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Spantree issue

Hi all,

we faced a problem with stp on our switch, one of our switch port is oftenly blocked on specific mst on different time intervals, bellow is our existing setup:-

we have 4 switches operates purely as layer-2 with MST configuration. we provide layer-2 vlan connectivity to customers, all switches has identical mst configuration and multiple instances configured for a range of vlans.

The problem is we have one port is oftenly blocked on mst00 at single switch due to receiving superior hellows from a customer running PVST and have been given a layer-2 vlan ID: 62 which is part of mstoo.

we have configured rootguard, bpdu guard on the port and the problem still exist occuring from time to time. I am attaching here a capture of the switch log.

Any feedback will be appreciated

New Member

Re: Spantree issue

Hi Medo,

Can you please tell me what is configured on GigabitEthernet0/1 interface .

If you had configured spanning tree port fast on a interface which is connected with Network device. then the chances of port getting into blocking state occurs.



Re: Spantree issue

Hi Akbar,

Thanks for reply, port fast is not configured on the interface. The interface is configured as a trunk to carry multiple vlans, vlan ID: 62 is part of those Vlans.


Re: Spantree issue

In your logs, I see that the root still have the default SPT priority(32768).

I would begin by changing that on each VLAN. Put it a 0 on the root.

This will probably fix your issue.

Re: Spantree issue

Hi Dominic,

could you tell me how did u figure the root of mst0 has the default priority (32768)?

have a look at the attached config for mst on the switches. also I have another question, If I use MST instances for arange of vlans , I beleive I should change the priority of specific mst rather than changing each vlan priority.

In the attached, vlan from 1-99 are not included in the mst config, but thoses vlans were considered from mst0.


Re: Spantree issue

Sorry, attached file here..

Re: Spantree issue

Il your first post, you have this

May 4 03:53:51: %SPANTREE-2-PVSTSIM_FAIL: Superior PVST BPDU received on VLAN 6

2 port Gi0/1, claiming root 32768:0013.c30a.16a8. Invoking root guard to block t

he port

The bridge claiming root as a prio of 32768...the default. Your root must have a higher or equal prio for this to append. I concluded that you did not lower the root prio in your network.

I would lower the prio on your mst root. It's always better to have a predictive L2 network.

spanning-tree mst instance-id root {{primary | secondary} | {priority prio}} [diameter dia [hello-time hello-time]]

Re: Spantree issue

Hi dominic,

yes u are right, thanx for the helpfull post.

although other MST instances were configured with the approprate priorities for network optimization, but only mst0 was not included.

we will rectify it,

Thanks again..

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