Products - Migrating Cisco Catalyst 4500-X and 6880/6840-X Series to 9500 Series Switches Guide". Cisco. Retrieved 2025-09-13.
12. "StackWise Virtual for Catalyst 9000 Series". Cisco Systems. 2023-02-10. Retrieved September 13, 2025.
13. "Cisco Switching & Spanning Tree Protocol (STP)". www.pluralsight.com. Retrieved 2025-09-13.
14. Schule, Mike (2024-08-25). "Cisco VSS Fundamentals: Key Concepts and Operational Benefits". orhanergun.net. Retrieved 2025-09-13.
15. DCT_vinod (2011-12-08). "Achieving High Availability using Cisco VSS". Data Center News, Trend, Analysis, Articles and Services. Retrieved 2025-09-13.
16. "Cisco's virtual switch smashes throughput records". Network World. Retrieved 2025-09-13.
17. Molenaar, Rene (2013-08-14). "Cisco 6500 VSS Configuration Example". NetworkLessons.com. Retrieved 2025-09-13.
18. "What is Cisco Virtual Switching - Best Explained (2025)". www.nwkings.com. Retrieved 2025-09-13.
19. "High Availability Configuration Guide, Cisco IOS XE Cupertino 17.8.x (Catalyst 9400 Switches) - Configuring Cisco StackWise Virtual [Support]". Cisco. Retrieved 2025-09-13.
20. "Virtual switching system (VSS) Configuration For Cisco 4500 series switches". community.cisco.com. 2013-01-24. Retrieved 2025-09-13.
21. Oppenheimer, Priscilla (2018). Top-Down Network Design. Cisco Press. pp. 189–195. ISBN   978-1587202834.
22. Schule, Mike (2024-08-25). "Understanding VSS: An Introduction to Cisco's Virtual Switching System". orhanergun.net. Retrieved 2025-09-13.
23. link, Get; Facebook; X; Pinterest; Apps, Other (2017-11-23). "CCIE 400-101: Layer 2 Technologies - VSS, StackWise Concepts" . Retrieved 2025-09-13.{{cite web}}: |last2= has generic name (help)
24. Molenaar, Rene (2013-08-14). "Cisco 6500 VSS Configuration Example". NetworkLessons.com. Retrieved 2025-09-13.
25. "What is Cisco Virtual Switching - Best Explained (2025)". www.nwkings.com. Retrieved 2025-09-13.
26. "How to Configure a Cisco Virtual Switching System". info.pivitglobal.com. Retrieved 2025-09-13.
27. "Catalyst 4500E Supervisor Engine 7-E: Leading Borderless Network Access and Aggregation Supervisor Engine Data Sheet". Cisco. Retrieved 2025-09-13.
28. "Release Notes for the Catalyst 4500E Series Switch, Cisco IOS XE 3.8.xE". Cisco. Retrieved 2025-09-13.
29. "Virtual switching system (VSS) Configuration For Cisco 4500 series switches". community.cisco.com. 2013-01-24. Retrieved 2025-09-13.
30. "Catalyst 4500 Series VSS Configuration Guide". Cisco Systems. Retrieved September 13, 2025.
31. "Link Aggregation Control Protocol (LACP) (802.3ad) for Gigabit Interfaces". Cisco. Retrieved 2025-09-14.
32. "EtherChannel in Computer Network". GeeksforGeeks. 2018-05-03. Retrieved 2025-09-14.
33. "VSS - Virtual Switch Domain Uniqueness Doubt". community.cisco.com. 2014-06-03. Retrieved 2025-09-14.
34. "Setting switch priority in a 4500x VSS pair in virtual switch mode". community.cisco.com. 2016-05-19. Retrieved 2025-09-14.
35. John (2021-10-19). "Multichassis EtherChannel (MEC) and MEC Options". Study CCNA. Retrieved 2025-09-14.
36. "Products - Cisco Catalyst 9000 Platform StackWise Virtual White Paper". Cisco. Retrieved 2025-09-14.
37. "https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/XE3-11-0E/configuration/guide/xe-311-cg/vss.html". Cisco. Retrieved 2025-09-14.{{cite web}}: External link in |title= (help)
38. Bishop, Judson (2012-03-18). "Virtual Switching System (VSS)". ChainRingCircus. Retrieved 2025-09-14.
39. "Virtual switching system (VSS) Configuration For Cisco 4500 series switches". community.cisco.com. 2013-01-24. Retrieved 2025-09-14.
40. "What Is Virtual Switching System (VSS)? - ITU Online IT Training". 2024-06-11. Retrieved 2025-09-14.
41. Schule, Mike (2024-08-25). "Understanding VSS: An Introduction to Cisco's Virtual Switching System". orhanergun.net. Retrieved 2025-09-14.
42. "What is Cisco Virtual Switching - Best Explained (2025)". www.nwkings.com. Retrieved 2025-09-14.
43. "Virtual switching system (VSS) Configuration For Cisco 4500 series switches". community.cisco.com. 2013-01-24. Retrieved 2025-09-14.
44. "Cisco Catalyst 6500 Virtual Switching System 1440". Cisco. Retrieved 2025-09-14.
45. "Cisco VSS (virtual switching system) or Cisco vPC (virtual port channels)? | DX NetOps". community.broadcom.com. Retrieved 2025-09-14.
46. "Virtual Networking". Cisco. Retrieved 2025-09-14.
47. "Single control plane vs separate control planes". community.cisco.com. 2014-04-25. Retrieved 2025-09-14.

External Links

• Cisco VSS Configuration Guide
• Cisco StackWise Virtual Overview