Dell PowerConnect B-FCXs, Configuration Manual

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PowerConnect B-Series FCX Configuration Guide 233
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Configuring STP related features
8
Edge port and non-edge port states
As soon as a port is configured as an Edge port using the CLI, it goes into a forwarding state
instantly (in less than 100 msec).
When the link to a port comes up and 802.1W detects that the port is an Edge port, that port
instantly goes into a forwarding state.
If 802.1W detects that port as a non-edge port, the port state is changed as determined by the
result of processing the received RST BPDU. The port state change occurs within four seconds of
link up or after two hello timer expires on the port.
Changes to port roles and states
To achieve convergence in a topology, a port role and state changes as it receives and transmits
new RST BPDUs. Changes in a port role and state constitute a topology change. Besides the
superiority and inferiority of the RST BPDU, bridge-wide and per-port state machines are used to
determine a port role as well as a port state. Port state machines also determine when port role
and state changes occur.
State machines
The bridge uses the Port Role Selection state machine to determine if port role changes are
required on the bridge. This state machine performs a computation when one of the following
events occur:
•
New information is received on any port on the bridge
•
The timer expires for the current information on a port on the bridge
Each port uses the following state machines:
•
Port Information – This state machine keeps track of spanning-tree information currently used
by the port. It records the origin of the information and ages out any information that was
derived from an incoming BPDU.
•
Port Role Transition – This state machine keeps track of the current port role and transitions
the port to the appropriate role when required. It moves the Root port and the Designated port
into forwarding states and moves the Alternate and Backup ports into discarding states.
•
Port Transmit – This state machine is responsible for BPDU transmission. It checks to ensure
only the maximum number of BPDUs per hello interval are sent every second. Based on what
mode it is operating in, it sends out either legacy BPDUs or RST BPDUs. In this document
legacy BPDUs are also referred to as STP BPDUs.
•
Port Protocol Migration – This state machine deals with compatibility with 802.1D bridges.
When a legacy BPDU is detected on a port, this state machine configures the port to transmit
and receive legacy BPDUs and operate in the legacy mode.
•
Topology Change – This state machine detects, generates, and propagates topology change
notifications. It acknowledges Topology Change Notice (TCN) messages when operating in
802.1D mode. It also flushes the MAC table when a topology change event takes place.
•
Port State Transition – This state machine transitions the port to a discarding, learning, or
forwarding state and performs any necessary processing associated with the state changes.
•
Port Timers – This state machine is responsible for triggering any of the state machines
described above, based on expiration of specific port timers.