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VLAN, STP, loop

Occasional Contributor

VLAN, STP, loop


I'm designing a new network architecture and I have a big question about VLAN and STP. As you can see on the diagram, I have a loop on my network which STP block immediatly.

My question is how can I do to avoid STP from blocking it. As you my noticed, it's not a real loop as there is only one VLAN on that link. Does spanning tree cares about VLAN when detecting loop ?

For the story, I would like to use my 2 core switch as redundant router. As these 2910al HP switch don't provide VRRP or any other gateway redundancy protocol, I'm going to use these 2 switchs as a gateway on my client with different metric to use both at the same time. These switch are setup with routing enabled.

But with STP blocking the link between both switch, the switch B cannot connect to the gateway.

What can I do to stop STP acting like that ?

Thanks a lot for your ideas.


Respected Contributor

Re: VLAN, STP, loop

Start by lowering STP priority on SW A and SW B, making them primary and secondary STP root.
If SW A and SW B only have a single interface in VLAN 100, it can never be part of a loop, so you can safely disable STP on that interface. 
If you add additional access switches later, create extra VLAN (101) and new IP subnet

-shouldn't the gateway be connected using VRRP too ?
-Shouldn't the link between 2 switch be a pure L3 link , with routing protocol advertising subnets?

Honored Contributor

Re: VLAN, STP, loop

Out of the box, spanning tree doesn't care about VLANs. You can configure multiple spanning tree (MSTP) which knows about VLANs but you need to spend some effort designing the configuration to do what you want. Some of the Procurves support per VLAN spanning tree (PVST), which is more plug and play but isn't actually a standard.

Or you can do what the other poster suggested, just disable STP on some of the ports.