Hello Messages in STP

urvi

Hi , Can anyone please tell me if Hello Messages are created only by Root Switch in STP? Is it also true for RSTP? Or any switch i.e. whether its a root or non-root switch,anyone can create hello messages? I will be thankful if someone can please clarify. With Regards

Node Man

I believe that the root switch determines the hello frequency for the other switches.

tomtom1

Correct me if I'm wrong, but this is my understanding:

Classic Spanning Tree (802.1d)
In classic spanning tree, the root bridge for a given STP instance (whether PVST, CST, etc) generated STP BPDU's every 2 seconds by default. These BPDU's are relayed to the non-root switches in the STP topology. The non-root switches use the BPDU generated via the root bridge to calculate their STP cost to the root bridge and therefore select a root port (the port on the non-root switch used to reach the root).

RSTP (802.1w)
In rapid spanning tree, the root bridge and the non-root switches (or bridges) both create BPDU's with the default interval of 2 seconds. The root bridge functionallity is the same as with 802.1d but the non-root switches also create BPDU's. When switches running RSTP miss 3 BPDU's in a row, they can age out their information. This is one of the things that make RSTP superior over STP.

xnx

Understanding and Tuning Spanning Tree Protocol Timers - Cisco

urvi

Hi Tomtom , Thank you for such a nice answer. Thanks to others as well. But from your reply Tomtom, do you mean that BPDUs are not generated by non-root switches in Classic STP? Also,what do you mean by "miss 3 consecutive BPDUs"?

urvi

xnx wrote: »

Understanding and Tuning Spanning Tree Protocol Timers - Cisco

Hi Tomtom , Thank you for such a nice answer. Thanks to others as well. But from your reply Tomtom, it seems that you mean that BPDUs are not generated by non-root switches in Classic STP? Also,what do you mean by "miss 3 BPDU's in a row"? Why will a switch miss 3 consecutive bpdus? And xnx,thanks for the very helpful link. so kind of you

urvi

Hey mod ,i cant edit my posts ! Whats happening?? Please check ur website as it has bugs. The programmer seems to have forgotten to code the edit ,go advanced ,cancel,delete button below each post !!

urvi

When i click on the "save" button after editing nothing happens! Its strange that such a technical forum like techexams.com has such flaws in their design and programming!

tomtom1

urvi wrote: »

Hi Tomtom , Thank you for such a nice answer. Thanks to others as well. But from your reply Tomtom, it seems that you mean that BPDUs are not generated by non-root switches in Classic STP? Also,what do you mean by "miss 3 BPDU's in a row"? Why will a switch miss 3 consecutive bpdus? And xnx,thanks for the very helpful link. so kind of you

In classic STP (802.1d) BPDU's are only generated by the root-bridge for a given instance.

What I meant is, when RSTP (802.1w) is running, BPDU's are generated by every switch in the topology. They use these BPDU's also as a hello / lifeness check. When 3 BPDU's are missed from the switch, the local switch knows that something is wrong and ages out the BPDU.