White Paper

Configuring a Backup Master

It is a good practice to configure another switch to be the backup should the primary master fail. Configuring a

backup master enables a predictable master election in case of primary master failure. To configure a secondary

(or backup) master, assign a priority value less than that of the primary master but greater than that of the other

members. In the example just given, the primary master has priority 14, and all other members have priority 1. Any

member will be secondary master with a value greater than 1 and less than 14.

Determining the Member Number

For brand-new switches the order in which switches are powered up initially is also the order the member numbers

will be assigned. Brand-new switches do not have a member number and default to member 1. As new switches

join an existing stack, the new member is assigned the next available member number. When the second switch is

powered on, it is assigned member number 2. When the third switch is powered on, it is assigned member number

3. And so on.

If new members are not powered on in any order, then member numbers can appear to be randomly assigned.

Understanding FlexStack connections between members can potentially be confusing when the member numbers

are not in an order that matches physical layout.

It is possible to change member numbers of switches in a stack. It is not required for stack operation. The stack will

operate just fine when member numbers do not match the physical order of the stack members. In spite of this, it is

desirable to have member numbers match the physical layout to ease managing the stack and its members.

Use the Cisco IOS Software CLI to change member numbers. The switches must be rebooted for the member

number change to take effect. The following example will renumber member 4 to member 3:

Switch(config#) switch 4 renumber 3

Caution: Changing member numbers causes any configuration on the interfaces of the old member to be lost. All

interface configurations are based on the member number. When the member is changed, the configuration for

current members is NOT changed or copied to the new member number. In the preceding example, the

configuration for all of the Ethernet interfaces on member 4 will be lost, and the configuration for member 3 is

unknown. Take care when changing member numbers and be prepared to reconfigure the interfaces.

Each member retains its own member number through a reboot or a power cycle. When a member boots into a

stack, the member announces its member number to the stack. If there is no conflict, the member number is

retained. If there is a conflict with two different physical members having the same number, then the first member

to join the stack retains its number, and the newest member must change its number. The next available member

number is assigned to the switch in such a conflict situation.

Since members retain their member number through a reboot, when a member moves from one stack to another,

the potential for member number conflict exists. The ability to modify the member number is critical to resolve

member number conflicts.

Adding Members to an Existing Stack

The relative ease with which new members can be dynamically added to an existing stack is a real strength of

FlexStack. A brand-new switch added to an existing stack will join as a member. The new member will be given the

next available member number. When a new member is added to a stack, it will be rebooted to clear any

configuration it might have. When the new member completes the reboot, it will have the configuration of the stack.