Multiple Spanning Tree (MST) questions

[jonathan.maxwell]

Please see below...

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Switch 2s1
Po1 Root FWD 10000 128.456 P2p Bound(RSTP)
Po2 Desg FWD 20000 128.464 P2p
Po3 Desg FWD 20000 128.472 P2p

##### MST0 vlans mapped: 1-4094
Bridge address 007e.9525.e900 priority 32768 (32768 sysid 0)
Root address 683a.1e8c.d6ee priority 8192 (8192 sysid 0)
port Po1 path cost 10000
Regional Root this switch
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

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Questions:

1. What means "sysid 0"?

2. "Po1 Root FWD 10000 128.456 P2p Bound(RSTP)" What means "Bound(RSTP)"?

3. "Operational hello time 2 , forward delay 15, max age 20," These timers seem to be vanilla STP slow converging timers that take up to 50s to resolve. RSTP takes maximum 6 seconds. So...

What is the MST default forward delay timer?

Shouldn't modern MST technology also use 802.1w RSTP modern timers that converge within a maximum of 6 seconds?

Thank you!

[MHM Cisco World]

What means "sysid 0"

the sysid 0meanig you run only one MST instance include all vlan form 1-4094, which is default MST0

"Po1 Root FWD 10000 128.456 P2p Bound(RSTP)" What means "Bound(RSTP)"? this is boundary Port between this SW run MST and other SW run RSTP.

for the hello time I will check and return to You later.

[jonathan.maxwell]

Thank you sir! I await the final response!

[David Ruess]

Hello,

AS per your questions:

"Sysid 0" - Is the Default MST instance of 0 (its not like a vlan identifier like other STP outputs)

Bound (RSTP) - means this port is connected to another switch that is running RSTP (its an MST boundary port)

MST timers - I'm pretty sure the timers are the same as other STP standards. But you should be able to modify them just the same to meet faster convergence times.

The benefit of MST (if I remember correctly) is not so much the "faster" convergence, but the reduced network congestion. Instead of sending a STP packet per VLAN to traverse the network like PVST+/RSTP does, it sends out notifications per instance. So you have all VLANS in one instance so it will only generate one notification for that region.

Here is some CISCO documentation that looks like it only carries those timers to be backward compatible with legacy STP. 

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html#stp_timers

Hope that helps

-David

[Flavio Miranda]

Hi

 Actually it does. If you read the Cisco doc for MST, you are going to see this:

"
Multiple Spanning Tree (MST) is an IEEE standard inspired from the Cisco proprietary Multiple Instances Spanning Tree Protocol (MISTP) implementation.
This document assumes that the reader is familiar with Rapid STP (RSTP) (802.1w), as MST heavily relies on this other IEEE standard. "

"Shouldn't modern MST technology also use 802.1w RSTP modern timers that converge within a maximum of 6 seconds?"

Any flavor os STP is old technology and should be replaced.  Modern technology is Software Defined Networks.

[jonathan.maxwell]

Thank you Flavio for your reply.

You say "Actually it does."

I assume you are replying to my question "Shouldn't modern MST technology also use 802.1w RSTP modern timers that converge within a maximum of 6 seconds?"

If that is the case, should a healthy MST instance be exhibiting timers of 2 seconds, to support convergence times of maximum 6 seconds, as per RSTP? 

Then in this example--"Operational hello time 2 , forward delay 15, max age 20," implies that in this network some switch is running pvst or stp, thus slowing down most other switches?

Thank you!