STP - Blocking State and Electing the Root Switch

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Jul 18th, 2010
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I have been reading various Cisco docs about STP.  The only thing I can't understand so far is how BPDU's are sent to other switches on active interfaces after initialisation when everything I've read so far says that interfaces in the Blocking state (after initialisation) can only receive BPDU's.  Is their some weird and intersting interface state between initialisation and blocking?



Correct Answer by Nagaraja Thanthry about 6 years 10 months ago


When the switches initialize the port and recognize the connections, they

will put the port in listening state. It is this phase where you will

actually hear and send BPDUs.


Hope this helps.



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Nagaraja Thanthry Sun, 07/18/2010 - 21:07
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I am not clear about your question. However, if I understand you right, you

want to know about STP calculations once the network settles down and there

is a change in the topology. When the network initializes, only the ports

that have an active connection (connected to another switch) will

participate in the spanning-tree calculations. Once the topology is

determined, certain ports will put in blocking mode. The blocking ports will

get BPDU from the other end of the connection i.e. if switch 1 and switch 2

are connected via ports 1 and 2 (on both sides), then we see a loop over

there. Let us assume that port 2 on Switch 2 is in blocking mode. That means

Port 2 on the switch 1 (connected to port 2 on switch 2) will be in

forwarding state and is the root port for that LAN segment. So, it will

forward the BPDUs which will be received by the port 2 on Switch 2. In case

the port 1 goes down, spanning tree will recalculate and activate port 2 on

second switch.

Hope this answers your questions.



tbogie_gvds Mon, 07/19/2010 - 00:47
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To put it another way:

STP initialises and then at some point all the active interfaces go into Blocking.  When are Hello BPDU's sent to work out who the root switch is and what is the STP state of the interfaces when this happens?


Timothy Bogie

Nagaraja Thanthry Mon, 07/19/2010 - 07:46
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You are partially correct. When there is a change in the topology, the

switch that is closest to the topology change (lets say you connect a new

switch, the adjacent switch in this case) would generate a TCN (Topology

change notification) and sends it towards the root. This will trigger a

Spanning-tree recalculation.


When the recalculation takes place, all ports go to listening/learning mode

and will not pass any traffic. Once they go back to forwarding mode, then

only they will become active and start forwarding traffic.

Hope this helps.



tbogie_gvds Mon, 07/19/2010 - 10:04
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Thanks.  I understand what you are saying.  What would happen if the switches were, for example, just powered up?  The switches would need to work out who the root switch is going to be.  So STP starts in an initialising state and then, according to the docs, interfaces move into a Blocking state.  As I see it, somewhere between initialisation and blocking, BPDU's are sent to elect the root.  So my question is, when does the election occur during which STP phase?  It can't be blaocking (as I see it) as only incoming BDPU's are processed and none are transmitted, 

Thanks again,



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