The default spanning tree version on a Cisco switch is PVST+ which runs a separate spanning tree instance for every VLAN. The Swiss army knife command for checking your spanning tree configuration is the command: How do we figure out how the spanning tree is laid out in a production network? That's what we're going to cover here, using the same example topology. Obviously, in the real world, you're not going to have a diagram that shows you exactly how the spanning tree is configured. A potential loop between CD1, CD2, and Acc4 has been broken by blocking the port FastEthernet0/21 on Acc4. Both of the possible loops, going from CD1, CD2, Acc3, that has been broken by blocking Gig0/2 on CD1. I can also see from the diagram that the ports that are being blocked are Gig0/2 on CD1, and port FastEthernet0/21 on Acc4. I can see on Acc3 all the links going to it are green on both sides. CD2 can't be the root bridge, it's going to be Acc3.

On CD1, it's blocking a port going towards CD2. So, one of those two is going to be the root bridge. Just from looking at it here, you see that both Acc3 and CD2, their links are all green. I've got the link lights enabled so you can actually see easily where the root bridge is, and the path that traffic is going to go over. The diagram above is a screenshot from Packet Tracer. Therefore, we can then check that spanning tree has eliminated any loops in the Layer 2 part of the network and we can also see the tree that traffic is going to be traveling over. Then from there, we can figure out our root ports on the other switches, our designated ports, and our blocking ports. What we want to do here is to determine which is the root bridge first. In this example, the switches have been configured with VLANs but the spanning tree has not been configured at all, so they're all going to be using the default priority. What we want to do here is to map out how the spanning tree has been configured. There's obviously Layer 3 connections going from the PCs up to the routers as their default gateways as well. We've also got the Layer 2 part of the network with our core distribution switches, CD1 and CD2, and our access layer switches Acc3 and Acc4.

In the topology below, we've got the Layer 3 part of the network up at the top with our routers, R1 and R2, and going northbound. Spanning Tree Verification Example: Root Bridge

Packet tracer labs spanning tree