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Spanning tree with Portchannel (Loop incoming?)

Dear Cisco Friends


I'm currently configuring 2 Switches to an additional Switch ( all 48 Ports, Catalyst 3560G) and i want to configure them as following (red = new):



PC = Portchannel

As you see, i already configured the ports on every switch and the last step i want to do is to ensure that when i will connect these 2 switches to my root switch, that there will be zero network issues like spanning tree recalculations.

currently the root switch ( doesn't have any spanning tree configurations so do switches 2 and 3.

I'm afraid to just connect them together to make a huge chaos, so how can i ensure myself that there is no Loop or spanning tree issues without just trying it out?

(also maybe which spanning tree configurations you might find appropriate? simple ones are prefered))


Thanks for your help!



Jon Marshall
Hall of Fame Guru

There will be an STP recalculation for all vlans that are common to the switches.

You can't prevent that and you should have an outage for this.

You should also enable STP on all switches because as you say you have a loop.

Not having STP enabled would be a very bad idea.

Make your existing switch the STP root for all vlans and if you want pick one of the others as secondary.

The root is more important.

You should use RSTP if available on all switches.

There are additional things like rootguard which could be enabled but if you want to keep it simple just enable STP and set the existing switch as root and that should work in your setup.

Do not bring up the ports without STP.

The outage should be short ie. with RSTP less than a minute but there will be a convergence because currently you don't have STP running anywhere.