Appendix B | About Switch Stacking |
the Console port, Telnet or
The unit that is assigned number 1 will act as the Master; this is indicated by the Stack Master LED on its front panel being lit amber. The unit that is assigned number 2 will act as the Backup Master.
Stack Resiliency
A stack’s topology may be either Ring or Chain. The best practice is to configure the stack in Ring topology, due to its higher resiliency in case of unit failure or stacking link failure.
Additionally, if a redundant power supply is used, it is recommended to make sure that the Master and Backup Master units are connected to the redundant power supply.
Advanced Stacking
In order to understand the operation of the stack it is necessary to understand two key concepts:
•Unit IDs, and how they are allocated
•Stack unit start up process
Unit IDs
Each unit in a stack has an assigned unique Unit ID number. Unit ID numbers are meaningful as follows:
•The unit that is assigned Unit ID=1 will serve as the Stack Master. All other units will be stack members.
The stack Master provides a single point of control, configuration and management for the entire stack, and stores the configuration for all stack members (which themselves do not store any configuration information at all).
•The unit that is assigned Unit ID=2 is a special stack member, which serves as the Stack Backup Master.
A stack Backup Master, in addition to being a stack member, serves as a Backup in case the stack Master fails or is disconnected for any reason. If that should happen, the Backup Master takes over the role of stack Master for the remaining stack members.
To make this possible, the stack Master will store a copy of the active configuration on the Backup Master, but this copy will only be used if and when it takes over the role of stack Master. Note that only the configuration file is copied. Any dynamically filled tables, e.g. addresses learned, are not copied from the Master to the Backup Master. If the Backup Master takes over the role of stack Master, it will start building its own dynamic tables from scratch.
•Units that are assigned Unit IDs
A stack member will only operate as a member of the stack under the direction of an operational stack Master (or a Backup Master that has taken over the role).
Stack members are not directly manageable and configurable, and must be managed through the stack Master, and do not contain any meaningful configuration information (not even their own configuration). If an operational master is not present and reachable, these units will not be functional.
•Master Enabled units
Units that are assigned a unit ID number of 1 or 2 are calledmasterenabledunits.Onlymasterenabledunits participate in master election (see below) whenever they are initiated, inserted into a new stack or lose connectivity with the existing master. Only master enabled units can become the stack master or backup master. Units with assigned IDs of
Unit ID Allocation
Units are shipped from the factory with no Unit ID, and must be assigned a unique Unit ID before they can operate as part of a stack. Unit ID numbers are assigned to units in one of two ways:
•Assigned by the system administrator, in which case they can only be changed manually by the system administrator
•Allocated to a stack member unit by the stack Master during system initialization
In general, a unit that was assigned a Unit ID will tend to keep this number even after it is rebooted. The stack Master may reallocate Unit IDs during system initialization to resolve duplicate Unit ID conflicts (see below). Manually assigned Unit IDs cannot be changed by the stack Master, even if they are in conflict.
Unit ID assignment/change takes effect only during system initialization and does not take place during run- time of the system.
Units of a stack do not have to be numbered in sequence or in order, and may be interconnected as desired, as long as each unit has a unique ID and at least one unit of the stack serves as stack Master.
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