Chandu,
Q1. HOw much time the swiches need to calculate the Root switch, will it depend on the size of the network ? Or else there is any fixed time saying that the root switch will be elected within this time.
You can assume a switch to reevaluate its personal choice of a root switch, root port, designated ports and non-designated ports any time it receives a BPDU - because it checks whether the received BPDU on a port is different from the one already stored on it. To choose a root switch means simply to evaluate all BPDUs stored on ports and choose the switch with the lowest BID. This comparison can be done extremely quickly - it is just about comparing integers.
Propagating the information about a root switch is another issue. If we assume that a new BPDU from a switch is emitted every 2 seconds, the information about a new (i.e. changed) root switch propagates to a distant switch in approximately 2*N seconds where N is the number of "hops" the switch is distant from the root switch.
After the Root switch is elected, the root swtich will start sending BPDUs to reflect
the cost to reach root.
Q2. Are these the configuration BPDUs ?
Yes. In fact, all switches send only configuration BPDUs until they detect a topology change event.
Q3. HOw much time the switches need to calculate the root ports ?
It is a negligible time. A root port is defined as the port receiving the best BPDU after adding the port's cost to the BDPU's Root Path Cost value. You can perform this comparison almost immediately - in fractions of a second.
Q4.HOw much time the switches need to calculate the designated ports ?
And again time for selecting Non designated ports ?
Q5. HOw much time the switches need to calculate the designated ports ?
The same negligible time.
As per my knowledge once the Root bridge and port states on non root switches are
selected then the ports move into blocking (20 secs), listening (15 secs), learning (15 secs) and forwarding state.
In fact, a port starts in Listening state, so it takes it 30 seconds (2x15) to arrive into Forwarding state.
Q6. So for STP to stabilize, the total time required is X + 50 secs. What is the value of X ?
It is either 30+X or 50+X, depending on the nature of the topological change (whether the topological change requires a port to age out the older BPDU to start accepting an inferior one). You can consider X to be 2*N where N is the diameter of the network.
Perhaps you have seen the following document?
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml
Best regards,
Peter
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