Users Guide

Stacking 255
It is recommended that operators provision large stacking topologies such
that it is unlikely that a significant portion of the stack capacity will transit
stacking links. One technique for achieving this is to distribute downlinks and
transit links evenly across the stack vs. connecting all downlinks/transit links
to a single stack unit or to adjacent stacking units.
A single switch in the stack manages all the units in the stack (the
management unit in the stack), and the stack is managed by using a single IP
address. The IP address of the stack does not change, even if the the
management unit in the stack changes.
A stack is created by daisy-chaining stacking links on adjacent units. If
available, up to eight links per stack unit can be used for stacking (four in
each direction). A stack of units is manageable as a single entity when the
units are connected together. If a unit cannot detect a stacking partner on any
port enabled for stacking, the unit automatically operates as a standalone
unit. If a stacking partner is detected, the switch always operates in stacking
mode. One unit in the stack is designated as the management unit in the
stack. The primary manages all the units in the stack. The management unit
in the stack runs the user interface and switch software, and propagates
changes to the member units. To manage a stack using the serial interface,
the administrator must connect to the management unit in the stack via the
connect command or by physically connecting the cable to the management
unit in the stack.
A second switch is designated as the standby unit, which becomes the
primary if the management unit in the stack is unavailable. The unit to be
selected as the standby can be manually configured, or the system can select
the standby automatically.
When units are in a stack, the following activities occur:
• All units are checked for software version consistency.
• The switch Control Plane is active only on the primary. The Control Plane
is a software layer that manages system and hardware configuration and
runs the network control protocols to set system configuration and state.
• The switch Data Plane is active on all units in the stack, including the
primary. The Data Plane is the set of hardware components that forward
data packets without intervention from a control CPU.