STP - How can switches shout "I'm Root" during initialization when in blocking state

[Anthony L]

One question I've often asked myself but haven't been able to find an answer to yet...

When a switch enabled with STP boots up, all switch ports transition through a series of STP port states before finally reaching a forwarding state (for those that can without causing a loop).

Crucially, all these ports start out in the blocking state and lasts for 20 seconds by default.

 Switching, Catalyst 2000

As I've read in numerous networking books on STP that cover the original IEEE 802.1D specification and PVST/PVST+, ports in the blocking STP state receive/process incoming BPDUs to the port, but do not send BPDUs.

These same sources also mention that when switches enabled with STP start up, they will all start claiming to be the root switch by sending out BPDUs that lists their BID in both the Sender BID and Root ID fields.

I can't see how both above points can be true, unless the point at which a switch claims to be the root after boot up occurs during the listening state that follows the blocking state (when a port can both forward and receive BPDUs). Unless this is just an exception to the rule for the blocking state...

Hoping someone can clarify for me please!

Thanks

[Steven Case, MBA-ITM]

It's been a bit, so I hope I can do this well enough.

Here's the point I get from the document here https://www.cisco.com/c/en/us/td/docs/routers/access/3200/software/wireless/SpanningTree.html#wp1040546 :

Blocking State

An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to the bridge's Ethernet and radio ports. A bridge initially functions as the spanning-tree root until it exchanges BPDUs with other bridges.

Also, there is this in the same document:

When the bridges in a network are powered up, each bridge functions as the STP root. The bridges send configuration BPDUs through the Ethernet and radio ports.

So, yes, I believe this fits your exception to the rule assumption.

[Martin L]

well, It's true, Blocking port only listens and does not send any BPDUs but it does not take 20 seconds to go into next state.  It is usually must faster, 2-3 seconds. 

I have seen that 20 sec default value somewhere but I have not catch port being "blocking" state once switch came up.  It might go back into Blocking state once it has chance to figure out its role in topo.

Port must move to Listening state to send its BPDUs out. Unless a port has received Superior BPDU from neighbor, it will start sending its own BPDUs to claim to be new Root Sw. 

see process examples https://community.cisco.com/t5/switching/switch-still-send-bpdu-in-whole-listening-state/td-p/3906335

Regards, ML
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