HP ProCurve 9304M Installation And Configuration Manual page 234

Installation and Basic Configuration Guide
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.
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 8.4, Port1 of Routing Switch 200 is the
peer port of Port2 of Routing 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:
• Proposing – The Designated port on the root bridge sends an RST BPDU packet to its peer port that contains
a proposal flag. The proposal flag is a signal that indicates that the Designated port is ready to put itself in a
forwarding state (Figure 8.4). The Designated port continues to send this flag in its RST BPDU until it is
placed in a forwarding state (Figure 8.7) or is forced to operate in 802.1D mode. (See "Compatibility of
802.1W with 802.1D" on page 48.)
• Proposed – When a port receives an RST BPDU with a proposal flag from the Designated port on its point-to-
8 - 28