A critical aspect of managing a network switch stack is understanding the concept of the “Auto Slave” role. While not a formal term, “auto slave” refers to a switch within a stack that automatically assumes a subordinate role to the elected master switch. This article delves into the stack master election process, outlining the criteria that determine which switch becomes the master and, consequently, which switches become auto slaves.
How is the Stack Master Elected?
The stack master is crucial for managing and controlling the entire stack. Several factors determine which switch is elected:
- Incumbency: The current stack master has a significant advantage in retaining its role. This ensures stability and prevents frequent leadership changes.
- Priority: Administrators can manually configure the stack member priority. The switch with the highest priority value is favored to become master. This allows for controlled selection and ensures the desired switch leads the stack. Cisco recommends assigning the highest priority to your preferred master switch.
- Configuration: A switch with a non-default interface-level configuration takes precedence over switches with default settings. This indicates a more specialized role and potentially greater importance within the network.
- Hardware/Software Priority: Switches running more advanced software images, such as cryptographic IP services, have higher priority. This reflects the greater processing power and capabilities of these images.
- System Uptime: If other factors are equal, the switch with the longest uptime is preferred. This suggests stability and reliability.
- MAC Address: As a last resort, the switch with the lowest MAC address is chosen. This acts as a tiebreaker when all other criteria are equal. However, relying on MAC addresses isn’t recommended for predictable master election.
When Does Stack Master Election Occur?
Several events trigger a stack master election:
- Full Stack Reset: A complete reset of the stack initiates a new election.
- Master Switch Reset/Power Off: Resetting or powering off the master switch forces the remaining switches to elect a new leader. Note that resetting the master often resets the entire stack.
- Master Switch Removal/Failure: Removing or experiencing a failure on the master switch necessitates a new election.
- Stack Expansion: Adding new switches to an existing stack triggers a re-election to incorporate the new members.
In scenarios where the entire stack or the master switch is reset, the current master has a higher likelihood of being re-elected due to incumbency. However, best practice dictates configuring priority to ensure the desired switch consistently assumes the master role, relegating other switches to the auto slave function. This eliminates reliance on less predictable factors like MAC addresses. Properly managing the stack master election process ensures network stability and predictable performance.
