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New Member

STP processing time questions

Once the intial BPDUs are exchanged between the switches,

they'll come to know who should be the ROOT.

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.

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 ?

If yes. So, after the config BPDUs are received by other switches. They will now try

to select the Root ports.

Q3. HOw much time the switches need to calculate the root ports ?

After the root ports are selected. Now time for designated ports.

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 ?

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.

So after the ports are selected it takes 50 sec's for them to move into forwarding state.

So inorder for all the selections like root bridge, all port states switches need sometime. (Let us say X).

Q6. So for STP to stabilize, the total time required is X + 50 secs. What is the value of X ?

Please don't mind if my understanding is wrong somewhere.

Regards,

Chandu

Regards, Chandu
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

STP processing time questions

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

4 REPLIES
New Member

STP processing time questions

No answers yet. Please do needful.

Regards, Chandu
Cisco Employee

STP processing time questions

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

New Member

STP processing time questions

Peter & All, do you think mastering all these stuff is neccessary to pass CCNA. I'm thinking that all these are CCNP level.

Please let me know.

Regards,

Chandu

Regards, Chandu
New Member

STP processing time questions

It is more than 20 days now.. i'm learning about STP... as go on analysing thing i'm getting numerous questions.

Regards, Chandu
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