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MSTP Design


Diagram of our network attached.

We have 2 core switches and 2 access switches connected in a resilient design. Access Switch 1 is configured with Data VLAN 101 and Voice VLAN 102 and Access Switch 2 is configured with Data VLAN 201 and Voice VLAN 202.

We are looking to configure MSTP to load balance the Data and Voice VLANs over the Access Switch uplinks so no links are blocking. With this scenario, would it be ok to configure 2 MSTP intances and map the Data VLANs 101 and 201 to instance 1 and Voice VLANs 102 and 202 to instance 2 with Core 1 as Root for instance 1 and Core 2 Root for instance 2? Or would I need to configure 4 MSTP instances one for each possible topology?

I will also have VLANs that will only need to transit Core 1 and Core 2, should these VLANs map to a dedicated Instance or is the IST ok for this?


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Re: MSTP Design


You would need 2 instances on each switch. Instance one will have vlan 101 and instance two have vlan 102. Vlan 101 and instance one will use the link towards core-1 and vlan 102  and instance  two will use the link to core-2.  The same for other switch.  If you are using HSRP or VRRP, you would need to adjust the primary VRRP or HSRP per instance.  For example, for vlan 101 the VRRP primary is core-1 and the stand-by is core-2.  For vlan 102 core-2 is primary and core-1 is stand-by.

In my opinion, since you only have 2 vlans on each switch, is it not worth the headache of changing everything unless your forwarding link on each switch is completely saturated.  Load balancing vlans per link is a good idea if you have hundreds of vlans and not a handful.


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