07-18-2010 07:12 PM - edited 03-06-2019 12:04 PM
Folks,
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?
Regards,
Timothy
Solved! Go to Solution.
07-19-2010 10:19 AM
Hello,
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.
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cw
si2/cwsiug2/vlan2/stpapp.htm
Hope this helps.
Regards,
NT
07-18-2010 09:07 PM
Hello,
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.
Regards,
NT
07-19-2010 12:47 AM
NT,
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?
Thanks,
Timothy Bogie
07-19-2010 07:46 AM
Hello,
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.
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080
094797.shtml
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.
Regards,
NT
07-19-2010 10:04 AM
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,
Timothy
07-19-2010 10:19 AM
Hello,
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.
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cw
si2/cwsiug2/vlan2/stpapp.htm
Hope this helps.
Regards,
NT
07-18-2010 11:39 PM
Any Block Port in the STP domain = receive BPDU.
if one side in block mode must other side it will be designated port
DS(designated port) port will send BPDU to Block port , if the DS port stop sending BPDU , the block port will calculate and send BPDU then come DS for the segment
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