How to add a switch to a Cisco C2960X stack without breaking anything

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Up to 8 members can be combined in a Cisco C2960X stack, but there can only be one stack master. Adding a switch to a stack without some precautions can be disastrous.

Preconditions

To begin with, all members of the stack must have the same Cisco IOS software image and the same feature set.

Not all software images are capable of being part of the stack:

Stacking is not supported on switches that have a LAN Lite image installed.

And finally, can we use different C2960 series at the same time?

Yes, all C2960X models are supported. You can also mix C2960X and C2960S in the same stack, but there are some limitations. More on this below.

Mixed C2960X / C2960S stack

We can create a mixed 2960X / 2960S stack, but there are a few important points to consider:

• The number of stack members supported has been reduced from eight to four.

• Stack bandwidth has been reduced from 20 Gbps to 10 Gbps because the 2960S stack ports are only 10 Gbps throughput.

• Stack convergence time increases from milliseconds to 1-2 seconds.

Before adding a C2960S switch to a mixed stack of Catalyst 2960X and 2960S switches, you must set the C2960X stack port speed to 10 Gbps. Otherwise, the switches will not stack. Here’s how you can do it:

eng-lab-ch-bel-s-008(config)#switch stack port-speed 10
WARNING: Changing the stack speed may result in a
stack speed mismatch .
Do you want to continue?[confirm]
New stack speed will be effective after next reload

Choosing a stack master

Each member on the stack can be a stack master. If the stack master becomes unavailable, a new master is selected from the remaining stack members based on one of these conditions in the following order:

1. The switch that is currently the stack master.

2. The switch with the highest stack member priority value (1 through 15).

3. Switch with configuration file.

4. The switch with the lowest built-in MAC address.

Stack member priority value

To avoid problems when adding a new switch to the stack, you must ensure that the priority value of the new switch is lower than the stack master and, if necessary, lower than the other members of the stack.

The default switch priority value is 1. If you have never prioritized the already installed switches in the stack, their priority is also 1. It is not recommended to leave these priority values.

Cisco recommends setting the highest priority value (15) to the switch that you would like to be the stack master. This ensures that the switch is selected when a new master is selected, should the need arise.

To do this, go to the stack and monitor its status:

test#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00
 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
*1 Master 2c86.d217.6f00 10 4 Ready 
 2 Member f8a5.c5d9.f300 1 4 Ready

We can see that the priority value of the current master is 10 and the priority value of the second member of the stack is 1. Let’s change the priority value to 15 for the master and 14 for the second member:

test#config t
Enter configuration commands, one per line. End with CNTL/Z.
test(config)#switch 1 priority 15
Changing the Switch Priority of Switch Number 1 to 15
Do you want to continue?[confirm]
New Priority has been set successfully

test(config)#switch 2 priority 14
Changing the Switch Priority of Switch Number 2 to 14
Do you want to continue?[confirm]
New Priority has been set successfully

test(config)#end
test#show sw
test#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00
 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
*1 Master 2c86.d217.6f00 15 4 Ready 
 2 Member f8a5.c5d9.f300 14 4 Ready

Note that this change does not require a reboot.

Stack member numbers

Stack member numbers (1 through 8) identify each stack member on a switch.

The member number also defines the configuration number at the interface level that the stack member is using: Gione/ 0 / x = stack member one, Gi2/ 0/1 = stack member 2 etc.

You can display the stack member numbers with the show switch command:

test#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00

 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
*1 Master 2c86.d217.6f00 15 4 Ready 
 2 Member f8a5.c5d9.f300 14 4 Ready

The default stack member number of a new switch is 1. When it joins a switch stack, its default stack member number is changed to the smallest available value in the stack.

When adding a new switch to the stack, it is best to set its number before addition. You can manually change the stack member number using the command:

eng-lab-ch-bel-s-008(config)#switch 2 renumber 3
WARNING: Changing the switch number may result in a
configuration change for that switch.
The interface configuration associated with the old switch
number will remain as a provisioned configuration.
Do you want to continue?[confirm]
Changing Switch Number 2 to Switch Number 3
New Switch Number will be effective after next reboot

After rebooting the switch / stack member, the new stack number will be used.

To reload a specific member of the stack, use the command:

test#reload slot 2
Proceed with reload? [confirm]

In this example, I have reloaded stack member # 2 and its value is now # 3.

If you manually change the stack member number and no interface-level configuration is associated with the new member number, that stack member value is reset to its default configuration. Therefore, if you want to “shift” the stack number from # 2 to # 3, you will lose your existing interface configuration.

Instead of using the show switch command, you can use the stack mode switch port LEDs to visually identify the number of each stack member.

Preparing a new member before adding to the switch stack (optional)

If necessary, you can prepare a new switch before adding it to the stack.

Then, even before adding the switch to the stack, you can configure the interfaces. To do this, specify the switch number and model. For example:

test#config t
Enter configuration commands, one per line. End with CNTL/Z.
test(config)#switch 3 provision ws-c2960x-24td-l
test(config)#end
test#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00
 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
*1 Master 2c86.d217.6f00 15 4 Ready 
 2 Member f8a5.c5d9.f300 14 4 Ready 
 3 Member 0000.0000.0000 0 0 Provisioned

In this example, we can see that switch # 3 is not physically present, but its interfaces can be configured:

test#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00
 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
*1 Master 2c86.d217.6f00 15 4 Ready 
 2 Member f8a5.c5d9.f300 14 4 Ready 
 3 Member 0000.0000.0000 0 0 Provisioned

test#!-- Then see if the interface of the new switch exists:
test#show run int gi3/0/22
Building configuration...

Current configuration : 39 bytes
!
interface GigabitEthernet3/0/22
end

So how to add a new member to an already existing stack

1. Check the model of the new switch for compatibility. If you mix the C2960X and C2960S switches, you must adapt the stack port speed to the 2960X: lower switch stack port speed up to 10.

2. Check the IOS version of the new switch (or stack), update / downgrade if necessary. I will expand on the topic of stack updates in more detail in future posts.

3. Check the stack status using the command: show switch

4. Change the priority of the stack master (and members) as needed: switch x priority y

5. Change the stack member numbers as needed: switch x renumber y (in that case, don’t forget to reload the member).

6. (Optional) set the priority and number of the new switch (preferably to the minimum value of the priority and number).

7. Make a note of the configuration of the new switch and turn it off!

8. Connect the stack cables. We usually connect cables crosswise, for example:

9. Connect or connect a console cable to the stack master to see console messages.

10. Power on the new switch.

Troubleshooting

Enabling a permanent MAC address

The MAC address of the stack master defines the MAC address of the entire stack.

When the stack is initialized, the MAC address of the stack master determines the bridge ID, which identifies the stack on the network.

If the stack master changes, the MAC address of the new stack master determines the new bridge ID and stack MAC address.

test(config)#stack-mac persistent timer 7
WARNING: The stack continues to use the base MAC of the old Master
WARNING: as the stack-mac after a master switchover until the MAC 
WARNING: persistency timer expires. During this time the Network 
WARNING: Administrators must make sure that the old stack-mac does
WARNING: not appear elsewhere in this network domain. If it does, 
WARNING: user traffic may be blackholed.

When this function is used without any value, the MAC address of the stack changes after 4 minutes (default). Or we can set the time (like in the above example: 7 minutes).

During this time, if the previous stack master rejoins the stack, the stack will continue to use its MAC address as the stack’s MAC address, even if that switch is now a simple stack member rather than a master. If the previous stack master does not join the stack within this period, the switch stack takes the MAC address of the new stack master as the MAC address of the entire stack.

test(config)#stack-mac persistent timer 0 
WARNING: Stack MAC persistency timer value of 0 means that, after a 
WARNING: master switchover, the current stack-mac will continue 
WARNING: to be used indefinitely.
WARNING: The Network Administrators must make sure that the old 
WARNING: stack-mac does not appear elsewhere in this network
WARNING: domain. If it does, user traffic may be blackholed.

In this case, if the entire switch stack is rebooted, it uses the MAC address of the stack master.

Accessing the CLI of a specific stack member

test#session 2

test-2#
test-2#show switch 
Switch/Stack Mac Address : 2c86.d217.6f00
Mac persistency wait time: 4 mins
 H/W Current
Switch# Role Mac Address Priority Version State 
----------------------------------------------------------
 1 Master 2c86.d217.6f00 15 4 Ready 
*2 Member f8a5.c5d9.f300 14 4 Ready 
 3 Member 0000.0000.0000 0 0 Provisioned

test-2#exit
test#

Enter exit to return to the CLI session of the stack master. Only the show and debug commands apply to a specific member.

Switch Stack Command Overview