Rapid Spanning Tree Protocol (802.1w)

Port roles

1. Root port – STP path that is “shortest” to root bridge.

2. Designated port – Port that is forwarding and is not a root port.

3. Alternate port – Backup root port. Present the alternate path to the root bridge.

4. Backup port – Backup designated port.

Bridge Protocol Data Unit

1. Type 2 (or version 2) BPDU is used, this is to make RSTP enabled switch to distinguish STP BPDU easily.

2. BPDU is generated by every RSTP enabled switches, BPDU works like a keep alive among switches. BPDU is generated every 2 seconds by default.

3. An inferior BPDU contains RSTP information that is worse than the currently stored BPDU.

4. A superior BPDU contains RSTP information that is better than the currently stored BPDU.

5. If a switch receives a superior BPDU on its port, the port becomes root/designated port.

6. A switch considers its neighbour switch “dead” when the switch does not receive BPDU for 3 consecutive times. In other words, if no BPDU is received from neighbour for 3 times, stored BPDU will be discarded.

Simplified port states

1. Discarding state – Combines blocking and listening states of 802.1D together. No mac address is learned, no frames is sent or received, only BPDU is exchanged. (802.1w)Discarding state = (802.1D)Disabled + Blocking + Listening.

2. Learning state – Learn mac address, no data frames is received or sent. BPDU is exchanged.

3. Forwarding state – Learn mac address, data frames is sent and received. BPDU is exchanged.

Proposal and Agreement

Reference: http://blog.ine.com/wp-content/uploads/2010/04/understanding-stp-rstp-convergence.pdf

This is the best RSTP document I have found.

The huge difference between STP and RSTP is this proposal and agreement mechanism.

Switch A sends proposal downstream to two switches B and C. While sending proposal to the switches Switch A ports are in designated blocking state.

Switch B and C ports received a superior BPDU from switch A, transit their port to root port.

Switch B and C sends agreement to Switch A to allow Switch A ports to transit to designated forwarding state, while during this time Switch B and C block their downstream designated port to prevent bridging loop. This process is known as sync, switch will block downtream non-edge designated port during sync process.

Switch B then sends proposal to switch C. Switch C port received a superior BPDU, changes its port to alternate port and blocking state. Switch B does not receive agreement from Switch C will transit its port to designated forwarding state.


*Mar  1 09:36:47.822: RSTP(1): initializing port Fa0/1
*Mar  1 09:36:47.822: RSTP(1): Fa0/1 is now designated
*Mar  1 09:36:47.830: RSTP(1): transmitting a proposal on Fa0/1
*Mar  1 09:36:47.838: RSTP(1): updt roles, received superior bpdu on Fa0/1
*Mar  1 09:36:47.838: RSTP(1): Fa0/1 is now root port
*Mar  1 09:36:47.838: RSTP(1): Po1 blocked by re-root
*Mar  1 09:36:47.838: RSTP(1): synced Fa0/1
*Mar  1 09:36:47.838: RSTP(1): Po1 is now designated
*Mar  1 09:36:47.847: RSTP(1): updt roles, received superior bpdu on Po1
*Mar  1 09:36:47.847: RSTP(1): synced Fa0/1
*Mar  1 09:36:47.847: RSTP(1): Po1 is now alternate
*Mar  1 09:36:47.847: RSTP(1): transmitting an agreement on Fa0/1 as a response to a proposal
*Mar  1 09:36:48.828: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
Sw2-3560#

Topology change

1. ONLY non-edge port changing to forwarding state will trigger a topology change.

2. RSTP enabled switches do not use topology change notification BPDU anymore, this is only used to be backward compatible with STP enabled switches.

3. The era for a switch to notify root bridge about topology change and root bridge informs all switches about the changes is over. When a RSTP enabled switch detects topology change, the RSTP enabled switch sets TC-while timer to 2x hello timer.

4. RSTP enabled switch sends BPDU to its neighbour with TC bit set.

5. RSTP enabled switch received BPDU with TC bit set flushes their mac address except for edge port associated mac address.

6. This process will have BPDU flooding until all mac addresses are re-learned, the proposal and agreement process will be involved as well.

Max Age and Forward delay

These parameters are practically useless to RSTP, they exist to accommodate possible legacy switches that only understand STP.

Risk

For small network using RSTP is relatively fast…As network grows like a 7 switch network, RSTP recovery time varies according to which link has failed. A RSTP race condition may occur which has risk of counting to infinity should the root bridge fails. For RSTP to be less problematic keep the RSTP topology small and with as minimum number of redundant path as possible. A better solution is not to use RSTP for huge switch network arranging in ring, use protocol such as Media Redundancy Protocol (MRP, developed by Siemens and Hirschmann), Hiper Ring (Hirschmann), Resilient Ethernet Protocol (REP, Cisco).

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