Cisco Catalyst Switch Stacks
A switch stack is a set of up to nine Catalyst 3750, 3750-E, 3750-X switches connected through their StackWise ports, or up to four 2960-S switches connected through their FlexStack connectors. One of the switches controls the operation of the stack and is called the stack master. The stack master and the other switches in the stack are stack members. The stack members use the Cisco StackWise (or FlexStack) technology to behave and work together as a unified system. Layer 2 and Layer 3 protocols present the entire switch stack as a single entity to the network. The stack master is the single point of stack-wide management.
NOTE: Catalyst 3850 switches cannot be stacked with any other switch model and maximum stack size is 4. As with 3750X switches all members must be running the same feature set.
Adding 3750-X Switches to Existing Stacks of 3750/3750-E Switches
Adding 3750-x switches to existing stacks can cause issues. Please note the following recommendations:-
1. An existing stack needs to be running the minimum level of IOS (minimum IOS version is 12.2(53)SE2 but IOS version 12.2(55)SE and later recommended) to support the 3750-X hardware before any attempt is made to add the 3750-X to the stack.
2. Ensure that the 3750-X switch is running the same IOS release (IOS 12.2(55)SE or later recommended) and has the same licence enabled as the existing stack members before it is added to the stack.
3. If the existing stack is running multiple IOS feature sets (e.g. IP Services on the master and standby and IP Base on the other members of the stack) then the stack will require to be upgraded so that all stack members are running the same feature-set. The 3750-X switch that is being added to the stack will also have to have the same licence to be enabled (IP Services in the example).
4. Due to issues found when carrying out this testing it is not recommended to use the archive command to upgrade all stack members at the same time. The recommendation is to use the archive command with the /destination-system option and upgrade each member of the stack individually.
Stacking Quick Start Configuration
This section has been added for engineers to refer to as a quick reminder for installing new stacks:-
• It’s important to ensure that the switches are cabled up correctly, that all switches are running the same version of software and the same feature-set, and that any previous configuration has been cleared.
• The switches should then be powered on in order that they are stacked with the top switch first, then the second switch in the stack and so on. The top switch should then be used to configure the stack.
• Configure the stack master with a priority of 15, the backup master with a priority of 10 and the other switches with a priority on 1.
The following script configures the switches
Use the ‘show switch’ command to ensure that the numbering is consistent with the switch position i.e. the top switch (stack master) is switch number 1, second switch (backup master) is switch number 2 etc.
If you wish to use a Cisco Catalyst 3750G – 12S (12 SFP ports) as a switch Master please refer to the SDM Templates section in this document.
Each member in the stack must have the same IOS image and feature-set. Otherwise upgrades of software become difficult, and may result in a breach of license.
Switch Stacks should confirm to the following standards:
Stack Numbering and Priority
The Stack Master will be the top switch in a stack and will have a member number of 1. All remaining switches will be positioned underneath the Stack Master and will be numbered sequentially.
The Master will have a priority of 15 and (where a stack of 3 or more switches exist) the second switch (member number 2) will have a priority of 10. All remaining switch members will have a default priority of 1.
The following procedure should be used for configuring a stack:-
1. Before connecting switches in a stack confirm all switches are running the same IOS version. The minimum version to support LACP is 12.2(25)SEC. The minimum recommended version to support LLDP is 12.2(55)SE or later.
Do not connect switches to the network or to each other at this point.
2. Ensure that all switches have been cleared of any previous configuration (If switch is not new out of the box).
Hints & Tips:
To quickly check if a switch has been configured previously press and hold the Mode button for 3 seconds. When all of the LEDs left of the Mode button turn green, release the Mode button. If the LEDs left of the Mode button begin to blink after you press the button, release it. Blinking LEDs mean that the switch has already been configured
3. To prevent incompatible configurations, follow these steps on a previously configured switch that you plan to add to the stack:
STEP 1 Press and hold the Mode button, as shown below to delete the configuration
The switch LEDs begin blinking after about 2 seconds. If the switch is not configured, the mode buttons are all green.
STEP 2 Continue holding down the Mode button. The LEDs stop blinking after 8 additional seconds, and then the switch reboots.
4. Ensure all switches in the stack are powered off and connect the switches via StackWise cables. See the section below for detailed information on connecting the switches together.
5. Power the top switch first. Once this switch is powered up power the next switch below. Once this is powered up continue until all switches have been powered up. This will ensure the switches will be numbered incrementally from top to bottom.
6. Apply the configuration below to Switch 1 (Stack Master).
switch 1 priority 15
switch 2 priority 10
Confirm the configuration is saved and then reload the switch
7. Apply the command below to maintain the same MAC address on the stack even if the stack Master fails. If you enter ’0’, the stack MAC address of the previous master is used until you enter the no stack-mac persistent timer global configuration command, which changes the stack MAC address to that of the current master. If you do not enter this command, the stack MAC address does not change. The new active master switch can continue to use the MAC addresses assigned by the old master switch, which prevents ARP and routing outages in the network.
stack-mac persistent timer 0
Note: When you configure this feature, a warning message displays the consequences of your configuration: if the stack master is removed from the stack and re-used elsewhere in the network this will cause networking issues (duplicate MAC address) and could be difficult to fault find.
To check which MAC address is being used, and check if it’s persistent, you can use the show switch command
8. Check the status of Stack with the ‘show switch’ command. Other useful commands are : –
show platform stack-manager all
show switch stack-port
show sdm prefer
At this point the configuration can now be applied to the Stack Master.
Provision a Stack Member
A provisioned configuration is automatically created when a switch is added to a switch stack that runs Cisco IOS Release 12.2(20)SE or later and when no provisioned configuration exists.
To configure a stack in advance of a new ‘slave’ switch (stack member) being added to the stack it is possible to manually provision a stack member in advance of the switch being physically connected to the network. You can configure the stack member number, switch type, and interfaces associated with a switch that is not currently part of the stack. The configuration that you create on the switch stack is called the provisioned configuration. The switch that is added to the switch stack and that receives this configuration is called the provisioned switch.
You can save the provisioned configuration to the startup configuration file as normal using the the copy running-config startup-config command.
You cannot use the switch current-stack-member-number renumber new-stack-member-number global configuration command on a provisioned switch. If you do, the command is rejected.
If you add a provisioned switch that is a different type than specified in the provisioned configuration to a powered-down switch stack and then apply power, the switch stack rejects the (now incorrect) switch stack-member-number provision type global configuration command in the startup configuration file. Within stack initialization, the nondefault interface configuration information in the startup configuration file for the provisioned interfaces (potentially of the wrong type) are executed. Dependent upon how different the actual switch type is from the previously provisioned switch type, some commands are rejected, and some commands are accepted.
This example shows how to provision a Catalyst 3750G-48PS switch with a stack member number of 2 for the stack. The show running-config command output shows the interfaces associated with the provisioned switch:
switch 2 provision ws-c3750g-48ts
You can now view and configure the interfaces of the switch in the running configuration:-
Remove a Provisioned Switch from the Stack
If a switch stack runs Cisco IOS Release 12.2(20)SE or later and you remove a provisioned switch from the switch stack, the configuration associated with the removed stack member remains in the running configuration as provisioned information. In order to delete all configuration information associated with the removed switch (a stack member that has left the stack), apply the no switch provision command in global configuration mode.
In this example, switch number 2 was removed from a stack. The model number of the switch is WS-C3750-48TS:
no switch 2 provision ws-c3750-48ts
Note: In order to avoid the receipt of an error message, you must remove the specified switch from the switch stack before you use the no form of this command to delete a provisioned configuration.
The output of a ‘show switch’ command is shown below
If there is a version mismatch and the master cannot update the SW it will not be possible to session across to the affected switch. The only way to update the SW is to power down the switch, disconnect it from the stack and to then manually update the IOS whilst offline. The interface configuration will still be present on the remaining stack, therefore ensure that when the switch is returned to the stack it has the correct switch number and priority.