Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 277)

If a port on one bridge has a Designated role and that port is connected to a port on another bridge that
has an Alternate or Backup role, the port with a Designated role cannot be given a Root port role until
two instances of the forward delay timer expires on that port.
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.
Port Role Selection 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.
In contrast to the 802.1D standard, the 802.1W standard does not have any bridge specific timers. All
timers in the CLI are applied on a per-port basis, even though they are configured under bridge
parameters.
Edge port and non-edge port states
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