Spanning-Tree PVST+ different VLANs on different Root ports on same switch

[VinceWhirlwind]

Can anybody help me get my head around this? I have never needed to use PVST before but now I'm having to consider it due to the horrible LAN design I'm facing.

I have a ring of 4 switches.
PVST+ enabled.
Switch 1 is at the top, 2 & 3 are each connected to 1, and 4 is at the bottom connected to 2 & 3.

Switch 1 is priority 4K and always wins Root bridge election.
Switch 1 is the router "core". All others are layer2 only.
All links are 10Gb = cost 2k

I have 2 user VLANs.

The Users are all on Switch4.
User VLAN1 mostly accesses a server that is on Switch 3
User VLAN2 mostly accesses a server that is on Switch 2

Each VLAN is carried between switches on a dedicated physical link.
ie, there are two links between Switch1 & Switch2, one is in VLAN1 and the other is in VLAN2.

So, from a traffic management point of view, I can avoid all traffic going through the "core" switch if I can have
- the link between Switch 4 & Switch 2 active for User VLAN2
- the link between Switch 4 & Switch 3 active for User VLAN1.

So, I put VLAN1 in PVST instance 1 and VLAN2 in PVST instance 2.
Then, I put Switch4 VLAN1 switchport connecting Switch2 a PVST cost = 20,000
I put Switch4 VLAN2 switchport connecting Switch3 a PVST cost = 20,000

So the higher cost interfaces become blocked, and I end up on Switch4 with the a Root port for VLAN1 that's facing one way, and the root port for VLAN2 facing the other way.

Is that how it works?

 

[vipergg]

A little fuzzy on what you are trying to do here. Maybe clarify. The core l3 switch should be the root for all vlans. Anything that has to travel between different vlans must go to the core anyway. Anything that is in the same vlan will not go to the core.

 

[VinceWhirlwind]

Things aren't accessing between VLANs. (If they do, they'll go through the core)

.......CORE Switch1
.........//........\\
........//..........\\
Switch2........Switch3
........\\..........//
.........\\........//
..........Switch4

VLAN1 physical link
VLAN2 physical link

Users are on Switch4
Users on VLAN1 use a Server on VLAN1 on switch3
Users on VLAN2 use a Server on VLAN2 on switch2
Each switch has a physical connection carrying VLAN1 and another physical connection carrying VLAN2 linking it to each of its neighbour switches

So they want VLAN1 active "East" out of Switch4
and VLAN2 active "West" out of Switch4

I've never used PVST because it has always seemed like a really dumb idea. This scenario that has been thrust upon me seems to call for it though.

I'm thinking I just change the cost to 20,000 on the Switch4 VLAN1 "West" port
and 20,000 on the VLAN2 "East" port.

So the VLAN1 link becomes blocked on the "East" side.
And VLAN2 becomes blocked on the "West" side.