Stackwise Technology of Cisco IOS Switches

Stackwise technology is used to combine several physical switches into a one logical switch.
Layer 3 switches like Cisco 3750, 3750-X, 3850 and layer 2 switches like 2960-X support this technology. 3750 supports maximum of 9 physical switches per stack..

Special cable type called stack cables are used to create a daisy-chain (loop) between switches. They are connected to the stack ports at the back of the switch. Each supported switch has 2 stack ports.
Following picture shows how it is to be done when it has 3 or more switches.

Either way it works..

Anyhow what required is that all the switches must be connected in a ring by stack cables.

I am using 2 Cisco 3750 series switches to experience how it works.. IOS version must be equal.

Before stacking, lets see some the stack details of both switches.. I reset both the switches before starting this lab, but remember even resetting does not change the pre-configured stack priorities..

Default priority is 1 and the highest is 15. Switch with the highest priority becomes the master.

Issuing show switch command;

As you can see, both the switches think they are the Master. Let's stack them with a one cable and see what happens..

SW1 went to an auto reboot, and it became just a member. Now I'm consoling to the SW1 but I can see its name as SW2. This means all the configuration of SW1 is overwritten from the configuration of SW2. Issuing show switch command;

Let's see why it became a member.

Stack Master election process is like the following..

1. Configured priority
we can configure a priority to decide which switch becomes the master.

2. Hardware / software priority
The switch with the most extensive feature set has a higher priority than another switch (for example: IP Services vs IP base).

3. Default Configuration 
A switch that already has a configuration will take precedence over switches with no configuration.

4. Uptime
The switch with the longest uptime.

5. MAC address
The switch with the lowest MAC address.

It looks like the MAC address is the tie breaker here.

Anyhow only one stack cable was needed to bring the stack up for 2 switches..

But why we always see 2 stack cables are used to bring a 2 switch stack in production?

Let's issue another command; show switch stack-ring speed

Now let's connect another stack cable and see what happens..

You can see the ring speed has become 32G and the ring configuration is full. This is the reason we always prefer to add 2 cables even for 2 switches..

My real physical connection is like this..

You can see which switch is connected to which switch by issuing show switch neighbors

Best practice is to change the priority of the switches to be master.  Let's say I want to make the switch 1 to be master,

SW2(config)#switch 1 priority 15

As you can see, it changed the priority, but it did not took over the master role. It will only be the master if the current master is down.

Things to keep in mind:-

1. IOS version must be equal to begin with.
2. Even resetting does not change stack priority values configured.
3. Default priority is 1 and highest is 15 and the highest will be the master.
4. There is no preemption here.
5. Master will overwrite member configuration by its running config.
6. Though you issued a reload command by consoling to a switch in the stack, all the switches will reboot.