What is switch stacking, and how is it different from a chassis or a pair of switches running MLAG?
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A stack joins several fixed switches with dedicated stack cables into one logical switch: one control plane on the active member, a hot standby, one config and one management IP, and EtherChannels that can span members. A chassis does the same with line cards and redundant supervisors. MLAG (vPC on Nexus) keeps two separate control planes that cooperate to present one port-channel, so each switch can be upgraded on its own.
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How a Catalyst stack behaves
- Members are numbered, and the number is in the interface name:
Gi2/0/1is port 1 on member 2. A Catalyst 9300 stack holds up to 8 members. - Roles are active, standby and member. At stack formation the highest stack priority wins (1 to 15, default 1), with the lowest MAC as the final tie breaker. A running active is not pre-empted when you add a higher-priority switch.
- Active and standby run SSO. Every member forwards in hardware locally, so traffic between two ports on the same member never crosses the stack ring.
- Cross-stack EtherChannel: one uplink from each of two members, so losing a member keeps the uplink.
Tradeoffs to say out loud
| Stack | Chassis | MLAG / vPC | |
|---|---|---|---|
| Control planes | 1 | 1 (dual supervisor) | 2 |
| Upgrade | Whole stack, usually one reload | Supervisor by supervisor on many platforms | One switch at a time |
| Config | One | One | Two, kept consistent |
| Failure domain | Software bug hits all members | Same | Smaller |
StackWise Virtual on Catalyst 9000 joins two switches over ordinary Ethernet links into one control plane, the modern form of VSS.
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What happens when the active member fails?
The standby takes over with SSO and a new standby is elected. Ports on the failed member go down; the rest keep forwarding while protocols resume on the new active.
How do you choose which member becomes active?
Set its stack priority, for example
switch 1 priority 15from privileged EXEC on Catalyst 9300. It takes effect at the next election, not immediately.
Traps
- Saying a stack removes all single points of failure. One control plane means one bad image or bug affects every member.