The Problem of STP

The transform from blocking state to forwarding state it takes 2 times forwarding delay. → use RSTP

→ allocating two port roles alternate port and backup port for root port and designated root, for fast state changing. When the root port is invalid, the alternate port will become the new root port and switch to forwarding state without delay. When the designated port is invalid, the backup port will become the new designated port and switch to forwarding state without delay.

→ in the point to point link only connecting two switch ports, just after one way handshake to the downstream bridge, the designated port could change to forwarding state without time delay. If more than three bridges are connected by the shared lnk, the downstream bridge will not respond to the handshake request sent from upstream designated port; only after two times forward delay, it could change to forwarding state.

→ the port is defined as edge port if it is connected with terminal directly instead of other bridges. The edge port could enter forwarding state without any time delay. However, it should be configured manually since the bridge cannot know whether the port is directly connected with the terminal or not.

The second problem of STP : in the case of VLAN, STP makes the host in some VLAN lost communication. How to resolve it? → Multiple Spanning Tree Protocol (MSTP) → IEEE 802.1s

STP/RSTP is based on port, but MSTP based on instance. The instance is a collection of multiple VLANs. Through bounding multiple VLANs into a single instance, the communication cost and network resources could be saved. In MSTP, the topology calculation of every instance independent. The load balancing could be implemented in these instances. In use, multiple VLANs with the same topology could be mapped into the same instance.

Q/A :

How does STP calculate a tree without loop in the network?

→ STP elects a root bridge, and then elects a root port for each non-root switch and elects a designated port for each network segment. The ports that are neither the root port nor the designated port are set to be in blocking state.

How does STP solve temporary loop problem?

→ Before switching from non-Forwarding state to Forwarding state, a port needs to wait two times as long as the forward delay. This ensures that other switches have enough time to calculate the spanning tree.