How is Spanning Tree Protocol Established?
Spanning Tree Protocol
Imagine you are a switch. You are connected to another switch, and both of you are connected to a third switch. If you send a broadcast message to the internet, it will go through all of the switches connected to you, and then back. This causes a loop, and the broadcast message will continue to loop through the switches until the network is congested.
Spanning Tree Protocol (STP) is a layer 2 protocol that prevents loops in a network by disabling redundant paths between switches.
How is Spanning Tree Protocol Established?
Spanning Tree Protocol is established by the following steps:
1. Determine the Root Bridge
The Root Bridge is selected from the lowest Bridge ID, which consists of the default BID (32768) + VLAN ID + MAC Address.
2. Determine the Root Ports
Root Ports are calculated from the Root Path Cost, which is the following:
| Link Speed | STP Cost– 802.1D |
|---|---|
| 10 Gbps | 2 |
| 1 Gbps | 4 |
| 100 Mbps | 19 |
| 10 Mbps | 100 |
If there is an equal cost path, the root port is elected from:
- Lowest SENDER BID
- Lowest SENDER port priority
- Lowest SENDER port ID
3. Determine the Designated Ports
The Designated Ports are the best path to receive traffic leading to the root bridge. If one end is a root port, the other end is a designated port.
4. Determine the Blocked Ports
Any port that is not a root or designated port is a blocked port.