298 G8264 Application Guide for ENOS 8.4Stack MembershipA stack contains up to eight switches, interconnected by a stack LAG in a local ringtopology (see Figure 25 on page 304). With this topology, only a single stack linkfailure is allowed.An operational stack must contain one Master and one or more Members, asfollows: MasterOne switch controls the operation of the stack and is called the Master. TheMaster provides a single point to manage the stack. A stack must have one andonly one Master. The firmware image, configuration information, and run‐timedata are maintained by the Master and pushed to each switch in the stack asnecessary. MemberMember switches provide additional port capacity to the stack. Membersreceive configuration changes, run‐time information, and software updatesfrom the Master. BackupOne member switch can be designated as a Backup to the Master. The Backuptakes over control of the stack if the Master fails. Configuration information andrun‐time data are synchronized with the Master.The Master SwitchAn operational stack can have only one active Master at any given time. In anormal stack configuration, one switch is configured as a Master and all others areconfigured as Members.When adding new switches to an existing stack, the administrator must explicitlyconfigure each new switch for its intended role as a Master (only when replacing aprevious Master) or as a Member. All stack configuration procedures in thischapter depict proper role specification.However, although uncommon, there are scenarios in which a stack maytemporarily have more than one Master switch. If this occurs, the switch with thelowestMAC address will be chosen as the active Master for the entire stack. Theselection process is designed to promote stable, predictable stack operation andminimize stack reboots and other disruptions.Splitting and Merging One StackIf stack links or Member switches fail, any Member which cannot access either theMaster or Backup is considered isolated and will not process network traffic (see“No Backup” on page 301). Members which have access to a Master or Backup (orboth), despite other link or Member failures, will continue to operate as part oftheir active stack. A Member that is isolated due to link failure resets itself. After itis up, if the link failure still exits, the Member stays in isolated state keeping all itsdata links disabled. Only the management and stacking links are enabled. If theMember was not configured when it went to isolated state, the Master pushes theconfiguration when the Member joins back the stack.
