BPDU exchange between RSTP switches

Question

According to the documentation:

BPDU are sent every hello-time, and not simply relayed anymore. With 802.1D, a non-root bridge only generates BPDUs when it receives one on the root port. In fact, a bridge relays BPDUs more than it actually generates them. This is not the case with 802.1w. A bridge now sends a BPDU with its current information every seconds (2 by default), even if it does not receive any from the root bridge.

But what about the BPDU exchange between two non-root RSTP switches?
Let's say we have a 3 switch star topology like this:
.

Will the segment between SW2 and SW3 exchange BPDUs every 2 seconds?
Are the non-root bridges going to send BPDUs to the root bridge in RSTP?

Answer 1

In RSTP when the topology is synchronized, the non-root switches will not exchange BPDUs unless one of the links change. If that happens, a new synchronization process starts.

The synchronization process is as follows:

RSTP protocol’s fast convergence depends on the use of point-to-point links connecting switches. In order to quickly transition a designated port into non-discarding state, the upstream switch needs to make sure that the downstream neighbor agrees with that idea. This constitutes the process known as handshake (or proposal/agreement):

1. Upstream bridge sends a proposal out of a designated port. As a matter of fact, it just sets the proposal bit in outgoing configuration BPDUs.
2. Downstream bridge receives the proposal, and if it agrees with the upstream port role, it starts the process known as synchronization.
3. Synchronization implies the downstream bridge blocking all non-edge designated ports, prior to sending an agreement to the upstream bridge.
4. Synchronization is needed to make sure there are no loops in the topology, after the upstream bridge unblocks its designated port.
5. If the downstream bridge does not agree with the proposal, it will continues sending it’s own configuration BPDUs with the proposal bit set. Eventually one of the bridges will accept the superior information and send an agreement.

There's a great document explaining the process of the different types of STP: http://blog.ine.com/wp-content/uploads/2010/04/understanding-stp-rstp-convergence.pdf

Answer 2

Actually a STP/RSTP blocked port requires BPDUs to be received, otherwise it will become unblocked. Here's the output of a show command on one of my production switches, which is a blocked port for this vlan. I typed these two commands a couple of minutes apart, and you can that the received BPDUs incremented by 70 or so:

SW1# sh spann int f0/24 det | sec VLAN003

Port 24 (FastEthernet0/24) of VLAN0033 is alternate blocking
   Port path cost 100, Port priority 128, Port Identifier 128.24.
   Designated root has priority 4129, address 5c83.8f39.c280
   Designated bridge has priority 4129, address 5c83.8f39.c280
   Designated port id is 128.8, designated path cost 0
   Timers: message age 16, forward delay 0, hold 0
   Number of transitions to forwarding state: 0
   Link type is point-to-point by default
   Loop guard is enabled by default on the port
   BPDU: sent 2, received **48426**


SW1# sh spann int f0/24 det | sec VLAN003

Port 24 (FastEthernet0/24) of VLAN0033 is alternate blocking
   Port path cost 100, Port priority 128, Port Identifier 128.24.
   Designated root has priority 4129, address 5c83.8f39.c280
   Designated bridge has priority 4129, address 5c83.8f39.c280
   Designated port id is 128.8, designated path cost 0
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 0
   Link type is point-to-point by default
   Loop guard is enabled by default on the port
   BPDU: sent 2, received **48497**

Answer 3

As said in the standard IEEE 802.1w-2001, 17.7:

"[...] Configuration Messages (BPDU) are transmited if the information to be transmitted by a Designated Port changes. In addition, Designated Ports transmit Configuration Messages at regular intervals to guard against loss and to assist in the detection of failed components (LANs, Bridges or Bridge Ports) [...]"

So only Designated Ports transmit BPDU at regular intervals. Included SW2 and SW3 in your picture.