A duplex mismatch is a situation where two devices on a network have different duplex settings, such as full-duplex or half-duplex. Full-duplex means that a device can send and receive data simultaneously, while half-duplex means that a device can only send or receive data at a time. A duplex mismatch can cause performance issues, such as collisions, errors, or slow throughput. In this scenario, the customer added a hub between the switch and the router. A hub is a device that operates at half-duplex and broadcasts all traffic to all ports. A switch and a router are devices that operate at full-duplex and forward traffic to specific ports.
Therefore, adding a hub between the switch and the router can cause a duplex mismatch and slow down all the other network components that were connected to the switch.
References: https://www.comparitech.com/net-admin/hub-vs-switch-vs-router/
https://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/10561-3.html

MDIX (メディア依存インターフェイス クロスオーバー) は、ネットワーク デバイスが適切なケーブル タイプを自動的に検出して構成できるようにする機能であり、クロスオーバー ケーブルが不要になります。スイッチで MDIX を有効にすると、技術者は元の Cat 6 ケーブルを使用してクロスオーバー接続を作成できます。
参考文献: CompTIA Network+ 認定試験学習ガイド、第 6 版、Glen E. Clarke 著

Explanation
CRC stands for Cyclic Redundancy Check, and it is a type of error-detecting code used to detect accidental changes to raw data. If the CRC count is increasing on a particular interface, it indicates that there might be an issue with the cabling, which is causing data corruption. References:
Network+ N10-008 Objectives: 2.1 Given a scenario, troubleshoot common physical connectivity issues.

192.168.0.0/20 CIDR は 3 番目のオクテットにあります。
/1-8 (第 1 オクテット)、/9-16 (第 2 オクテット)、/17-24 (第 3 オクテット)、/25-32 (第 4 オクテット)。
/17 = 128
/18 = 192
/19 = 224
/20 = 240が答えです
/21 = 248
/22 = 252
/23 = 254
/24 = 255