The Spanning Tree (SPT) Algorithm explained in detail

Spanning Tree Protocol SPT

Note: The phrase “bridge” can be read as switch as well in the explanantion below.

1. Elect the Bridge with the LOWEST Bridge – ID (BID = Priority. MAC) as root bridge. The default Priority is 32768.

All ports of the Root Bridge will transition to DESIGNATED + FORWARDING.

 

2. For each None Root Bridge (“loosers”), select a Root Port based upon:

-The lowest cumulative bandwith costs to the Root – Bridge

-The port towards a neigbouring bridge with the lowest BID in the UPLINK to the Root Bridge.

-The lowest Port priority.Port number, (default priority =128) of the Port in the UPLINK connection. (so the adjacent bridge / switch)

Example: fa0/1 has 128.1 and fa0/2 has 128.2 → select fa 0/1

The Root Port will transit to Forwarding and for this bridge represents the optimal path towards the Root Bridge.

 

3. Select a Designated port to the Root Bridge for each segment, provided there are unused ports available within the segment. The Designated port provides an optimal path to the Root Bridge for that segment.

Base the decision upon:

-The lowest cumulative bandwith costs to the Root – Bridge

-The port towards a neigbouring bridge with the lowest BID in the UPLINK to the Root Bridge.

-The lowest Port priority.Port number, (default priority =128) in the UPLINK to the Root Bridge.

The Designated Port will transit to Designated + Forwarding.

 

4. All remaining ports will transit to BLOCKING / None Designated.

Blocking ports will not transport ANY data, but will expect to receive BPDU’s from neigbouring Bridges.

 

HINT:

With a direct connection between tWo switches, once the Root port is determined the port on the other side will always be a Designated port.

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