P/A Mechanism
The Proposal/Agreement (P/A) mechanism helps a designated port to enter the Forwarding state as soon as possible. As shown in Figure 1, the P/A negotiation is performed based on the following port variables:
Figure 1 BPDU exchange during the P/A negotiation
proposing: When a port is in the Discarding or Learning state, this variable is set to 1. Additionally, an RST BPDU with the Proposal field being 1 is sent to the downstream switching device.
proposed: After a port receives an RST BPDU with the Proposal field being 1 from the designated port on the peer device, this variable is set to 1, urging the designated port on this network segment to enter the Forwarding state.
sync: After the proposed variable is set to 1, the root port receiving the proposal sets the sync variable to 1 for the other ports on the same device; a non-edge port receiving the proposal enters the Discarding state.
synced: After a port enters the Discarding state, it sets its synced variable to 1 in the following manner: If this port is the alternate, backup, or edge port, it will immediately set its synced variable to 1. If this port is the root port, it will monitor the synced variables of the other ports. After the synced variables of all the other ports are set to 1, the root port sets its synced variable to 1, and sends an RST BPDU with the Agreement field being 1.
agreed: After the designated port receives an RST BPDU with the Agreement field being 1 and the port role field indicating the root port, this variable is set to 1. Once the agreed variable is set to 1, this designated port immediately enters the Forwarding state.
Figure 2 Schematic diagram for the P/A negotiation
As shown in Figure 2, a new link is established between the root bridges S1 and S2. On S2, p2 is an alternate port; p3 is a designated port in the Forwarding state; p4 is an edge port. The P/A mechanism works in the following process:
p0 and p1 become designated ports and send RST BPDUs.
After receiving an RST BPDU with a higher priority, p1 realizes that it will become a root port but not a designated port, and thus it stops sending RST BPDUs.
p0 enters the Discarding state, and sends RST BPDUs with the Proposal field being 1.
After receiving an RST BPDU with the Proposal field being 1, S2 sets the sync variable to 1 for all its ports.
As p2 has been blocked, its status keeps unchanged; p4 is an edge port, and thus it does not participate in calculation. Therefore, only the non-edge designated port p3 needs to be blocked.
After p2, p3, and p4 enter the Discarding state, their synced variables are set to 1. The synced variable of the root port p1 is then set to 1, and p1 sends an RST BPDU with the Agreement field being 1 to S1. Except for the Agreement field, which is set to 1, and the Proposal field, which is set to 0, the RST BPDU is the same as that was received.
After receiving this RST BPDU, S1 identifies it as a reply to the proposal that it just sent, and thus p0 immediately enters the Forwarding state.
This P/A negotiation process finishes, and S2 continues to perform the P/A negotiation with its downstream device.
Theoretically, STP can quickly select a designated port. To prevent loops, STP has to wait for a period of time long enough to determine the status of all ports on the network. All ports can enter the Forwarding state at least one forward delay later. RSTP is developed to eliminate this bottleneck by blocking non-root ports to prevent loops. By using the P/A mechanism, the upstream port can rapidly enter the Forwarding state.
To use the P/A mechanism, ensure that the link between the two devices is a P2P link in full-duplex mode. Once the P/A negotiation fails, a designated port can be selected by performing the STP negotiation after the forwarding delay timer expires twice.
In RSTP, if a non-edge port changes to the Forwarding state, the topology changes.
After a switching device detects the topology change (TC), it performs the following procedures:
Start a TC While Timer for every non-edge port. The TC While Timer value doubles the Hello Timer value.
All MAC addresses learned by the ports whose status changes are cleared before the timer expires.
These ports send RST BPDUs with the TC field being 1. Once the TC While Timer expires, they stop sending the RST BPDUs.
After another switching device receives the RST BPDU, it clears the MAC addresses learned by all ports excluding the one that receives the RST BPDU. The device then starts a TC While Timer for all non-edge ports and the root port, the same as the preceding process.
In this manner, RST BPDUs flood the network.
Interoperability Between RSTP and STP
When RSTP switches to STP, RSTP loses its advantages such as fast convergence.
On a network where both STP-capable and RSTP-capable devices are deployed, STP-capable devices ignore RST BPDUs; if a port on an RSTP-capable device receives a configuration BPDU from an STP-capable device, the port switches to the STP mode after two Hello intervals and starts to send configuration BPDUs. In this manner, RSTP and STP are interoperable.
After STP-capable devices are removed, Huawei RSTP-capable datacom devices can switch back to the RSTP mode.
Related post Evolution from STP to RSTP
