MSTP vlan-map-to-instance

[minh_hieu]

Hi community,

We know that inside one MSTP region, all switches should have the common mapping vlans to instances. However I wonder what happens if some switches have the different mapping such as instead of adding all VLANs,  they only add some. Our customer argue that because their switches are allowed trunking some VLANs such as 101, 102, 103 so they only have to add these. Please advise what will happen?

Thanks and regards,

Hieu

[paul driver · Accepted solution]

Hello,

When you invoke mst all vlans should be mapped to the instances you are going to use ( multiple is requiring load balancing ( but not instance 0) and these mst instances should be root for all the vlans (its bad practice to have vlans outside the mst region to be root for specific vlans)

These vlans now become part of an instance, so if you have all vlans in one instance and you try to prune some vlans of specific trunks then your basically pruning the mst instance and you will lose connectivity.

Cisco statement:

"A simple rule to follow to steer clear of this problem is to never manually prune VLANs off a trunk. If you decide to remove some VLANs off a trunk, remove all the VLANs mapped to a given instance together. Never remove an individual VLAN from a trunk and not remove all the VLANs that are mapped to the same instance"

hope this helps

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[minh_hieu]

Thank Paul for your help.

A further discussion is that in case all VLANs of the customer switches belong to a separate instance, do they still need to define the other instances and VLAN of them (of course, they are still define the instance 0).    to Cisco statement, there is only trouble if we prune some VLANs off a trunk. So if we prune all VLANs of all the other instances, it should be fine?

To be clear, assume we have instance X with VLAN 101, 102, 103; instance Y with VLAN 201, 202, 203, instance Z with VLAN 301, 302, 303. Now the customer switch are SwA, SwB.... They will define instance X with VLAN 101, 102, 103 only although in SwC, SwD, we define instance Y, instance Z. Is it OK?

Sw A --------------------------SwB

|                                     |

|                                     |                           

Sw C ---------------------------SwD

Br,

Hieu

[InayathUlla Sharieff]

Hi Hieu,

Yes this should be okay.

http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_19_ea1/configuration/guide/swmstp.html#wp1035894

Only the CST instance sends and receives BPDUs, and MST instances add their spanning-tree information into the BPDUs to interact with neighboring switches and compute the final spanning-tree topology. Because of this, the spanning-tree parameters related to BPDU transmission (for example, hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MST instances. Parameters related to the spanning-tree topology (for example, switch priority, port VLAN cost, port VLAN priority) can be configured on both the CST instance and the MST instance.

MSTP switches use version 3 RSTP BPDUs or 802.1D STP BPDUs to communicate with legacy 802.1D switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches

Regards

Inayath'

[minh_hieu]

Hi Inayath,

What you mention is about using 2 separate MSTP regions which is out of my ability at the moment (I may ask about it into another topic later). Here I'm concerning if we can use the only one MSTP region but some switches define few mstp instances and the others configure all. Is it possible?