D-Link Xstack DES-3528 Series Reference Manual page 39

xStack® DES-3528/DES-3552 Series Layer 2 Managed Stackable Fast Ethernet Switch Web UI Reference Guide
Backup Master – The Backup Master is the backup to the Primary Master, and will take over the functions of the
Primary Master if the Primary Master fails or is removed from the Stack. It also monitors the status of
neighboring switches in the stack, will perform commands assigned to it by the Primary Master and will monitor
the running status of the Primary Master. The Backup Master can be set by the user by assigning this Switch
the second highest priority before physically assembling the stack, or it can be determined automatically by the
stack through an election process which determines the second lowest MAC address and then will assign that
switch as the Backup Master, if all priorities are the same.
Slave – Slave switches constitute the rest of the switch stack and although not Primary or Backup Masters, they
can be placed into these roles when these other two roles fail or are removed from the stack. Slave switches
perform operations requested by the master, monitor the status of neighbor switches in the stack and the stack
topology and adhere to the Backup Master's commands once it becomes a Primary Master. Slave switches will
do a self-check to determine if it is to become the Backup Master if the Backup Master is promoted to the
Primary Master, or if the Backup Master fails or is removed from the switch stack. If both Primary and Backup
masters fail, or are removed from the Switch stack, it will determine if it is to become the Primary Master. These
roles will be determined, first by priority and if the priority is the same, the lowest MAC address.
Once switches have been assembled in the topology desired by the user and powered on, the stack will undergo
three processes until it reaches a functioning state.
Initialization State – This is the first state of the stack, where the runtime codes are set and initialized and the
system conducts a peripheral diagnosis to determine each individual switch is functioning properly.
Master Election State – Once the codes are loaded and initialized, the stack will undergo the Master Election
State where it will discover the type of topology used, elect a Primary Master and then a Backup Master.
Synchronization State – Once the Primary Master and the Backup Master have been established, the Primary
Master will assign Stacking Unit IDs to switches in the stack, synchronize configurations for all switches and
then transmit commands to the rest of the switches based on the users configurations of the Primary Master.
Once these steps have been completed, the switch stack will enter a normal operating mode.
Stack Switch Swapping
The stacking feature of the Switch supports "hot swapping" of switches in and out of the running stack. Users may
remove or add switches to the stack without powering down or largely affecting the transfer of data between
switches in the stack, with a few minor provisions.
When switches are "hot inserted" into the running stack, the new switch may take on the Primary Master, Backup
Master or Slave role, depending on configurations set on the newly added switch, such as configured priority or
MAC address. Yet, if adding two stacks together that have both previously undergone the election process, and
therefore both have a Primary Master and a Backup master, a new Primary Master will be elected from one of the
already existing Primary Masters, based on priority or MAC address. This Primary Master will take over all of the
Primary Master's roles for all new switches that were hot inserted. This process is done using discovery packets
that circulate through the switch stack every 1.5 seconds until the discovery process has been completed.
The "hot remove" action means removing a device from the stack while the stack is still running. The hot removal is
detected by the stack when it fails to receive heartbeat packets during its specified interval from a device, or when
one of the stacking ports links is down. Once the device has been removed, the remaining switches will update
their stacking topology database to reflect the change. Any one of the three roles, Primary Master, Backup Master
or Slave, may be removed from the stack, yet different processes occur for each specific device removal.
If a Slave device has been removed, the Primary Master will inform other switches of the hot remove of this device
through the use of unit leave messages. Switches in the stack will clear the configurations of the unit removed, and
dynamically learned databases, such as ARP, will be cleared as well.
If the Backup Master has been hot removed, a new Backup Master will be chosen through the election process
previously described. Switches in the stack will clear the configurations of the unit removed, and dynamically
learned databases, such as ARP, will be cleared as well. Then the Backup Master will begin backing up the
Primary Master when the database synchronization has been completed by the stack.
If the Primary Master is removed, the Backup Master will assume the Primary Master's role and a new Backup
Master will be chosen using the election process. Switches in the stack will clear the configurations of the unit
removed, and dynamically learned databases, such as ARP, will be cleared as well. The new Primary Master will
inherit the MAC and IP address of the previous Primary Master to avoid conflict within the stack and the network
itself.
If both the Primary Master and the Backup Master are removed, the election process is immediately processed and
a new Primary Master and Backup Master is determined. Switches in the stack will clear the configurations of the
units removed, and dynamically learned databases, such as ARP, will be cleared as well. Static switch
configurations still remain in the database of the remaining switches in the stack and those functions will not be
affected.
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