RSTP Convergence

• Convergence of RSTP is a sequence of handshakes (proposal and agreement)  propagated over point-to-point links.
  • When a switch needs to make an STP decision, a handshake is made with its nearest neighbor.
  • When that is successful, the handshake sequence is moved to the next switch and the next and moves on the rest of the spanning tree towards the edge.
• During the handshake sequence the switch takes proper steps to make sure loops are prevented before moving on to the next handshake sequence.

Example

Step 1

• Assume in figure 1 that the topology on the left is a non-redundant topology.
• An administrator decides to add a link between the Root and Switch A for redundancy.
• As soon as the links come up, the ports on the link between A and the root are put in blocking.

Figure 1a: Step 1 

Proposal and Agreement

• A negotiation (handshake) sequence is started between A and the root.
  • Both bridges exchange BPDUs.
• As soon as A receives the BPDU of the root, non-edge designated ports on A are blocked.
• This process is called synchronization.
• The following diagram illustrates the handshake sequence between A and the Root bridge:

Figure 1b: Proposal/Agreement Between Switch A and Root Bridge

1. In the figure above, when the new link is created between the Root and Switch A, both ports are initially put on blocking state. 
   • They both start in designate blocking status and both sends BPDUs with the proposal bit.
   • Port p0 of the Root bridge sends a superior BPDU.
2. Port  p1 of Switch A immediately knows that it is the new root port when it receives the superior information. Switch A starts a sync to verify that all of its ports are in-sync* with this new information.
   • Switch A proceeds to block the other non-edge designate ports (p2 and p3).
3. Switch A sends a BPDU back to the Root bridge - this time with the agreement bit set.
   • Switch A then tells the root bridge that it can go ahead and put its port in forwarding status.

*A port is in sync if it meets either of these criteria:

• The port is in blocking state, which means discarding in a stable topology.
• The port is an edge port. 

Step 2

• After the links between the Root and Switch A has converged, a new negotiation sequence is started on the links between Switch A’s neighbors – B and C.
• As you can see on the diagram on Figure 2, instead of blocking on the segment above Switch A, the network now blocks below Switch A.  

Figure 2: RSTP Convergence Step 2

• At this stage, the same proposal/agreement sequence discussed above happens between Switch A and Switch B.
  • Because there is no other non-edge designated ports (assume that the nodes on bottom of Switch A are edge devices) on Switch B, it has no ports to block in order to authorize Switch A to go to the forwarding state.
  • It immediately transitions to forwarding state.
• Switch A and Switch C initiate the same sync operation.
  • This time, when Switch C receives the proposal bit from Switch A along with the superior BPDU.
  • Switch C blocks its non-edge designated port (link between C and D)
  • Switch C then sends agreement BPDU to A and tells it to start forwarding.

Step 3

• The end of convergence process ends here, where the port on D stays blocking.
• The time it takes for the whole operation to complete happens within the speed of one BPDU transmission.

Figure 3: Final Topology

In Summary…

• RSTP convergence starts of with the switches sending proposals BPDUs.
• The recipient of the proposal makes sure it is synchronized by putting all its non-edge designated ports in blocking, effectively isolating itself from the rest of the topology.
• These blocked ports also send proposal messages to their nearest neighbors and those neighbors perform the synchronization operation themselves.
• This action creates a sort of moving “wave” of switches synchronizing all the way down the tree.
• Each “wave” independently makes decisions whether to forward or not, depending on the agreement message received from the neighboring switch. This way, the transition happens rapidly as opposed to waiting for the BPDU message to propagate to the whole topology before making a decision which port will ultimately by blocking and forwarding.

Topology Change

• With 802.1D, when a topology change is detected:
  • The switch that detects the change sends a Topology Change Notification to the root.
  • The Root sends a Configuration BPDU with the TC flag bit set towards the rest of the topology.
  • The switches down the tree ages out their CAM table (MAC address table) in 15 seconds (forward-delay time) to quickly delete old information.
• In 802.1w, topology change is only detected when a non-edge port transitions to the Forwarding State.
  • A port moving to blocking does not generate a TC BPDU.
• If a topology change is detected:
  1. A switch sends out BPDUs with their TC bit set out to all non-edge designated ports.
     • This happens for the duration of the TC While timer, which is 2 times the Hello interval.
  2. MAC addresses associated with the non-edge designated ports are flushed from the CAM table, thereby forcing the bridges to re-learn and re-populate its CAM database.
     • The port that received the TC message does not have to clear its learned MAC address.
  3. This process happens for every switch that receives the TC message. Each receiving switch starts the TC While timer and must also send out TC messages out their non-edge designated ports.
  4. While the TC While timer is active, the switches send BPDUs even on the root port. 
• This process floods the TCN to the entire much more quickly because the initiator immediately sends the TC information to the network instead of waiting for the Root bridge to take care of the notification. 

802.1D Compatibility

• In the event a designated port sends a proposal message and does not receive an agreement reply, it reverts to using the 802.1D rules of convergence – it mus wait the forward delay time before forwarding.
• 802.1D cannot understand 802.1w.
• If an RSTP switch receives an STP BPDU, the RSTP switch changes its mode to 802.1D on the port where it receives the STP BPDU. If the STP switch is removed from the topology, an administrator must re-configure the port manually back to RSTP – that is because it doesn’t know whether the STP switch is still present or not.

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BCMSN course, consider purchasing Building Cisco Multilayer Switched Networks (BCMSN) (Authorized Self-Study Guide) (4th Edition)by Richard Froom, Balaji Sivasubramanian, and Erum Frahim and CCNP BCMSN Official Exam Certification Guide (4th Edition) by Dave Hucaby ; as well as following the links on the reference section of this entry.