Spanning Tree election process summerized

SPANNING TREE election process

Step 1 ) Root bridge election

1. All routers & switches identify them selves as root bridge

2. They will have there bridge id and root id the same
3. Each router will send a BPDU to the other router , BPDU contains the bridge ID and router id, the receiving switch will verify if the root id of the foreign BPDU is lower than its own bridge id, if its lower it will assign foreign BPDU root  ID as its Root id.

4. Tie breaker is the lowest bridge  id

Step 2) Root port election.

1. Each non root bridge needs to elect a root port
2. Root port election is based on least cost to the  root bridge.
3. Tie are broken on lowest upstream bridge id
4. Tie  are broker on lowest port id

Step 3) Designated port/device  election.

1. Elect a designated device/ port for each segment
2. Ports with the lest cost to root  are elected as designated ports.
3. Tie are broken based on bridge id
4. Ties are broken on lowest port id

Step 4) ports which are neither designated ports or root ports go into the blocking state, root and designated ports first go into the learning state and eventually go into the forwarding state.