Question

Describe the process of STP from implementation to convergence.

Answer 1

STP-(SPANNING TREE PROTOCOL)

To prevent loops from causing broadcast storms and making the MAC address table unstable, the Spanning Tree Protocol (STP) runs switches when redundant links are used on the network.

STP is a basic feature of datacom products including switches, routers, and WLAN products

STP Implementation

1. During network initialization, every device considers itself the root bridge and sets the root ID to its own BID. Then devices exchange configuration BPDUs and compare their root IDs to find the device with the smallest BID, which becomes the root bridge. All the ports on the root bridge are in Forwarding state.

2. A non-bridge device selects the port that receives the optimal configuration BPDU as the root port. The root port is in the Forwarding state.
3. The device generates a configuration BPDU for each port based on the configuration BPDU on the root port and path cost of the root port. Then the device compares the calculated configuration BPDU with the configuration BPDU received on the port.

   • If the calculated configuration BPDU is superior, the port is selected as the designated port and periodically sends the calculated configuration BPDU.

   • If the port's own configuration BPDU is superior, the configuration BPDU on the port is not updated and the port is blocked. Then, the port only receives BPDUs, and does not forward data or send BPDUs.

BPDU

A BPDU carries the BID, root path cost, and PID. A BPDU is encapsulated in an Ethernet frame. Its destination MAC address is a multicast MAC address, 01-80-C2-00-00-00. The Length or Type field specifies the MAC data length, and is followed by the LLC header and BPDU header.

Each bridge actively sends configuration BPDUs during initialization. After the network topology becomes stable, only the root bridge proactively sends configuration BPDUs. Other bridges send configuration BPDUs only after receiving configuration BPDUs from upstream devices. A configuration BPDU is at least 35 bytes long and includes the parameters such as the BID, root path cost, and PID. A bridge processes a received configuration BPDU only if either the sender BID or PID is different from that on the local bridge receive port. If both fields are the same as those on the receive port, the bridge discards the configuration BPDU. Therefore, the bridge does not need to process BPDUs with the same information as the local port.

STP CONVERGENCE

STEP-1 : Elect Root Bridge.

STEP -2 :Elect Root Port.

STEP-3 :Elect Designated Port

• Root bridge: There is only one root bridge on the entire STP network. The root bridge is the logical center, but not necessarily the physical center, of the network. Typically, the root bridge should have high performance and be located at a higher layer of the network. When the network topology changes, the root bridge will be re-elected accordingly. It is recommended that you manually configure the root bridge and secondary root bridge. Configure an optimal core switch as the root bridge to ensure stability of the STP Layer 2 network. Otherwise, new access devices may trigger STP root bridge switching, causing short service interruptions.
• Root port: The root port on an STP device has the smallest path cost to the root bridge and is responsible for forwarding data to the root bridge. Among all STP-capable ports on a device, the port with the smallest root path cost is a root port. An STP device has only one root port, and there is no root port on the root bridge.
• Designated port: The designated port is located on a designated bridge and forwards configuration BPDUs to the device or LAN.