There are typically multiple IP subnets on a typical data communication network. Layer 3 devices are required to exchange data between these IP subnets. These devices have the routing capability and can forward data across subnets. Routing is the basic element of data communication networks. It is the process of selecting paths on a network along which packets are sent from a source to a destination. This note introduces the basic concepts of routing.

Static routes are manually configured. If a network topology changes, static routes have to be manually adjusted, which restricts the large-scale application of static routes on the live network. Dynamic routing protocols are widely used on live networks because of their high flexibility, high reliability, and easy scalability. The Open Shortest Path First (OSPF) protocol is a widely used dynamic routing protocol. This course describes basic concepts,working mechanism, and basic configurations of OSPF.

Data transmission on networks must comply with certain standards. Ethernet protocols define how data frames are transmitted over an Ethernet network. Understanding Ethernet protocols is the basis for fully understanding communication at the data link layer. An Ethernet switch is the main device for implementing data link layer communication. It is essential to understand how an Ethernet switch works. This note describes the concepts related to Ethernet protocols, MAC address types, and working process and mechanism of Layer 2 switches.

Ethernet technology implements data communication over shared media based on carrier sense multiple access with collision detection (CSMA/CD). If there are a large number of PCs on the Ethernet, security risks and broadcast storms may occur, deteriorating network performance and even causing network breakdowns. The virtual local area network (VLAN) technology is therefore introduced to solve the preceding problem. This note describes basic VLAN principles, working principles of different Layer 2 interfaces, VLAN applications, data forwarding principles, and basic VLAN configuration methods.

On an Ethernet switching network, redundant links are used to implement link backup and enhance network reliability. However, the use of redundant links may produce loops, leading to broadcast storms and an unstable MAC address table. As a result, communication on the network may deteriorate or even be interrupted. To prevent loops, IEEE introduced the Spanning Tree Protocol (STP). Devices running STP exchange STP Bridge Protocol Data Units (BPDUs) to discover loops on the network and block appropriate ports. This enables a ring topology to be trimmed into a loop-free tree topology, preventing infinite looping of packets and ensuring packet processing capabilities of devices. IEEE introduced the Rapid Spanning Tree Protocol (RSTP) to improve the network convergence speed.

By default, a Layer 2 switching network is a broadcast domain, which brings many problems. Virtual local area network (VLAN) technology isolates such broadcast domains, preventing users in different VLANs from communicating with each other. However, such users sometimes need to communicate. This course describes how to implement inter-VLAN communication.

As services develop and the campus network scale expands, users have increasingly demanding requirements on network bandwidth and reliability. Traditional solutions improve network bandwidth by upgrading devices and implement high reliability by deploying redundant links and using the Spanning Tree Protocol (STP), leading to low flexibility, time-consuming troubleshooting, and complex configuration. This chapter describes how to use Eth-Trunk, intelligent stack (iStack), and cluster switch system (CSS) technologies to improve network bandwidth and reliability.

Rapid network development brings challenges to network security and quality of service(QoS). Access control lists (ACLs) are closely related to network security and QoS. By accurately identifying packet flows on a network and working with other technologies, ACLs can control network access behaviors, prevent network attacks, and improve network bandwidth utilization, thereby ensuring network environment security and QoS reliability. This course describes the basic principles and functions of ACLs, types and characteristics of ACLs, basic composition of ACLs, ACL rule ID matching order, usage of wildcards, and ACL configurations.

User management is one of the most basic security management requirements for any network. Authentication, authorization, and accounting (AAA) is a management framework that provides a security mechanism for authorizing some users to access specified resources and recording the operations of these users. AAA is widely used because of its good scalability and easy implementation of centralized management of user information. AAA can be implemented through multiple protocols. In actual applications, the Remote Authentication Dial-In User Service (RADIUS) protocol is the most commonly used to implement AAA. This course describes the basic concepts, implementation, basic configurations, and typical application scenarios of AAA.

With the development of the Internet and the increase of network applications, limited public IPv4 addresses have become the bottleneck of network development. To solve this problem, Network Address Translation (NAT) was introduced. NAT enables hosts on an internal network to access an external network. It not only helps alleviate IPv4 address shortage but also improves the security of the internal network as NAT prevents devices on the external network from directly communicating with hosts on the internal network that uses private addresses. This course describes the motivation behind NAT, and implementations and application scenarios of different types of NAT.