BigIron RX Series Configuration Guide 35953-1001986-01State machines 13• Topology Change – This state machine detects, generates, and propagates topology changenotifications. It acknowledges Topology Change Notice (TCN) messages when operating in802.1D 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, orforwarding state and performs any necessary processing associated with the state changes.• Port Timers – This state machine is responsible for triggering any of the state machinesdescribed above, based on expiration of specific port timers.In contrast to the 802.1D standard, the RSTP standard does not have any bridge specific timers. Alltimers in the CLI are applied on a per-port basis, even though they are configured under bridgeparameters.RSTP 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 sentby 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. Theold Root port goes into a discarding state and negotiates with its peer port for a new role and a newstate. A peer port is the port on the other bridge to which the port is connected. For example, inFigure 43, 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 portHowever, a Designated port that is attached to an Alternate port or a Backup port must wait untilthe forward delay timer expires twice on that port while it is still in a Designated role, before it canproceed to the forwarding state.Backup ports are quickly placed into discarding states.Alternate ports are quickly placed into discarding states.A port operating in RSTP mode may enter a learning state to allow MAC address entries to be addedto the filtering database; however, this state is transient and lasts only a few milliseconds, if theport is operating in RSTP mode and if the port meets the conditions for rapid transition.Handshake mechanismsTo rapidly transition a Designated or Root port into a forwarding state, the Port Role Transition statemachine uses handshake mechanisms to ensure loop free operations. It uses one type ofhandshake if no Root port has been assigned on a bridge, and another type if a Root port hasalready been assigned.Handshake when no root port is electedIf a Root port has not been assigned on a bridge, RSTP uses the Proposing -> Proposed -> Sync ->Synced -> Agreed handshake:
