Different root switches for IST in a region?

[MarkGi]

Hello,

we have four ProCurve switches that run MSTP over MPLS (VPLS). SWI-1 & SWI-2 and SWI-3 & SWI-4 have an ethernet interconnect as well.
They all have the same spanning-tree config-name and config-revision configured.
All of these switches agree on who is the CST root switch, but the IST root switches chosen confuse me:

SWI-1
IST Regional Root MAC Address : SWI-1

SWI-2
IST Regional Root MAC Address : SWI-1

SWI-3
IST Regional Root MAC Address : SWI-3

SWI-4
IST Regional Root MAC Address : SWI-4


Can anyone explain why there is not one IST root chosen as they are all part of the same region?

Regards,

MG

[Shadow13]

IST is Internal Spanning Tree, it includes all the vlans that are not assigned to any intance (ex: Vlan 1)
so the IST is localy for the switch itself and every switch is the root for it's IST, since this IST is not propagated to the other switches.

Regards

[Mohammed Faiz]

Have you checked that your MST-VLAN mappings are the same across all of your switches?
Run "sh span mst" on each switch and make sure that the MST Configuration Digest matches.

[MarkGi]

Hi,

I checked the digest and only SWI-4 has a different one. This because it has an (unused) VLAN less mapped to the IST.

Could the different digests cause it to see SWI-4 as a different region from the other 3 switches?

And if so, any idea why SWI-3 still chose itself as IST root instead of SWI-1?


Already thanks for your help on this.


Regards,

MG

[Mohammed Faiz]

Hi,

Yes, the three conditions that need to match for switches to be in the same region are the config-name, revision and mst-vlan mappings (the digest). So I'd correct that first and see what you get.
Have you changed the IST priorities for any of the switches?
What does SWI-3 list as it's port costs for the connections to neighbouring switches?

[MarkGi]

Hi,

thanks for the clarification. I will change the mappings during a maintenance window.

The costs are the default ones (20 000) and the priorities weren't changed for STP instance 0 (IST).

Regarding SWI-3, its interconnect to SWI-4 is in blocking and the one to the cloud forwarding. The designated bridge is in both cases SWI-4, so I don't know what the difference with SWI-1 and SWI-2 is.


Regards,

Mark

[Mohammed Faiz]

Can you see SWI-1 or SWI-2 via lldp from SWI-3 or SWI-4?

[Shadow13]

Mark is talking about IST, regardless the MSTP config and instance the IST does not have anything related to that, he has mentioned
SWI-1
IST Regional Root MAC Address : SWI-1

SWI-2
IST Regional Root MAC Address : SWI-1

SWI-3
IST Regional Root MAC Address : SWI-3

SWI-4
IST Regional Root MAC Address : SWI-4


so even for the switches that are configured for the same region( as there is one switch is mis-configured) he is still seeing the IST root for each device as it self, so why are you talking about MSTP now when his question is about IST.

[Mohammed Faiz · Accepted solution]

Ok, well I don't pretend to know everything about spanning tree but here's my understanding:

The IST is the default instance within an MSTP region (any vlans that aren't added to an MSTI are by default added to the IST). For switches to correctly belong to the same region the vlan to instance mappings must match (including the IST).
On the MSTP setup I am running each switch is not the root for the IST, there is one root IST switch.
I'm not sure I see the difference between the IST and MSTP that you are hinting at.

[MarkGi]

Hi,

I think Mohammed is right about the IST working as he described. I will make sure the VLAN to instance mappings are correct for all the switches and do some testing from there on.

Thanks for your help.

Regards,

MR

[Shadow13]

mmmmm then can anybody explain why the 2 switches that are in the same region then each one of them is the root for it's IST then ??

SWI-1
IST Regional Root MAC Address : SWI-1

SWI-2
IST Regional Root MAC Address : SWI-1

SWI-3
IST Regional Root MAC Address : SWI-3

SWI-4
IST Regional Root MAC Address : SWI-4

Please check the Digest on the MSTP
#show spanning-tree mst config
or something like that i'm not sure about the command, there you will find everything, check the DIGEST, it must match in all the switches on the same vlan. if not then there is a mis-configuration on these switches

[Shadow13]

one more thing. Mohd is right :).