If you want to increase your network’s switching capacity, look no further than the Cisco VSS (Virtual Switching System). It’s a technology that effortlessly combines two switches into one, making management simple and offering increased bandwidth.
But beyond just learning what a VSS is, there’s a lot to learn about configuring the technology. There’s also much to learn about where to start if you’ve decided this is the route you want to go in.
In this article, we will discuss the following:
- What a Cisco VSS is.
- How to configure a Cisco VSS.
- Where to look to get started.
What Is a Virtual Switching System?
A Virtual Switching System (VSS) is a Cisco proprietary technology that interconnects two physical switches and logically makes them one switch with interconnected control and management planes. This architecture simplifies the design process and reduces overhead on the units as they logically process as one unit. VSS optimizes operational efficiency and allows for more scalable bandwidth up to 1.4
As shown below, the two physical units are logically joined to create one virtual switch. VSS eliminates the need for protocols like HSRP (Hot Standby Routing Protocol) or VRRP (Virtual Router Redundancy Protocol), which are generally put in place to help with redundancy. These protocols still require each switch to have its own control plane, which is where VSS is more optimal.
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Instead of an Active/Passive relationship, the two switches form more of a Master/Slave relationship, with one switch becoming the primary.
The primary switch is then responsible for the configuration of both switches. One switch within the chassis will act as the Active Virtual Switch, while the other sits as a Standby Virtual Switch.
Why Should I Use VSS?
One of the downsides to HSRP or VRRP is that Spanning Tree will block one of the uplinks from the Access Layer to the Core because it sees the switches as separate entities.
VSS eliminates this as it logically combines the two switches and takes advantage of port-channeling, which creates one logical link between the Access and Core, ultimately optimizing bandwidth. Spanning Tree convergence time is also reduced to sub-200ms.
As shown above, two independent Catalyst 6500 switches become one virtual chassis, essentially doubling the slot capacity from nine to 18. Adjacent devices in your network will view the VSS-enabled switch as one logical switch (or router).
The Supervisor Engine on the Active Virtual Switch will manage the control plane, which includes the following:
- Layer 2 Protocols (STP, LACP, BPDUs, PDUs, etc.)
- Layer 3 Protocols (EIGRP, OSPF, etc.)
- Management Functions (SNMP, SSH, etc.)
The data planes on both switches will stay active as traffic flows physically through both while logically intertwined. The Control plane on the VSS Standby Switch will become active when the VSS Active Switch becomes non-responsive.
Cisco VSS: A Configuration Guide
Here’s a step-by-step guide to configuring two switches into a single virtual switch by configuring VSS. In this example, we’re using Cisco 6500 Series switches. The exact process and commands may vary slightly depending on the series/model of switches you use.
Configure Port Channel
The first step is to set up the port channel for each switch using the commands in the following image.
Configuration of VSS
A prerequisite to configuring VSS is enabling SSO for the switches. Otherwise, access to the switches may be limited or disabled.
SSO Configuration
VSS Configuration
To complete the conversion, execute the following:
PiVIT-6500SW1 # switch convert mode virtual
To view the configuration:
PiVIT-6500SW1 # show virtual switch
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FAQs
How to check Cisco VSS status?
When using Cisco VSS with two switches, you can use various command to check the status of the switches and get configuration information.
Here are some commands you can use in different contexts to determine the status of the logical or physical switches.
Command to get information on the switch virtual link (SVL):
show switch virtual link
Command to get modules on the switches:
show module
Command to see the status of the switch (each physical switch):
show switch
Command to show uptime (useful for identifying changes or restarts in VSS)
show version | include uptime
What Cisco switches support VSS?
Cisco VSS can be configured on various Cisco switches in the 4500 and 6500 series. The technology in newer platforms like the Cisco Catalyst 9000 switches is called StackWise and is pretty similar to VSS.
What is the difference between VSS and vPC?
Although similar in concept to VSS, vPC is a protocol exclusive to Cisco Nexus switches. These technologies help make two physical switches into one logical one. The main difference is that in VSS, there’s only one control plane, but in vPC, the switches are controlled independently.