Cisco Module 2: Switching Concepts

Module 2: Switching Concepts

Module Objective

  • Switching Concepts: Explain how Layer 2 switches forward data.

Topics and Objectives

  • Frame Forwarding: Explain how frames are forwarded in a switched network.

  • Switching Domains: Compare a collision domain to a broadcast domain.

2.1 Frame Forwarding

Overview of Frame Forwarding

  • Terms associated with frames entering or leaving an interface:

    • Ingress: Refers to data entering the interface.

    • Egress: Refers to data exiting the interface.

  • A switch forwards frames based on the ingress interface and the destination MAC address.

  • Forwarding Decision: A switch makes its forwarding decisions using its MAC address table.

    • An important note: A switch will never forward traffic out the same interface that it received the traffic from.

Switch MAC Address Table

  • Purpose: The switch uses the destination MAC address to determine the egress interface.

  • Learning Process: A switch builds its MAC address table, also known as a Content Addressable Memory (CAM) table, by recording:

    • The source MAC address

    • The port it was received on.

Switch Learn and Forward Method

  • The switch employs a two-step process to forward frames:

    • Step 1: Learn

    • Examine Source Address: Adds the source MAC to the table if not already present.

    • Resets the timeout setting back to 5 minutes if the source MAC address is already in the table.

    • Step 2: Forward

    • Examine Destination Address:

      • If the destination MAC is in the MAC address table, the frame is forwarded out the specified port.

      • If the destination MAC is not in the table, the frame is flooded out all interfaces except for the one it was received on.

Switch Forwarding Methods

Forwarding Decision Making

  • Switches utilize software on Application-Specific Integrated Circuits (ASICs) to make rapid forwarding decisions.

Types of Switching Methods

  • After receiving a frame, a switch employs one of two methods for forwarding:

    1. Store-and-Forward Switching:

    • Process: Receives the entire frame before making a forwarding decision ensuring it is valid.

    • Preferred Method: Cisco’s preferred switching method for its ability to check frames for errors.

    1. Cut-Through Switching:

    • Process: Begins to forward the frame immediately after determining the destination MAC address and the egress port.

Characteristics of Store-and-Forward Switching

  • Error Checking: The switch checks the Frame Check Sequence (FCS) for CRC errors, discarding bad frames.

  • Buffering: The ingress interface buffers the frame while checking the FCS. This allows for adjustments if there is a speed difference between ingress and egress ports.

Characteristics of Cut-Through Switching

  • Forwards the frame immediately after determining the destination MAC address.

  • Fragment (Frag) Free Method: Checks that frames are at least 64 bytes to eliminate runt packets.

  • Use Cases:

    • Ideal for scenarios needing low latency (<10 microseconds).

  • Drawbacks:

    • Does not check FCS, potentially leading to error propagation.

    • May induce bandwidth issues if errors are propagated excessively.

    • Cannot support ports of differing speeds between ingress and egress.

2.2 Switching Domains

Collision Domains

  • Role of Switches: Switches eliminate collision domains and reduce congestion.

  • Full Duplex:

    • If there is full-duplex communication on the link, collision domains are eliminated.

  • Half-Duplex:

    • If one or more devices operate in half-duplex mode, it creates a collision domain where bandwidth contention occurs, making collisions possible.

  • Auto-Negotiation: Most devices, such as Cisco and Microsoft products, utilize auto-negotiation as the default setting for duplex and speed configurations.

Broadcast Domains

  • A Broadcast Domain encompasses all devices that can receive broadcast traffic across all Layer 1 or Layer 2 devices on a Local Area Network (LAN).

  • Only a Layer 3 device (Router) can break a broadcast domain, which is also referred to as a MAC broadcast domain.

  • Flooding Behavior: When a Layer 2 switch receives a broadcast, it floods the signal out on all interfaces except the ingress interface.

  • Potential Issues: At excessive levels, broadcast traffic can cause network congestion and poor performance.

  • Network Expansion: Increasing the number of devices at Layer 1 or Layer 2 extends the broadcast domain.

Alleviating Network Congestion

  • Switches utilize the MAC address table while employing full-duplex operation to eliminate collisions and thereby avoid congestion.

  • Features Relieving Congestion:

    • Fast Port Speeds: Certain switch models may offer up to 100Gbps.

    • Fast Internal Switching: Utilizes fast internal buses or shared memory for improved performance.

    • Large Frame Buffers: Provides temporary storage for handling large amounts of frames efficiently.

    • High Port Density: Allows many devices to connect to a LAN in a cost-effective manner, increasing local traffic handling and reducing congestion.

2.3 Module Practice and Quiz

Learning Summary from the Module

  • Frame Forwarding:

    • Ingress is the entry port; Egress is the exit port.

    • The switch constructs a MAC address table to effectively forward frames on the LAN.

    • The switch can operate using either the store-and-forward or cut-through method for frame forwarding.

  • Switching Domains:

    • Ethernet ports configured in half-duplex will be part of a collision domain, whereas full-duplex ports eliminate collision domains.

    • A switch will flood all interfaces except for the ingress port if the frame is a broadcast or if the destination MAC in unicast is unknown.

    • Broadcast domains can be segmented by a Layer 3 device like a router, while switches can eliminate collision domains but extend broadcast domains, helping to mitigate network congestion.