Need RSTP help with three switches

[ktsaved]

Working with two SG350 switches and one third switch - a Catalyst 3560.

When I connect two trunk ports between the two SG350 switches with RSTP enabled things work fine.

First switch is the root bridge. When I disconnect the main trunk, on the second switch RSTP switches to the alternate port. When I re-connect the main trunk the second switch changes back the first port being the root and the second port the alternate.

My issue is connecting a third switch into the configuration.

Main trunk port from the Root goes to the second switch.

The other trunk goes to a third switch. Then a trunk from that third switch goes to the second switch.

The second switch sees the third switch as the Alternate Bridge.

Using a Catalyst 3560 as the third switch, when I disconnect the main trunk to the second switch, the second switch changes to using the alternate port which now is coming from the third switch. That part works fine.

When I re-connect the main trunk from the Root bridge to the second switch, the second switch puts that port is BDPU Guard Disabled state. In continues to use the alternate port coming from the third switch.

I have to manually take the port out of the guard state.

I have disabled BDPU Guard on all ports on the second switch. I am not understanding why it re-enables itself on that port and puts it in the disabled state.

The first switch is the Root with 4096.

The third switch I am adding is the Alternate with 8192 set.

BDPU Guard, Root Guard and Loop Guard are off on all ports.

No BDPU filtering or flooding are on.

The reason for this is I am preparing to use it in a failover setup in which it will still pass traffic to other switches on the network if the Root fails, which is a combination Core/Access switch.

I already have a setup to feed data into the Alternate Bridge, so that is not the issue.

Thanks.

[balaji.bandi]

STP only come to major play, when the Layer 2 network has look, how is this switches connected ?

they connected traingle ? has looks, any network diagram help to understand.

If you want to Switch 1 be root all time set priority low, the one you dont like set them higher.

so based on the priority the alternative path elected, if the switch 1 is the exit point then the traiff will be blockhole.

[ktsaved]

I created a drawing of the layout.

The first example at the top with two switches works correctly. Changes to alternate port and back again to root ok.

The second example does not work. It changes over to the alternate port but when changing back, it instead puts the port into an error disabled state - BDPU Guard disable. I have disabled BDPU Guard on all ports. The switch turns it on and puts the port in that state anyway.

I have the priority set to 4096 for the Root Bridge, and 8192 for the Alternate Bridge.

SW2 is set at default priority 32768.

Thanks.