Roles

1. Bridges:

   1. root
   2. non-root

2. Ports:

   1. Root Port (RP)
   2. Designated Port (DP)
   3. Non-designated Port (NDP)

Overview

1. For each VLAN choose one root bridge

   1. The switch with the lowest Bridge ID (BID)
   2. BID consists of 4-bit priority value (multiple of 4096) + 12-bit VLAN ID + MAC address
   3. Priority defaults to 8 (8 * 4096 = 32768)
   4. If all BIDs are default, then switch with lowest MAC address will be the root

2. For each non-root bridge choose one root port (RP)

   1. This will be the port that is on the best path to the root bridge
   2. Root Path Cost is cumulative link cost on path to root bridge out that port
   3. First the switch compares cumulative Root Path Cost out each potential RP
      1. Lowest root path cost wins
   4. If root path cost is tied
      1. RP is port that is receiving BPDUs from the neighbor switch with the lowest BID
   5. If sender BID is tied
      1. Lowest sender port ID wins
      2. Port ID is 8 bit priority (default 128) then 8 bit interface number

3. For each link choose one designated port (DP)

   1. The one port on the link that is closest to the root bridge
   2. Lowest Root Path Cost
   3. If Root Path Cost is tied
      1. lowest sender BID
   4. If sender BID is tied
      1. lowest local port ID (looped to itself)

4. If a port is not classified as an RP or a DP, then it is a NDP and it blocks

States (Controlled by timers set on root bridge)

1. Blocking to Listening

2. Listening to Learning

3. Learning to Forwarding

Timers

1. Hello timer

   1. root bridge sends a BPDU every (default 2 seconds)
   2. non-root bridges update it and forward it

2. Max Age 

   1. Max Age is refreshed every time the port receives a BPDU
   2. non-designated ports will save a BPDU for (default 20 seconds)
   3. If port fails to hear 10 BPDUs in a row
      1. Max age expires
      2. port transitions to Listening state

3. Forward Delay

   1. time spent in Listening and Learning states (default 15 seconds)

4. Time to transition from blocking to forwarding

   1. Max Age + (2 * Forward Delay) =  (20 seconds) + (2 * 15 seconds) = 50 seconds