MSTP L2 VLAN Databse Query !

Answered Question
Jan 27th, 2012

Dear All

Regarding the MSTP configuration is it mandatory to create all the L2 VLANs in the VLAN Database for which we creating instance-to-vlan mapping.


In a bigger network with lots of switches why do we need to create
those L2 VLANS on a Switch for which its not catering any traffic.

Please help to confirm the same.

Regards

Varma

I have this problem too.
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Correct Answer by Peter Paluch about 2 years 2 months ago

Hello Varma,

Regarding the MSTP configuration is it mandatory to create all the L2  VLANs in the VLAN Database for which we creating instance-to-vlan  mapping.

No, creating the VLANs is not required at all. What is required is to have the VLANs mapped into individual instances, and have the mapping (along with the MST region name and revision number) identical throughout the MST region.

Mapping an VLAN into an MST instance and creating that VLAN are two independent things. You can map non-existent VLANs into MST instances beforehand, and create them later (or never create them, according to your needs).

There is a different issue not related to MSTP at all - if a switch is a transit switch for a VLAN, then this VLAN must be created on this switch if even if there are no access ports configured for that VLAN. A switch will not accept frames in VLANs (on trunks) that are not created on the switch itself.

Best regards,

Peter

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Correct Answer
Peter Paluch Fri, 01/27/2012 - 00:38

Hello Varma,

Regarding the MSTP configuration is it mandatory to create all the L2  VLANs in the VLAN Database for which we creating instance-to-vlan  mapping.

No, creating the VLANs is not required at all. What is required is to have the VLANs mapped into individual instances, and have the mapping (along with the MST region name and revision number) identical throughout the MST region.

Mapping an VLAN into an MST instance and creating that VLAN are two independent things. You can map non-existent VLANs into MST instances beforehand, and create them later (or never create them, according to your needs).

There is a different issue not related to MSTP at all - if a switch is a transit switch for a VLAN, then this VLAN must be created on this switch if even if there are no access ports configured for that VLAN. A switch will not accept frames in VLANs (on trunks) that are not created on the switch itself.

Best regards,

Peter

svaibhava Fri, 01/27/2012 - 00:41

Hi Peter

Thanks a lot for the quick response and clarifications. Thus helps in my current network scenario.

Regards

Varma

kishore.chennupati Sat, 01/28/2012 - 16:53

Hi Varma,

just to add to my Master(Peter)'s comments. the default MST instance 0 has all the vlans mapped to it. Does it mean that we create all the vlans in the database and on every transit switch? NO. We only create the ones we using and we create them on the switches transiting those vlans. see below what i mean.

S1(config)# spanning-tree mst configuration
S1(config-mst)# show current
Current MST configuration
Name      []
Revision  0     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
S1(config-mst)# instance 1 vlan 20,40
S1(config-mst)# show pending
Pending MST configuration
Name      []
Revision  0     Instances configured 2

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-19,21-39,41-4094
1         20,40

HTH

Kishore

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Posted January 27, 2012 at 12:30 AM
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