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.
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 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 Proposing and Proposed Stage, 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:
- 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 (Proposing and Proposed Stage). The Designated port continues to send this flag in its RST BPDU until it is placed in a forwarding state (Agree Stage) or is forced to operate in 802.1D mode. (Refer to Compatibility of 802.1W with 802.1D).
- Proposed: When a port receives an RST BPDU
with a proposal flag from the Designated port on its point-to-point link, it
asserts the Proposed signal and one of the following occurs (Proposing and Proposed Stage):
- If the RST BPDU that the port receives is superior to what it can transmit, the port assumes the role of a Root port. (Refer to the section on Bridges and Bridge Port Roles .)
- If the RST BPDU that the port receives is inferior to what it can transmit, then the port is given the role of Designated port.
In the following figure, Port3/Switch 200 is elected as the Root port.
- Sync: Once the Root port is elected, it sets a sync signal on all the ports on the bridge. The signal tells the ports to synchronize their roles and states (Sync Stage). Ports that are non-edge ports with a role of Designated port change into a discarding state. These ports have to negotiate with their peer ports to establish their new roles and states.
- Synced: Once the Designated port changes into a discarding state, it asserts a synced signal. Immediately, Alternate ports and Backup ports are synced. The Root port monitors the synced signals from all the bridge ports. Once all bridge ports asserts a synced signal, the Root port asserts its own synced signal as shown in the following figure.
- Agreed: The Root port sends back an RST BPDU containing an agreed flag to its peer Designated port and moves into the forwarding state. When the peer Designated port receives the RST BPDU, it rapidly transitions into a forwarding state.
At this point, the handshake mechanism is complete between Switch 100, the root bridge, and Switch 200.
Switch 200 updates the information on the Switch 200 Designated ports (Port2 and Port3) and identifies the new root bridge. The Designated ports send RST BPDUs, containing proposal flags, to their downstream bridges, without waiting for the hello timers to expire on them. This process starts the handshake with the downstream bridges.
For example, Port2/Switch 200 sends an RST BPDU to Port2/Switch 300 that contains a proposal flag. Port2/Switch 300 asserts a proposed signal. Ports in Switch 300 then set sync signals on the ports to synchronize and negotiate their roles and states. Then the ports assert a synced signal and when the Root port in Switch 300 asserts its synced signal, it sends an RST BPDU to Switch 200 with an agreed flag.
This handshake is repeated between Switch 200 and Switch 400 until all Designated and Root ports are in forwarding states.
Handshake When a Root Port Has Been Elected
If a non-root bridge already has a Root port, 802.1W uses a different type of handshake. For example, in the following figure, a new root bridge is added to the topology.
The handshake that occurs between Switch 60 and Switch 100 follows the one described in the previous section (Handshake When No Root Port is Elected). The former root bridge becomes a non-root bridge and establishes a Root port (New Root Bridge Sending a Proposal Flag).
However, since Switch 200 already had a Root port in a forwarding state, 802.1W uses the Proposing -> Proposed -> Sync and Reroot -> Sync and Rerooted -> Rerooted and Synced -> Agreed handshake:
- Proposing and Proposed: The Designated port on the new root bridge (Port4/Switch 60) sends an RST BPDU that contains a proposing signal to Port4/Switch 200 to inform the port that it is ready to put itself in a forwarding state (New Root Bridge Sending a Proposal Flag). 802.1W algorithm determines that the RST BPDU that Port4/Switch 200 received is superior to what it can generate, so Port4/Switch 200 assumes a Root port role.
- Sync and Reroot: The Root port then asserts a sync and a reroot signal on all the ports on the bridge. The signal tells the ports that a new Root port has been assigned and they are to renegotiate their new roles and states. The other ports on the bridge assert their sync and reroot signals. Information about the old Root port is discarded from all ports. Designated ports change into discarding states as shown in the following figure.
- Sync and Rerooted: When the ports on Switch 200 have completed the reroot phase, they assert their rerooted signals and continue to assert their sync signals as they continue in their discarding states. They also continue to negotiate their roles and states with their peer ports as shown in the following figure.
- Synced and Agree: When all the ports on the bridge assert their synced signals, the new Root port asserts its own synced signal and sends an RST BPDU to Port4/Switch 60 that contains an agreed flag as shown in the following figure. The Root port also moves into a forwarding state.
The old Root port on Switch 200 becomes an Alternate Port as shown in the following figure. Other ports on that bridge are elected to appropriate roles.
The Designated port on Switch 60 goes into a forwarding state once it receives the RST BPDU with the agreed flag.
Recall that Switch 200 sent the agreed flag to Port4/Switch 60 and not to Port1/Switch 100 (the port that connects Switch 100 to Switch 200). Therefore, Port1/Switch 100 does not go into forwarding state instantly. It waits until two instances of the forward delay timer expires on the port before it goes into forwarding state.
At this point the handshake between the Switch 60 and Switch 200 is complete.
The remaining bridges (Switch 300 and Switch 400) may have to go through the reroot handshake if a new Root port needs to be assigned.