Spanning-tree - Select alternate port

[rhaji]

Hi, 

I have a problem with Spanning-tree on mixed brand switch

FOR RESUME : 

There is one C3850 (SW1)  using only spanning tree and  2 Siemens Scalance Switch (SW2 and SW3)

In the configuration below, SW3 manage "Alternate port" between SW3/SW2

C3850 is the root,  but alternate port is manage by Scalance and there is a bug.

- SW3/SW1 Disconnect and reconnect :  No problem;

- SW3/SW2 Disconnect and reconnect :  No problem;

- SW2/SW1 Disconnect : No problem ...  Reconnect:  The scalance that manages "Alternate port" (SW3) disable directly the port between SW3/SW2 before the connection between "SW2/SW1" (C3850) is ready (5 seconds).

Is it possible to force the Cisco to use the Gi2/1/2 port to be alternate? When trying with the priority, I only managed to move the alternate port to SW2

Good da

[MHM Cisco World]

Show spanning tree 

Where is root for this spanning tree domain?

[rhaji]

On SW1, Cisco C3850

[rhaji]

On C3850 by default,  SW3 manage the alternate port between (SW2/SW3).

• When i disconnect the patch between SW1/SW2,  it's always the root;
• I reconnect the patch,  SW3 disconnect directly (before the connection is ok between SW1/SW2)
• SW2  is isolate (SW1/SW2 not ready and connection between SW2/SW3 disable by  SW3) => Second root on the network

[r.heitmann]

looks like „sw 2“ is the root bridge.

- just lower the STP root-priority of your cisco-stack

conf t

  spanning-tree vlan 1 priority 4096

end

…for example.

- than sw1 will act as the root of the stp topology with active interfaces

- the link between sw2 and sw3 will get „blocked“ at one of both switches sw2 or sw3.

[rhaji]

OK,  but is the simple drawing with for identical  VLAN,  there is 4 different loop with scalance connected on this network.   There is no risk to define on each subloop a Root ?