What is the working principle of the Ethernet switch board?

Raytrans Digital Technology Co., Ltd. has launched an Ethernet switch board on the IDM MSTP155-24 platform, which is based on the Spanning Tree Protocol and the Unidirectional Link Protocol. It enables Ethernet switching and achieves ring network protection even under unidirectional link conditions, effectively overcoming the issues in Ethernet ring network protection when a single-direction fiber break occurs in an SDH ring network.
The idea behind the Spanning Tree Protocol (STP) topology is that regardless of the physical connections between bridges (switches), the bridges (switches) can automatically discover a network with a loop-free topology, and this logical topology must be tree-structured. The Spanning Tree Protocol can also determine that there are sufficient connections to reach every part of the entire network. All network nodes either enter the forwarding state or the blocking state, thereby establishing a spanning tree for the entire LAN. When bridges are first connected or when the network structure changes, the bridges will recalculate the spanning tree topology. A root bridge is selected for a stable spanning tree topology, and when there are two or more paths for transmitting data from one point to another, only one active path with the shortest distance to the root bridge can be chosen. This control mechanism of the Spanning Tree Protocol can coordinate multiple bridges (switches) to work together, allowing the computer network to avoid the loss of overall network connectivity due to the failure of a single node, and the redundant network loops will not cause broadcast storms.
However, in ring network protection, STP alone is not sufficient, because there are situations where the Spanning Tree Protocol may fail in the network. If the Spanning Tree Protocol fails in a LAN with redundant links, the entire network may become blocked and paralyzed. It is generally believed that errors in the Spanning Tree Algorithm lead to bridging loops, and it is often assumed that Layer 2 link loops are caused by hardware device or system software bugs, but this is not necessarily the case.
In certain situations in an SDH ring network, undetected optical path failures may result in unidirectional links. Since STP has unidirectional BPDU (Bridge Protocol Data Unit) flows, it is quite sensitive to such failures. A link that can only communicate in one direction while still being in the LINKUP state is very dangerous for STP. If STP operates on a Layer 2 loop and the blocked port can no longer receive BPDUs from the upstream designated switch, then as the STP information on the port ages, the state of this blocked port will transition to the forwarding state. This creates a forwarding loop or an STP loop. If a loop running the Spanning Tree Protocol has no blocked ports, not only will ordinary broadcast packets form broadcast storms, but even BPDU packets will surge within the loop, causing multicast packet storms, which is even more fatal.
For the ports of two interconnected switches, if the auto-negotiation mechanism at both ends of the link detects the link as LINKUP, but the local end can receive information sent by the remote end through this link while the local end cannot receive information sent by the remote end, this constitutes a unidirectional link.
Based on the reasons described above, a unidirectional link detection mechanism is added at the link layer to promptly eliminate such unidirectional links before they form loops, thereby removing the hidden risks they pose to STP and ensuring the network is not affected by unidirectional links. The design purpose of unidirectional link detection mainly has two aspects. One is to detect unidirectional links existing in the network and prevent them from causing the Spanning Tree Protocol to fail; the other is to reduce the link's dependence on mechanisms such as physical layer auto-negotiation.
The protocol used for unidirectional link detection is DLDP (Device Link Detection Protocol). Its function is to detect, in a network with unidirectional links, the unidirectional links that would cause the Spanning Tree Protocol to fail before they do so, and to shut down such links, thereby effectively preventing network broadcast storms caused by Spanning Tree Protocol failure. In addition to being applied to fiber media, the Unidirectional Link Detection Protocol can also be applied to other media, such as twisted pair and coaxial cable.