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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter5 Managing Switch Stacks
Understanding Switch Stacks
A stack master retains its role unless one of these events occurs:
•The switch stack is reset.*
•The stack master is removed from the switch stack.
•The stack master is reset or powered off.
•The stack master fails.
•The switch stack membership is increased by adding powered-on standalone switches or switch
stacks.*
In the events marked by an asterisk (*), the current stack master might be re-elected based on the listed
factors.
When you power on or reset an entire switch stack, some stack members might not participate in the
stack master election. Stack members that are powered on within the same 20-second time frame
participate in the stack master election and have a chance to become the stack master. Stack members
that are powered on after the 20-second time frame do not participate in this initial election and become
stack members. All stack members participate in re-elections. For all powering considerations that affect
stack-master elections, see the “Switch Installation” chapter in the hardware installation guide.
The new stack master becomes available after a few seconds. In the meantime, the switch stack uses the
forwarding tables in memory to minimize network disruption. The physical interfaces on the other
available stack members are not affected during a new stack master election and reset.
After a new stack master is elected and the previous stack master becomes available, the previous stack
master does not resume its role as stack master.
As described in the hardware installation guide, you can use the Master LED on the switch to see if the
switch is the stack master.
Switch Stack Bridge ID and Router MAC Address
The bridge ID and router MAC address identify the switch stack in the network. When the switch stack
initializes, the MAC address of the stack master determines the bridge ID and router MAC address.
If the stack master changes, the MAC address of the new stack master determines the new bridge ID and
router MAC address. However, when the persistent MAC address feature is enabled, the stack MAC
address changes in approximately 4 minutes. During this time period, if the previous stack master rejoins
the stack, the stack continues to use its MAC address as the stack MAC address, even if the switch is
now a stack member and not a stack master. If the previous stack master does not rejoin the stack during
this period, the switch stack takes the MAC address of the new stack master as the stack MAC address.
See Enabling Persistent MAC Address, page 5-20 for more information.
Stack Member Numbers
The stack member number (1to 9) identifies each member in the switch stack. The member number also
determines the interface-level configuration that a stack member uses. You can display the stack member
number by using the show switch user EXEC command.
A new, out-of-the-box switch (one that has not joined a switch stack or has not been manually assigned
a stack member number) ships with a default stack member number of 1. W hen it joins a switch stack,
its default stack member number changes to the lowest available member number in the stack.