Rapid Spanning-Tree (RSTP)

Lessons Discussion
1

This topic is to discuss the following lesson:

2

Thank you very much

3

You are welcome Raed!

4

Wow. Excellent lesson. Very clear and to the point. I am definetly enjoying this site and learning. Excellent refresher cource !
Please keep up the great work. BTW…Yes I am recommonding this site.

1 Like
5

Glad to hear you like it!

6

Excellent explanation! :slight_smile:

7

Hi Rene,
Very Good explanation on RSTP.
I think BPDU message flags are reversed in the diagram.

Thanks,
Srini

8

Hi Rene,
what is the characteristic of blocking mode in RSTP? Can send and receive BPDUs?
In most of the scenario Alternate / Backup port directly transition in to forwarding ?
On what condition the port will be in Learnging stat?
Thanks,
Srini

9

Hi Srini,

The port in blocking mode will only receive BPDUs, not send them. The same thing applies to the root port. Only the designated port sends BPDUs since this is downstream (to other switches).

The alternate port can go into forwarding when the switches loses its root port, the same thing occurs when you use uplinkfast with PVST.

Each interface has to go through the blocking, listening and learning mode unless you enable portfast:

https://networklessons.com/cisco/spanning-tree-port-states/

Rene

10

Hi Srini,

Which one do you mean?

Rene

11

Hi Rene,
Can you please explain bit more on TC while timer and Fd While Timer in RSTP?
Tc while timer 2xhello time = 4 sec , only 4 sec for flushing the Mac address learned through
topology change action ports. Please correct my understanding on Tc while timer.

Thanks,
Srini

12

Hi Rene,
I am seeing BPDU Message Flags in reverse order.

1 TCN
2 Proposol
.
.
.
7 Agreement
8. TCA.

Please correct me If my understanding is wrong.

Thanks,
Srini

13

Hi Rene,
You are correct . In RSTP a special case , designated port in the non-edege port of a switch will go in to blocking state and block all non-BPDU traffic until it gets agreement from the neighbour switch.

Thanks,
Srini

14

Hi Srini,

When a non-edge or root port moves into the forwarding state then this triggers as topology change in RSTP. When this happens it will start the TC while timer which 2x hello (4 seconds in total) and it will flush the MAC addresses that were learned on all non-edge interfaces.

When the TC while timer is active, all BPDUs that the switch will send will have the TC bit set. Other switches that receive this BPDU will start their own TC while timer and they will flush all MAC addresses except the interface where they received the BPDU with the TC bit.

I hope this helps.

Rene

15

awesome lesson! very detailed!

i have a question. you said that RSTP uses uplinkfast by default, so no need to configure it.

 

done some research on the internet, they are saying that RSTP doesnt use the BBfast and ULfast, but it has a similar feature like them which is set on by default. meaning, when you issue the “show span summary” you’ll see the Uplinkfast and BackboneFast disabled, because RSTP has a built-in feature like that. is this true? thanks!

16

Hi John,

That’s correct, RSTP has features that are similar to uplinkfast and backbonefast, there’s no need to activate these.

Rene

17

Hi Rene,

I read RSTP thoroughly.
Here is my understanding:
Port States are decided on point to point links by the negotiation method. If a switch receives a superior BPDU from a bridge claiming to be a ROOT, it will send an agreement.

But my question is, how is the ROOT BRIDGE decided in the first place ?
Is it done like the normal STP or does that also need a negotiation ?

Please let me know

18

Hi Vikas,

That’s right, it is similar to normal STP. Each switch will think it’s the root bridge until it hears a superior BPDU.

Rene

19

Hi Rene ,

Why this priority changing (8193 , 8194)

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    8193
             Address     0044.04ee.be02
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    8193   (priority 8192 sys-id-ext 1)
             Address     0044.04ee.be02
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

-

VLAN0002
  Spanning tree enabled protocol rstp
  Root ID    Priority    8194
             Address     0044.04ee.be02
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    8194   (priority 8192 sys-id-ext 2)
             Address     0044.04ee.be02
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Thanks

20

Sims,
Priority = Configured priority (8192) + vlan number