What will happen if both ends of the same network cable are plugged into the same switch?

If both ends of the same network cable are plugged into the same switch, a network loop failure will occur.

Network loops are also divided into Layer 2 loops and Layer 3 loops. All loops are formed due to confusion caused by unclear destination paths. (Recommended learning: web front-end video tutorial)

For example, in the second layer, when a broadcast message passes through two switches, it will continuously generate broadcasts in a vicious cycle, causing a loop. A layer loop occurs when the original route unexpectedly fails to work, causing route advertisement errors, forming a vicious cycle

Cause

Layer 2 loops in the network usually In an environment where the office area moves or network nodes are relatively dense, because the crystal plugs at both ends of the network jumper are consistent, there is no distinction between connecting to the Hub/switch or the PC, resulting in greater randomness in access.

This allows users to freely connect network jumpers to ports at the same time. Once this problem occurs, a loop will be formed. The harm of network loops is very serious, and in worst case, it will cause the entire network of a company to be damaged. Network outages, at least to the network center in a region, can cause huge losses to a company's production and operations.

Although Layer 2 physical loops are rare in ordinary offices or scenarios where network nodes are not dense, they are used in enterprises with dense network nodes such as IT manufacturing or school laboratories because Due to the mobility of personnel, the density of network nodes, and the consistency of RJ45 at both ends of the jumper, layer 2 network loops are not uncommon.

STP mostly does not take effect in this environment and cannot be enabled well. Because there are certain problems between STP and the production control program or implementation program, it leads to layer 2 network loops in such enterprises. It has become a hidden danger, like a time bomb, and you don’t know when it will explode.

Causes of interruption

1. Loops cause network broadcast storms, exhaust switching resources, and cause switch paralysis.

Broadcast reports in the network will continue to be forwarded and broadcast in a loop after entering the loop, and cannot be ended. A large number of data packets can cause the switch's CPU to reach 85-100%, causing the switch to be paralyzed.

2. The loop causes MAC address drift, causing network interruption.

Since the switch has a learning function, as long as the host in the network sends a broadcast message, the MAC address will be learned to the port with a network loop. An incorrect MAC address table will directly cause network interruption. MAC address drift is the main reason for immediate network outage.

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