Cisco Catalyst 9300: StackWise vs VSS vs StackWise Virtual

Cisco offers two major technologies for combining switches - StackWise stacking and Virtual Switching System (VSS) - plus the newer StackWise Virtual on Catalyst 9000 platforms. They all consolidate multiple switches into one logical unit but differ in platform support, redundancy model and the network layer they are designed for. This article compares the three approaches so you can pick the right one for access, distribution or core designs.

All three technologies solve the same operational problem from different angles: how do you get one management plane, one control plane and loop-free forwarding out of more than one physical box? The answer depends on the platform. StackWise was built for the wiring closet, VSS for the chassis-based core, and StackWise Virtual brings the VSS model to fixed, high-density switches. Knowing where each one belongs in the network - and where it does not - is the difference between a clean design and a 03:00 outage. This guide covers each technology, the hardware requirements, the configuration commands, the verification output, and the failure modes you should plan around.

What Is Cisco StackWise?

StackWise is a switch-stacking technology for the access layer (Catalyst 2960-X, 3750, 3850, 9300 and others). Multiple physical switches join into a single logical unit through dedicated StackWise cables, with one switch acting as master and the others as members:

• Single IP management for all stack members.
• Centralized configuration and software upgrades.
• Rapid failover to a standby master.
• Up to 8-9 members per stack depending on the model.

The stack ring is a dedicated backplane running on proprietary StackWise, StackWise-480 or StackWise-1T cables, physically separate from the data ports. It carries control traffic, stack management, and on StackWise-480 and StackWise-1T platforms also the data forwarded between members. Ring capacity matters more than most people expect: a Catalyst 9300 stack provides 480 Gbps of ring bandwidth, while the 9300X with StackWise-1T raises that to 1 Tbps. Once the aggregate member-to-member traffic approaches the ring capacity you will see drops that no amount of uplink bandwidth can fix.

One switch is elected active (master) and a second becomes standby; all remaining members are in ready state and forward traffic. The election runs on explicit priority first, then the highest MAC address as a tiebreaker. Setting priority on both the intended active and the intended standby makes the result deterministic across reloads and RMA events:

switch 1 priority 15
switch 2 priority 14
! Keep the stack MAC stable across a master failover
stack-mac persistent timer 7

The stack MAC address is normally the MAC of the active switch. When the master fails and the standby takes over, that address would change and trigger gratuitous ARP churn, new DHCP leases and MAC-notification-table updates across the campus. The stack-mac persistent timer command holds the original stack MAC for a configurable window so the transition is invisible upstream. Verify the stack health with:

show switch
show switch detail
show switch stack-ports
show switch stack-ring speed

What Is VSS?

Virtual Switching System combines two chassis-based switches (Catalyst 6500, 6800, 4500) into one logical switch for core or distribution redundancy. Each chassis keeps its own control and forwarding planes but appears as a single device to the network:

• Multichassis EtherChannel (MEC) aggregates links across both chassis.
• No spanning tree between VSS peers.
• Dual-active forwarding with faster convergence.
• Simplifies Layer 2 topologies at the core.

VSS is the chassis-based answer to the same problem. Two chassis are joined by a Virtual Switch Link (VSL) carried over one or more EtherChannels between supervisors or line cards. One chassis becomes active and the other standby, but both forward traffic. VSL capacity should always exceed the sum of the MEC uplinks: if the VSL is thinner than the combined MEC bandwidth, a VSL failure creates congestion before it creates a dual-active condition.

The genuine advantage of VSS is Multichassis EtherChannel. A downstream switch or a dual-homed server runs a single Port-Channel with one member landed on each chassis, and neither side sees a spanning-tree loop. Traffic is actively load-balanced across both chassis instead of half the links being blocked. With quad-supervisor VSS on the 6500/6800, you additionally get supervisor-level redundancy inside each chassis.

Dual-active detection is mandatory in any VSS design. If the VSL fails while both chassis stay powered, each chassis believes it is the active one and you have two devices answering to the same management IP. Cisco supports Enhanced PAgP (ePAgP), IP BFD, and on some platforms PAgP and BFD together, exchanged over MEC links to a neighbouring dual-active-capable device. When the neighbour answers on the MEC while the VSL is down, the losing chassis puts all of its interfaces into an error-disabled state. The build sequence looks like this:

! Both chassis: define the virtual domain and role
switch virtual domain 10
 switch 1
 switch priority 15
!
! Then convert the chassis to virtual mode
switch convert mode virtual
!
! Virtual Switch Link over a port-channel between the pair
interface range TenGigabitEthernet 1/1 - 2
 channel-group 10 mode on
!
interface Port-channel 10
 no shutdown
!
switch virtual link 1

After the reload, the active chassis keeps the configuration and the standby synchronises from it. Confirm the pair with:

show switch virtual
show switch virtual link
show switch virtual role
show switch virtual dual-active summary

What Is StackWise Virtual?

StackWise Virtual is the Catalyst 9000-era evolution: two Catalyst 9500/9600 (and supported 9300 models) switches operate as one using front-panel interfaces instead of a backplane or dedicated stack cables. It is effectively VSS for fixed switches - ideal for high-performance access or collapsed-core designs.

Two switches form one domain with one management IP and one control plane. The SVL runs over dedicated front-panel ports - typically 40G or 100G on Catalyst 9500/9600 and 100G on the 9300X - and must be a direct connection between the two members. No transit devices, no media converters, no patch panels that could break the link. You also need a separate dual-active detection path, usually a pair of 10G ports bundled as an EtherChannel to a downstream access switch, that does not share fate with the SVL. Configuration:

! Configure on both switches
stackwise-virtual
 domain 1
!
! SVL links on switch 1
interface range FortyGigabitEthernet 1/0/1 - 2
 stackwise-virtual link 1
!
! SVL links on switch 2
interface range FortyGigabitEthernet 2/0/1 - 2
 stackwise-virtual link 1
!
! Dual-active detection over the downstream EtherChannel
interface range TenGigabitEthernet 1/0/1 - 2/0/1
 stackwise-virtual dual-active-detection

Both switches must be reloaded to form the pair. Afterwards the standby is reachable only through the active management interface, and the member ports are addressed as 1/0/x and 2/0/x from the common CLI. Verification:

show stackwise-virtual
show stackwise-virtual link
show stackwise-virtual dual-active-detection
show switch

Comparison Table

Hardware, Licensing and Cabling Requirements

StackWise Virtual is not a checkbox you enable on any two Catalyst 9000 switches. Before you design around it, confirm the following:

• Exactly two members. StackWise Virtual is a two-switch technology. It is not a three- or four-member stack, so it does not scale a wiring closet the way classic StackWise does.
• Compatible platforms and models. Both switches must be the same platform family or a supported mixed pair (for example C9300 with C9300X, or C9500 with C9600). Model mixing is bounded by the release notes, not by wishful thinking.
• Licensing. The full feature set, including cross-member port-channels, needs the appropriate Cisco DNA or Network Advantage subscription level per switch.
• Dedicated SVL ports. A minimum of two ports, direct-attached, same speed class, using supported transceivers or DAC cables.
• A DAD path with independent fate. The dual-active detection link must not traverse the same fibre, conduit or patch panel as the SVL, or a single cable cut will produce dual-active.
• Matching software. Both switches should run the same IOS-XE major release before forming the pair; upgrade the standby to the target image first.

Traditional StackWise is more forgiving. You can add and remove members while the stack runs, mix supported models, and the cables cost a fraction of 40/100G optics. That cost difference is the single biggest reason StackWise still wins in most wiring closets, even on Catalyst 9300 hardware that supports StackWise Virtual.

Control Plane, Data Plane and Failover Behaviour

The three technologies behave quite differently the moment something fails, and this is where most designs are won or lost:

Note the asymmetry in the split risk. A failed StackWise cable on a ring stack normally results in a surviving stack that is simply a little degraded; a genuine stack split is a rare double-fault. A failed SVL is a routine single fault that produces two active switches, which is why the DAD path is not optional. Deploy the DAD link in the same change window as the SVL, never "later".

Which One Should You Use?

Choose StackWise for the access layer: easy configuration, single management, horizontal scaling in the wiring closet, and the cheapest inter-switch cabling of the three. Choose VSS (or StackWise Virtual on Catalyst 9000) when you need redundancy, MEC and STP-free topologies at the core or distribution layer. For a Catalyst 9300 access deployment with two switches, StackWise Virtual gives VSS-style benefits without the chassis form factor.

A simple decision path that works in practice:

• More than two switches in one closet, and no cross-switch port-channel requirement: StackWise.
• Chassis already installed in the core, or a large MEC fan-in from distribution: VSS.
• Two fixed Catalyst 9000 switches, dual-homed servers or access switches, and 40/100G ports available: StackWise Virtual.
• Room for growth beyond two switches in the same closet: StackWise today, or a routed design instead of a virtual one.

Do not lose sight of the alternative. A plain Layer 3 point-to-point link between two switches, with dynamic routing, has fewer moving parts than any virtualisation technology and cannot go dual-active. If you do not actually need a single management IP and cross-chassis EtherChannel, a routed pair is often the better engineering answer.

Migration and Design Considerations

Most upgrades follow one of two paths. The first is replacing a 3750/3850 stack with a Catalyst 9300 stack: the port-addressing model and the stack CLI are close enough that the configuration translates with moderate edits, and the cable plant changes from StackWise to StackWise-480. The second is replacing an ageing chassis pair with two Catalyst 9500s in a StackWise Virtual domain - the IP addressing, VLANs and MEC port-channels carry over conceptually, but the SVL replaces the VSL and you must build the DAD path from scratch.

Plan the cut-over around the failure domain. Build the new pair alongside the old one, migrate one downstream switch or server at a time, and only remove the legacy device once the port-channel on the new pair is fully populated. Keep a console connection to both new switches until the pair has reloaded cleanly, because a failed SVL formation leaves you with two standalone switches and no reachability to the standby.

Verification and Troubleshooting Commands

Keep this short list on hand for any of the three technologies:

! StackWise
show switch
show switch detail
show switch stack-ports
show switch stack-ring speed
show switch neighbors

! StackWise Virtual
show stackwise-virtual
show stackwise-virtual link
show stackwise-virtual dual-active-detection
show switch

! VSS
show switch virtual
show switch virtual link
show switch virtual role
show switch virtual dual-active summary

! Common
show version | include License
show spanning-tree summary
show etherchannel summary

If a port-channel refuses to come up after forming a StackWise Virtual pair, check the member numbering first: a port referenced as 2/0/1 only exists once both switches have joined the domain. A port-channel that was valid on a standalone switch, with both ends local, will fail immediately after the pair forms.

FAQ

Can you stack switches in a VSS configuration? No - VSS pairs two chassis; traditional stacking is a separate mechanism. StackWise Virtual combines ideas from both but is limited to two switches.

Does VSS replace stacking? Not directly - VSS targets core/distribution, stacking targets access. Pick per topology needs.

Is StackWise Virtual faster than StackWise-480? The SVL runs over front-panel 40/100G ports with dedicated capacity, while the stack ring is shared by all members. For two switches with heavy cross-member traffic, StackWise Virtual has the higher ceiling.

Do I need dual-active detection for StackWise Virtual? If you want a supported configuration, yes. Without it, an SVL failure leaves two switches with the same IP addresses and the same MAC addresses, which is worse than a simple outage.

Can I run more than two members with StackWise Virtual? No. Two is the limit - design for four switches in two SV pairs, or use classic StackWise with up to eight or nine members.

Does forming a virtual pair require a reload? Yes for both VSS conversion and StackWise Virtual formation. Schedule a maintenance window for the initial build, not just for the software upgrade.

Related articles: Cisco Nexus NX-OS VLAN and trunk configuration, Cisco Catalyst port security with sticky MAC, Data center ToR switch selection, Catalyst 9300 StackWise vs StackWise Virtual selection guide, Catalyst 9300 split stack troubleshooting, and Cisco EtherChannel PAgP vs LACP.

原文链接：https://www.router-switch.com/faq/cisco-stack-vs-vss-difference.html