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MSTP abnormalities

Hi all, I am experiencieng MSTP protocol abnormality in integrating 3rd party SW units connected to Cisco7609-1 and Cisco7609-2. Cisco 7609-1 is the bridge root since spanning tree priority is set to lowest value than Cisco 7609-2. The two SWU are connected to both Cisco 7609 routers for redundancy purposes with bridge priority set to 32768. SWU-1 i/f1 is connected to C7609-1 and i/f2 is connected to C7609-2 with higher path cost configured on SWU-1 i/f2. SWU-2 i/f1 is connected to C7609-1 with higher path cost and i/f2 of SWU-2 is connected to C7609-2.

All active vlan is assigned to instance 1 of MSTP. When checking the MSTP parameters on SWU1 and SWU2, showing instance 0 and 1 as "regional root".

Any suggestion/recomendation will be appreciated...

Hall of Fame Super Silver

Re: MSTP abnormalities

Hello Allan,

the third party switches are running MST that is IEEE 802.1s using RSTP 802.1w.

MSTP is a Cisco proprietary multi instance spanning tree prestandard.

Check if your current IOS version allows to use MST mode.

use in global config:

spanning-tree mode ?

in that case use MST where instance 0 is the IST.

Otherwise you need to upgrade the core switches to a release that support MST.

another thought: actually the IST instance 0 is the only one that sends out BPDUs so your problem could be that at least a vlan must be associated with IST.

In fact, only one BPDU is sent out every link and it contains fields that are associated to the instances (this is different then with PVST+ where each instance sends its own BDPUs).

So without any vlan associated to IST instance 0 the switches are isolated and each claims to be the root bridge for the region.


Hope to help


Community Member

Re: MSTP abnormalities

Dear Guiseppe,

Thanks for your reply. MST are supported on both C7609 using IOS c7600rsp72043-advipservicesk9-mz.122-33.SRC.bin.

Third party switches supported iee802.1s mstp implmentation.

Community Member

Re: MSTP abnormalities

Dear Guisseppe,

Cisco7609 supported MST and terms of configuration, both Cisco 7609 and third party SWU are within the same region and all MSTP parameters are the same.

But the problem is still there. Is there any other parameters that i need to look at?



Hall of Fame Super Silver

Re: MSTP abnormalities

Hello Allan,

as I've explained in the second part of my first post you need to associate at least one vlan with IST instance 0 otherwise no BPDUs will be sent out on the trunk port and every switch will claim to be the root bridge because they are isolated.

So I would suggest the following:

- create a new vlan 500 for example on all switches.

- associate the vlan with IST istance 0 this on the 6509 is done in mst region config mode

- add the vlan to permitted vlan list on the trunk ports on all devices

By doing so the BPDUs should start to flow and the correct root bridge ID should be accepted by all devices for the vlans of interest.

Hope to help


Re: MSTP abnormalities

Hi Allan, Giuseppe,

No, you don't need to configure a vlan on instance 0 in order to get the switch to send MST BPDUs. Instance 0 is sent on every link, regardless of the MST configuration.

Here, the SWU switches consider themselves as part of a different MST region, that's why they think they are regional root. The fact that the role of one of their uplink is "master" proves that (the master port is the "root" port for the region, when considering the region as a virtual bridge).

I would suggest you triple check your MST configuration on both side. Name, revision number and vlan to instance mapping must match on the switches. I think Giuseppe other suggestion (the pre-standard MST version) might be the issue if you are sure your config match.



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