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New Member

Three Switches connected in a triangle - how does Spanning tree block ports

I am working on a design that would link three Cats in a triangle configuration. 2 of the switches would connect up file servers. The third would connect up the client workstations. I would like to have it where the traffic would flow in the shortest path. Server to server traffic would not need to flow through the 3rd switch, and server to client traffic would not cross through a single switch, but rather go directly to switch 3.

The router is located in switch 3, and all ports on switches 1 and 2 are on the same vlan.

Is this possible with spanning tree? Does it block the entire port, or is it based on MAC address pairs? Is their something other than spanning tree that I should consider?



Cisco Employee

Re: Three Switches connected in a triangle - how does Spanning t

The Spanning-tree Topology is based on VLANs and Ports. It is not based on MAC address pairs. Each VLAN, with a few restrictions, can have its own autonomous STP topology.

New Member

Re: Three Switches connected in a triangle - how does Spanning t

A couple things come to mind:

If the clients and all servers are in the same VLAN and you have three switches and three links (triangle), then you have an Ethernet loop. It is the job of spanning tree alogrithm to provide a loop-free topology. Therefore, one the three links between the three switches that are in the same VLAN must have a port in blocking state to prevent the loop. Which port is in spanning tree blocking state is determined by several factors to include the relative bridge priority for the VLAN in question on each switch. Suggest you read these links:

As far as most direct path requirements you mentioned, spanning tree with triangle will not provide for that under the constraints you have listed (same VLAN). You might ask how critical that requirement is in this case. If you have enough bandwidth between switch 1 and switch 2, there is not a technical reason to worry about that issue. Just make sure switch 1 and switch 2 are the root and root+1 of the spanning tree domain and switch 3 will block on the uplink going towards switch 2 (run uplinkfast also).

If I understand your topology and might consider adding layer 3 switching capabilities to your design. Spanning tree has limitations that can be overcome with a very stable layer 3 design. For example if we were solving your problem (again, if I understand it), we might have layer 3 capabilities on switch 1 and 2 (where the servers connect) and leave switch 3 as a layer 2 device (for the clients). I can't see your entire topology, so there are variations on this theme that will provide a preferred design, but could require hardware purchases.

Take these suggestions as general guidelines only since I can't see your entire topology. Hope this helps.