Vlan Cascade without trunk port

[emi91]

Hi Guys,

Actually I'm working to a new configuration on our system. We have a lots of devices that has a redundant link (Primary and Secondary).

We have a fews rack mount system devices connected to a single switch using the vlan (Vlan #100 Primary and Vlan #200 Secondary).

Now I would like to connect these vlans to a two switches that has not vlan configuration: we use one switch for Primary and one switch for Secondary.

Now the problem is: If I connect one rack system (any rack) to these switches without vlan configuration we don't have any problems and the system works well, instead if I connect two rack in this way I lost the secondary link to one of the rack system.

Another problem is, If I connect just one rack to the switches all works properly, but If I disconnect and reconnect the secondary link, the primary link goes down for a few seconds and after turn up. This is strange because you should be able to connect and disconnect secondary link without interere with primary link.

All the ports on the rack switches are configured to access port and assigned to a respectively vlan.

I attached an image to show you better my set-up.

Thank you for your help and suggest.

Emi

[MHM Cisco World]

Disabled STP' that serious not good.

STP prevent looping if you disable STP then you will face l2 storm.

[emi91]

Sorry of my lack of knowledge, but, if I'm pretty sure that any loop there's in a LAN, I can disable the spanning tree or you suggest to keep on?

An important things that I miss, I disable the STP just for the switches with vlan, my "master" primary and secondary has STP active.

Thanks a lot

[Flavio Miranda]

Hi @emi91 

 First, glad you found the problem. 

Let me see your questions:

"But now, I'm not so expert to understand, could you please explain me why, without any loop, the STP create this problems? "

 As I stated earlier, it does not create problem but it will try to eliminate the possibly loops. 

maybe the Vlan#2 has a tag and when I connect both vlans#2 to the secondary switch STP block one of the two switch?

This is ont possibility. You could check who is the root for this vlan.

there's some configuration to do for keep STP active?

 You need to keep in mind that STP will shutdown ports that it consider to be a potencial loop. Rather then that, it will work fine and will not cause problems.

 If we look at the paths A,B,C and D there would be a potential loop and Spanning-tree will try to block interfaces in order to avoid the potential loop.

   There are a few config you could do in order to change  this, but, it does not make sense to keep Spanning-tree if you prevent it from blocking potential loop. 

[emi91]

But you agree that in this situation should not to be any loop? 

I'm not be able to understand the behavior. theoretically should works this set-up 

thanks a lot for the detailing reply

[Flavio Miranda]

If Vlan2 exist in Sw1, 2,3 and 4 the spanning-tree will see it as a potential loop as there will be 2 physical path between switches.

  But, if you can ensure that no loop can happen and you disable spanning-tree we can say you are good.