Step 1: elect the root bridge
Every switch has a bridge ID: a priority (default 32768) plus its MAC address. With PVST+, the VLAN number is added to the priority (the system ID extension), so in VLAN 1 the default shows as 32769. The switch with the lowest bridge ID becomes the root bridge: lowest priority wins, and on a tie, the lowest MAC address.
All three switches below have priority 32769, so the lowest MAC wins: SW3.
In NetLab
▶ SW3 SW3>enable SW3#show spanning-tree vlan 1 | include Root ID|Bridge ID|Address|This Root ID Priority 32769 Address 001a.423b.6ea3 This bridge is the root Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 001a.423b.6ea3
Step 2: every other switch picks one root port
Each non-root switch chooses the port with the lowest root path cost (the sum of the port costs on the way to the root). Ties are broken by the neighbour's bridge ID, then the neighbour's port ID (priority + number).
| Speed | Cost (short method) |
|---|---|
| 10 Mb/s | 100 |
| 100 Mb/s | 19 |
| 1 Gb/s | 4 |
| 10 Gb/s | 2 |
Step 3: every link gets one designated port
On each link, the port closest to the root (lowest root cost, then lowest bridge ID) is designated and forwards. All the root bridge's ports are designated. Any port that is neither root nor designated is blocked (alternate in Rapid PVST+).
In NetLab
▶ SW1 SW1>enable SW1#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 001a.423b.6ea3 Cost 4 Port 26 (GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 001a.b61f.d605 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi0/1 Altn BLK 4 128.25 P2p Gi0/2 Root FWD 4 128.26 P2p
SW1: root cost 4 via Gi0/2 (straight to SW3). On the SW1–SW2 link both switches are 4 away from the root, so the lower bridge ID wins: SW2's port is designated and SW1's Gi0/1 is the alternate.
Remember: Root bridge: lowest bridge ID. Root port (one per non-root switch): lowest cost to the root. Designated port (one per link): the side closest to the root. Everything else blocks.