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Spanning tree loop and other questions


We are going to have a network of eight switches in a loop. ie each switch connects to two others. Half the switches are cisco3550, the rest are nortel450. The cisco switches will connect to each other using gbic over fibre, the rest of the loop will be 100Mb using fibre and utp via media converters.

Will spanning tree prevent loops?

What sort of recovery time is likely if there is a break in the loop?

Will most of the traffic go via the gigabit side by default (network is layer 2 switching only)?

When we go to layer 3 switching in the future, (at present some protocols are non-routable), what are the traps to look out for?

Thanks for any replies,


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Re: Spanning tree loop and other questions


Please clarify what you mean by "loop"--do you mean one switch is connected to the next, which is connected to the next, forming a large ring? Also, will you have one VLAN, or multiple VLANs with trunks between the switches?

As long as spanning tree protocol (STP) is enabled on all the switches, there should be one port on one switch in blocking mode when the network converges. Normal STP operation takes up to 50 seconds to converge after a topology change such as a link failure, but Cisco features such as Backbonefast and Uplinkfast can help speed up the process in specific situations. Here are a couple useful links for STP for the Cat 3550:

I recommend manually selecting the root switch for the loop to be the middle of the Cisco-side of the ring. That way, the loop will be "broken" on the Nortel side with the 100Mb links and most of the traffic will traverse the GigE side of the ring. Unfortunately, any traffic between the switches sharing the blocked link will have to go all the way around the ring, so you might want to take that into consideration when selecting where your root bridge should be.

Hope this helps.


Community Member

Re: Spanning tree loop and other questions


thanks for your answers. The switches will be connected in a ring. There are no VLANs as there are only about 14 client devices across the network, so we didn't see an advantage in VLANs.

When we go to layer3, I thought of breaking the ring in 2 locations that have layer 3 switches, and implementing routing at these points. Any comments?


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