Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 113
53-1002269-02
Configuring STP related features
6
• 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.
802.1W state machines attempt to quickly place the ports into either a forwarding or discarding
state. Root ports are quickly placed in forwarding state when both of the following events occur:
• It is assigned to be the Root port.
• It receives an RST BPDU with a proposal flag from a Designated port. The proposal flag is sent
by ports with a Designated role when they are ready to move into a forwarding state.
When a the role of Root port is given to another port, the old Root port is instructed to reroot. The
old Root port goes into a discarding state and negotiates with its peer port for a new role and a new
state. A peer port is the port on the other bridge to which the port is connected. For example, in
Figure 5, Port1 of Switch 200 is the peer port of Port2 of Switch 100.
A port with a Designated role is quickly placed into a forwarding state if one of the following occurs:
• The Designated port receives an RST BPDU that contains an agreement flag from a Root port
• The Designated port is an Edge port
However, a Designated port that is attached to an Alternate port or a Backup port must wait until
the forward delay timer expires twice on that port while it is still in a Designated role, before it can
proceed to the forwarding state.
Backup ports are quickly placed into discarding states.
Alternate ports are quickly placed into discarding states.
A port operating in 802.1W mode may enter a learning state to allow MAC entries to be added to
the filtering database; however, this state is transient and lasts only a few milliseconds, if the port
is operating in 802.1W mode and if the port meets the conditions for rapid transition.
Handshake mechanisms
To rapidly transition a Designated or Root port into a forwarding state, the Port Role Transition state
machine uses handshake mechanisms to ensure loop free operations. It uses one type of
handshake if no Root port has been assigned on a bridge, and another type if a Root port has
already been assigned.
Handshake when no root port is elected
If a Root port has not been assigned on a bridge, 802.1W uses the Proposing -> Proposed -> Sync
-> Synced -> Agreed handshake: