Module 2

Module Overview

Module Title: Switching Concepts

Module Objective:

Explain how Layer 2 switches forward data efficiently and understand the intricacies of switching in a network environment.

Objectives:

• Frame Forwarding: Explain how frames are forwarded in a switched network, including the roles of ingress, egress, and the MAC address table.

• Switching Domains: Compare and contrast a collision domain to a broadcast domain, as well as the implications of each on network performance.

2.1 Frame Forwarding

Basic Terms:

• Ingress: Refers to the data entering a specific interface of a switch or router, marking the entry point in the frame's lifecycle.

• Egress: Denotes the data exiting from an interface, marking the exit point for frames being transmitted across the network.

Forwarding Mechanism:

• The forwarding process is fundamentally reliant on the ingress interface and the destination MAC address contained within the frame.

• MAC Address Table: A crucial component used to make forwarding decisions, it records previously observed source and destination MAC addresses to enable efficient traffic management.

• Traffic Directionality: A key rule in switching is that traffic will NOT be forwarded out of the same interface from which it was received, preventing loops and enhancing overall network efficiency.

2.2 The Switch MAC Address Table

MAC Address Table (CAM Table):

• The MAC Address Table, often referred to as the Content Addressable Memory (CAM) Table, is essential for storing destination MAC addresses detected from incoming frames in conjunction with their associated port numbers.

• Learning Process: The table is built dynamically by recording source MAC addresses when frames are received, thereby allowing the switch to learn the network topology and predict the best path for future frames.

2.3 Learn and Forward Method

Two-Step Process:

1. Learn: This initial phase involves examining the source address of incoming frames. If the source MAC address is not already present in the table, it is added. If it is already present, the timeout for this entry is reset, ensuring frequently used addresses remain in the memory.

2. Forward: The second phase requires examining the destination address. If the destination MAC is found in the MAC address table, the frame is forwarded to the corresponding port. If not found, the switch floods the frame out of all interfaces, excluding the ingress port, to reach the intended device.

2.4 Switch Forwarding Methods

Methods Used:

• Store-and-Forward Switching: This method involves receiving the entire frame and performing a validity check (Frame Check Sequence or FCS check) before forwarding. This approach minimizes errors and is considered Cisco’s preferred method due to its reliability.

• Cut-Through Switching: In contrast to store-and-forward, cut-through switching allows the switch to forward the frame immediately after determining the destination MAC address, without waiting for the entire frame to be received. While this leads to lower latency (under 10 microseconds), it risks propagating errors since no FCS checking is performed. It also utilizes the Fragment Free method, preventing issues with smaller frames less than 64 bytes.

3. Switching Domains

3.1 Collision Domains

• Collision Domain Definition: Collision domains arise in networks where devices communicate using half-duplex mode. In this configuration, only one device can transmit at a time, potentially leading to collisions.

• Full-Duplex Links: These configurations eliminate collision domains, allowing simultaneous sending and receiving of data, thus significantly reducing congestion and improving performance.

• Auto-Negotiation: Many devices, including Cisco switches, are equipped with auto-negotiation capabilities that automatically determine the optimal duplex setting, enhancing operational efficiency.

3.2 Broadcast Domains

• Broadcast Domain Definition: A broadcast domain encompasses all Layer 1 and Layer 2 devices on a Local Area Network (LAN) that receive broadcast packets sent by any single device. This requires careful management to prevent broadcast storms.

• Role of Layer 3 Devices: Only a Layer 3 device, such as a router, can segment broadcast domains, enabling better traffic management and reducing network congestion.

• Broadcast Traffic Handling: Switches will flood broadcast traffic to all ports except the ingress port, which is essential for certain communication protocols but can overwhelm the network if not managed properly.

Implications:

• An excessive number of broadcasts can lead to network congestion and poor performance, underscoring the importance of managing broadcast domains effectively.

3.3 Alleviated Network Congestion

Methods to Reduce Congestion:

• Fast Port Speeds: Technologies supporting port speeds of up to 100 Gbps depending on the model have been developed to accommodate increased data traffic.

• Fast Internal Switching: Improved internal switching capabilities enhance overall performance by maximizing the efficiency of internal resources.

• Large Frame Buffers: Expanded frame buffers that provide temporary storage for handling bursts of high frame volumes, preventing drops during peak traffic times.

• High Port Density: A higher density of ports allows for more devices to connect simultaneously while effectively managing the overall congestion within the network.

4. Module Practice and Quiz

Key Learnings:

• Understanding the difference between ingress and egress ports is crucial for effective traffic management in switched networks.

• The MAC address table plays an integral role in frame forwarding within local area networks (LANs) and influences switching behavior.

• Selecting between store-and-forward and cut-through methods impacts latency, error handling, and overall performance of the network.

• Recognizing the characteristics of collision and broadcast domains assists in designing and managing more effective network topologies.

5. New Terms and Commands

• CAM: Content Addressable Memory, commonly associated with the MAC address table.

• MAC Address Table: A directory that maps MAC addresses to specific ports on a switch.

• Store-and-Forward Switching: A validity checking method confirming data integrity before forwarding.

• Cut-Through Switching: An immediate forwarding method that skips validity checks, increasing speed at the risk of error propagation.

• Automatic Buffering: A technique involving temporary storage of frames during processing to manage traffic more efficiently.

• Fragment Free Switching: A variation of cut-through switching, ensuring that frames are of a minimum length to mitigate potential issues.