Ethernet Switch Board Principle

Raytrans launched the 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 under unidirectional link conditions and achieves ring network protection, effectively overcoming the issues in Ethernet ring protection when a single-direction fiber is broken in an SDH ring network.
The idea of the Spanning Tree Protocol (STP) topology is that, regardless of the physical connections between bridges (switches), the bridges (switches) can automatically discover a loop-free topological network, and this logical topology must be tree-shaped. 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 the spanning tree of 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. When transmitting data from one point to another, if more than two paths exist, only one active path with the shortest distance to the root bridge can be selected. Such a control mechanism of the Spanning Tree Protocol can coordinate multiple bridges (switches) to work together, enabling the computer network to avoid the loss of the entire network connectivity function 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 cases where the Spanning Tree Protocol fails in the network. If the Spanning Tree Protocol fails in a LAN providing redundant links, the entire network may be in a blocked and paralyzed state. Usually, errors in the spanning tree algorithm lead to bridging loops. It is generally believed that Layer 2 link loops are caused by bugs in device hardware or system software, but this is not necessarily the case.
In certain situations in an SDH ring network, undetected optical path failures may result in the existence of unidirectional links. Since the spanning tree has unidirectional BPDU (Bridge Protocol Data Unit) flows, it is quite sensitive to such failures. A link that can only communicate in one direction but remains in the LINKUP state is very dangerous for STP. If STP runs on a Layer 2 loop and the blocked port no longer receives 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 can be even more fatal.
For the ports of two interconnected switches, if the auto-negotiation mechanisms at both ends of the link detect the link as LINKUP, but the peer end can receive information sent by the local end through this link while the local end cannot receive information sent by the peer 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, eliminating the hidden dangers they pose to STP and ensuring that 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 failure of the Spanning Tree Protocol; the other is to reduce the dependence of links on mechanisms such as physical layer auto-negotiation.
The protocol adopted for the unidirectional link detection principle is DLDP (Device Link Detection Protocol). Its function is to detect, in a network with unidirectional links, the unidirectional links that would cause the failure of the Spanning Tree Protocol before they cause such failure, and to shut down these links, thereby effectively preventing network broadcast storms caused by the failure of the Spanning Tree Protocol. 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.