974 | Stacking
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Failover Roles
If the stack master fails (e.g., is powered off), it is removed from the stack topology. The standby unit
detects the loss of peering communication and takes ownership of the stack management, switching from
the standby role to the master role. The distributed forwarding tables are retained during the failover, as is
the stack MAC address. The lack of a standby unit triggers an election within the remaining units for a
standby role.
Once the former master switch recovers, despite having a higher priority or MAC address, it will not
recover its master role but instead takes the next available role.
To view failover details, use the
show redundancy command.
MAC Addressing on S-Series Stacks
The S-Series has three MAC addresses: the chassis MAC, interface MAC, and null interface MAC. All
interfaces in the stack use the interface MAC address of the management unit, and the chassis MAC for the
stack is the master’s chassis MAC. The stack continues to use the master’s chassis MAC address even after
a failover. The MAC address is not refreshed until the stack is reloaded and a different unit becomes the
stack manager.
In Figure 47-2 and Figure 47-3, a standalone is added to a stack. The standalone and the master unit have
the same priority, but the standalone has a lower MAC address, so the standalone reboots. In Figure 47-3, a
standalone is added to a stack. The standalone has a higher priority than the stack, so the stack (excluding
the new unit) reloads.
Note: If the removed management unit is brought up as a standalone unit or as part of a different stack,
there is a possibility of MAC address collisions.
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