What is and how to implement STP slow convergence

14

STP slow convergence is a concept relative to the fast convergence of RSTP and MSTP point-to-point links. It is not a new function but the initial convergence mode of the STP protocol.

The STP port is blocked by default. After the STP root bridge is selected and the port role calculation is performed, the port enters the listening state.

After the first ForwardDelay, the status switches from listening to learning. This ForwardDelay takes 15 seconds, during which the port is in the listening state.

After the second ForwardDelay, the status switches from learning to forwarding. This ForwardDelay takes 15 seconds, during which the MAC address learning is completed.

The two ForwardDelay takes total 30 seconds during which STP port enters the forwarding process. This whole process is called slow convergence.

NOTE:
During the 30 seconds' slow convergence, the port is blocked, and the sent DHCP packets are discarded for multiple times, making the DHCP protocol run abnormally. In this case, modify configuration to prevent slow convergence.

Other related questions:
How to prevent slow convergence on STP edge ports connected to terminals
Edge devices cannot participate in the STP calculation or respond to STP packets, causing slow convergence. You can use either of the following methods to prevent slow convergence on edge ports connected to user terminals or servers: 1. Run the stp disable command on the ports so that the ports always remain in the Forwarding state. 2. Run the stp edge-port enable command on the ports to configure them as edge ports, and then run the stp bpdu-protection command on the ports to enable BPDU protection. An edge port is in Forwarding state by default. The BPDU protection function shuts down an edge port after the port receives a BPDU. The edge ports that are shut down must be manually restored.

Why is the convergence for multiple MST regions slow
When a specified interface of the root bridge is faulty, the two packets that contain different root bridge information in an MST region may interact with each other continuously. The root bridge selection flaps for a period. As a result, the negotiation time is long and the network convergence is slow. Using the stp max-hops command, you can properly limit the maximum number of hops as required. Thus, the root bridge can be quickly confirmed and the convergence time can be shortened.

How do I implement multipoint convergence on the DP300?
For details about how to implement multipoint convergence on the DP300, see 7.3 Multipoint Convergence.

How does RSTP implement P/A fast convergence
RSTP implements fast convergence by improving the STP protocol. The link is point-to-point, and RSTP changes to the forwarding state based on the P/A fast convergence: The upstream device sends the Proposal packet and starts the waiting timer. The downstream device blocks all other ports and sends back the Agreement packet to the upstream device. The upstream device receives the Agreement packet and the port enters the forwarding state. The preceding process is repeated between all upstream devices and downstream devices, implementing the fast convergence on the entire RSTP network. NOTE: The P/A fast convergence cannot be implemented for non-point-to-point links. If a Huawei device is connected to a device from other manufacturers, run the stp no-agreement check command to implement fast convergence.

Why does the STP convergence fail for a switch
The switch STP calculation, convergence, and damage are implemented using BPDUs. The BPDU processing capacity must be enabled for the port. Otherwise, the switch discards the BPDUs by default, making the STP convergence fail. NOTE: Globally run the bpdu enable command for the S2700 switch. Run the bpdu enable command on the port for other devices.

If you have more questions, you can seek help from following ways:
To iKnow To Live Chat
Scroll to top