Different spanning tree mode between 2 switches

[mahesh18]

Hi Everyone,

I did test below at my home lab.

Access layer switch connects to the distribution layer switch.

Default gateway of Access layer is distribution switch.

Distribution switch rapid-pvst

Access layer  mst

Both distribution and access layer are running different spanning tree mode protocols.

When they do this then access switch is unable to ping its default gateway and access switch port that is trunk to distribution switch it is in bKN state.

Need to know when spanning tree mode is different between two connected switches does this mean that they are speaking different language and they

can not talk to each other completely?

Regards

MAhesh

[Rasika Nayanajith · Accepted solution]

Yes, PVST runs STP instances for each vlans & MST running stp instace for a group of vlans.  So your two different switches STP communication won't be understand by each other.

Following two post are very good reference if you interesting to go for details.

1. http://blog.ine.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/

2. http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/

HTH

Rasika

*** Pls rate all useful responses ***

[Aninda Chatterjee · Accepted solution]

Hi Mahesh,

When a MST running switch connects to another switch that is running PVST+ (or Rapid) then the boundary port (that connects to the non MST region) runs something called PVST simulation. This is needed so that the two different domains can understand each other and a common spanning tree can be built for your entire layer 2 topology.

Having said that, PVST simulation works with very specific rules. Failure to adhere to these rules will lead to the port being blocked due to PVST simulation failure.

Can you post the output of show spanning-tree interface from the access switch please?

Regards,
Aninda

Sent from Cisco Technical Support Android App

[Aninda Chatterjee · Accepted solution]

I recently wrote about PVST simulation and its functioning. You can find it here:

https://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080c180bd.shtml

This should give you insight into its workings and the reasoning behind it.

Regards,

Aninda

[cadet alain]

Hi Aninda,

Really cool paper

Regards

Alain

Don't forget to rate helpful posts.

[mahesh18]

Hi Aninda,

Thanks for very useful info.

I will put output -- show spanning-tree interface from the access switch later when i am at home.

Best reagrds

Mahesh

[mahesh18]

Hi Aninda,

HEre is required info

sh spanning-tree int fastEthernet 0/1

Mst Instance     Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

MST00            Desg BKN*100000    128.65   P2p Bound(PVST) *ROOT_Inc

Also here is morte info

sh spanning-tree int fastEthernet 0/1 detail

Port 65 (Port-channel1) of MST00 is broken  (Root Inconsistent)

   Port path cost 100000, Port priority 128, Port Identifier 128.65.

   Designated root has priority 32768, address 000b.bece.bbc0

   Designated bridge has priority 32768, address 000b.bece.bbc0

   Designated port id is 128.65, designated path cost 0

   Timers: message age 5, forward delay 0, hold 0

   Number of transitions to forwarding state: 0

   Link type is point-to-point by default, Boundary PVST

   BPDU: sent 3, received 472

sh spanning-tree mst 0

###### MST00        vlans mapped:   1-4094

Bridge      address 000b.bece.bbc0  priority  32768 (32768 sysid 0)

Root        this switch for CST and IST

Configured  hello time 2, forward delay 15, max age 20, max hops 20

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Po1              Desg BKN*100000    128.65   P2p Bound(PVST) *ROOT_Inc

Regards

MAhesh