RSTP convergence without PortFast

[iores]

Hi,

If we connect PC to a trunk port without portfast, how long does it take for RSTP to converge?

My guess is 21 seconds - 6 sec of max age timer, and 15 second of learning timer. Would that be correct?

[MHM Cisco World]

PC connect to new port 15 

Sw - Sw link change state need 21 

MHM

[MHM Cisco World]

Why SW to SW ? If one SW dont receive bpdu then max age timer (6) start count if still then the port start rstp election and need 15 (( total 15 plus 6 of age)

Where PC to SW there is no bpdu and hence no need max age timer but still SW port need to wait 15 before FWD  port. (Total 15 only).

MHM

[iores]

How switch knows that RSTP sync process failed for not receiving BPDUs without waiting for 6 sec?

[MHM Cisco World]

Why SW to SW ? If one SW dont receive bpdu then max age timer (6) start count if still then the port start rstp election and need 15 ( total 15 plus 6 of age)

MHM

[iores]

In my previous reply to you, I was refering to initial scenario with PC.

[MHM Cisco World]

Did SW ever receive any bdpu from PC to age it ?

The age is for recieve bpdu not for send bpdu and since PC never send any bpdu the max age time never use in port connect to PC

MHM

[iores]

BPDUs are sent every 2 seconds. How can switch be sure that there are no BPDUs coming to that port? It doesn't know that there is PC on the other side so I would suspect that it waits for some time before it continues with learning state. What am I missing?

[MHM Cisco World]

Friend 
BPDU send from SW every specific timer 
BUT BUT 
max age timer use by SW to age the BPDU it receive, if PC dont send BPDU how SW receive it ?

Forwarding  timer is time for port pass many STP states, each one have specific timer total is 15 
these state start with disable (shut down port)
then Blocking then listing then learning end with forwarding 

It doesn't know that there is PC on the other side so I would suspect that it waits for some time before it continues with learning state. What am I missing? Correct it wait a little in listening before start learning state.

MHM

[Joseph W. Doherty]

Your 5 state diagram is for STP but OP asked about rapid-STP, which uses a different 3 state diagram.

Although often Rapid's 3 states are shown as logical equivalents for STP's 5 states, it's not so much the reduced number of states that matters as much that how much more "rapidly" convergence can be done.

From my reading of the literature, STP convergence usually takes up to 50 seconds but Rapid's shouldn't exceed 10.

My PT lab results appear to be in the ballpark.

On my lab, two Rapid switches began forwarding in about 2 seconds.  Such time should include sync agreement and convergence.  When one switch replaced by host, we have to wait for timeout waiting for BPDUs (as hellos), which if default (3 x 2 sec.), takes 6 seconds and wait for convergence, if about the same as the two switches' 2 seconds, would be about 8 seconds total, so again PT's about 9 seconds seems in the ballpark.

[MHM Cisco World]

Dont depend on PT to calculate time.

We talk about optimal conditions.

MHM

[Joseph W. Doherty]

"Dont depend on PT to calculate time."

I don't, which is why I noted in my initial PT described reply, I don't consider PT "authoritative".

"We talk about optimal conditions."

In a case like my PT lab, indeed we do.  In the OP, full L2 network not described, so unknown where on the best to worst case it is.  Still, I would assume literature's mention of 10 seconds might be worse case.

To be clear, you believe for OP's case, it takes at least 21 seconds?

[MHM Cisco World]

I dont want to go deep do lab abd test but as I mention SW to SW  try lab below 

Three SW connect to each other in triangle' one of SW will have BLK and root port' 

This SW calculate time take for BLK port to become FWD when it connect SW dont send anymore bpdu (this SW lost it connect to root bridge 

This case exactly explains how long SW to SW port become FWD 

Do lab abd share result let other get more information 

Thanks in advance 

MHM

[Joseph W. Doherty]

You describe 3 switch triangle but OP's question is switch<>host (without portfast).

Also as your replies seem to focus on STP, want to confirm you're testing rapid-STP.

Of course, you could add a host to one those switches, and the benefit of doing so, you have a multi switch topology, i.e. possibly a larger switch topology will impact switch<>host time to forward.  (I suspect it matters little.)

[MHM Cisco World]

First @Joseph W. Doherty @iores When I mention stp in my previous comments I meaning rstp.

@Joseph W. Doherty his post ask about PC to SW but he also mentioned max age, that why I answer both cases.

MHM