Why RSTP / STP sending packet twice as hello time

[Ivan Mamka]

Hello,

Below is simulated in CML 2.4.1. Why STP packets are going every 9-10 seconds?

Thanks in advance

[MHM Cisco World]

time until the message appear in wiresharke can be different than hello time =5 sec. 
but 
you can debug BPDU and see the time between each STP hello message. 

[Ivan Mamka]

It is true. Cisco device is sending every 10 seconds [1] as shows suggested debug output.

But VM receives those packets in average every 16-17 seconds [2] (tcpdump)

If there is a delay in packet transfer because of some simulation issues, I believe still tcpdump should receive every 10 seconds.

Interestingly, when changed hello time to 1 second, tcpdump receiving BPDU in average every two seconds [3].

[1]

[Ivan Mamka]

[MHM Cisco World]

Yes, for hello try use debug, it immediate show you when the hello message send. 

[Ivan Mamka]

Why? I tried that and it is showing every 10 seconds (picture above). But VM is receiving it after 17 seconds (picture above).

[MHM Cisco World]

Yes, we can predict the delay between two node in simulate lab. 

[Ivan Mamka]

I don't agree that this is general simulation delay. At least it does not look like. Why simulator would delay hello BPDU's differently when hello timer is set different?

set hello time to 2 seconds : VM receiving it after 3-4 seconds

set hello time to 5 seconds: VM receiving it after 8-10 seconds

set hello time to 10 seconds: VM receiving it after 16-18 seconds

[Ivan Mamka]

In other words, when set hello timer to 10 seconds, and delay is 8 seconds - this would be a joke if its a normal thing:)

[MHM Cisco World]

""Why simulator would delay hello BPDU's""
friend, the simulator not delay the BPDU in same node, I am talking about delay between node. 

there is delay, and we can not predict it. 
try it in real life and see or even you can check the other SW that receive BPDU what is time interval between BPDU hello. 

again this is simulate so it not 100% accurate. 

[Ivan Mamka]

there is delay, and we can not predict it - Yes I can predict it: packet arrives ALWAYS to VM after 3-4 seconds when hello timer is set to 2 seconds. packet arrives ALWAYS after 16-18 seconds when hello timer is set to 10 seconds.

Hello timer set to 2 seconds. VM tcpdump:

Hello timer set to 10 seconds. VM tcpdump:

[Ivan Mamka]

Solved by installing all the updates through cockpit and rebooting CML. Now packets arriving every 10 seconds. Hope that is permanent. Thanks for all the effort!

Other forums suggested that above should not happen:

Why VM in CML receives RSTP packet almost twice later as hello time set in the switch? To reproduce problem: connect switch to VM (Ubuntu or CML), set hello time on switch to 10 seconds. start tcpdump on VM > it will receive the STP packets every 16-...

[Ivan Mamka]

Unfortunately, installing updates and rebooting CML was not permanent solution. After laptop (Windows 11) enters sleep and wakes up, the RSTPs are again received at 80-90% delayed time. VM and vSwitch reboot does not help, but reboot of CML solves problem until another sleep of a laptop where CML VM is running.

[MHM Cisco World]

first happy new year 
second it not predict as I mention before.

[Ivan Mamka]

Happy new years!

Will all respect Sir - the packets are predictably arriving within 30 milliseconds (0.003 seconds) as expected when in normal operation as I mentioned (screenshot below). Other forum user said also said that there should be no random delays in arrival of RSTP.

I'm not sure what you are trying to say in last comments. Please make a full statement with references to the official CML documentation if available or examples of packet capture from your own CML instance. The statements that this is simulator and anything can happen and everything there is unpredictable - is not helpful. Otherwise as for now, above looks like a defect in CML.