MST config on single 3560

Unanswered Question
Jul 13th, 2009


We have existing 3560's with multiple trunk ports to clients+upstreams - We will go very close to hitting the 128 STP instance limit, therefore MST looks to be like an option(Without upgrading the switches).

The 3560's also have a trunk port to 7200's(For dot1q subints), for clients that require L3 connectivity.

I'm just a little unsure how to group vlans into seperate instances(Or if it is entirely necessary?)

i.e. GE0/1 (From Provider A) has:

interface GigabitEthernet0/1


switchport trunk allowed vlan 112,172,208,211,240,309,315,385,537,547,550-552

switchport trunk allowed vlan add 554,623,635,687,690,694,696,697,867,879,980

switchport mode trunk

These vlan's are allocated by provider and represent individual services - These vlans are then either presented on client trunk ports for L2 services, or added to trunk port to 7200 for L3 services.

So as you can see, there is no "standard" for how the individual vlan's are treated, nor which trunk port they may be presented on.....hoping someone can provide guideance on how best to manage this?

Thanks in advance.

I have this problem too.
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Giuseppe Larosa Tue, 07/14/2009 - 05:00

Hello John,

even using a single MST instance would give you the scalability required in comparison with PVST+ limit of 128 instances.

If you want to use multiple instances you need to think also of vlans you are not using now:

one suggestion is to partition the 4094 vlan space in subsets and to associate each of them to an instance (also vlans not existing now)

By doing so you avoid the headache of changing the configuration on each device when new vlans are deployed.

Another approach is that each MST instance has its own topology or meaning: you could use two MST instances in your cases one for vlans for L2 services (not extended to C7200) and one for L3 services.

Hope to help


johnelliot6 Tue, 07/14/2009 - 14:19

Hi Giuseppe,

We wont be introducing additional switches(The 3560's are 48port, and if we are approaching capacity, we will upgrade to 4500's).

The 3560's do have QinQ links to other POPs(Also single 3560's w/ 7200's) - but they are not participating in vtp(vtp mode transparent).

We also regularly have to present a L2 service to our 7200 to 'prove' connectivity to grouping them into 2 instances as you have suggested may not be ideal?

Therefore, would the following config on each 3560 be sufficient?

spanning-tree mode mst

spanning-tree mst configuration

name LOC_A

revision 10

instance 0 : Vlan 1-4094

spanning-tree mst 0 root primary

Thanks again for your assistance.

Giuseppe Larosa Thu, 07/16/2009 - 02:25

Hello John,

sorry for late answer

the proposed configuration is correct and can serve the scalability purpose

Hope to help



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