Overview

Introduction

The Configure and Verify Switching Concepts module provides you with the instructions and Cisco hardware to develop your hands on skills in understanding, configuring and verifying various features of switches. This module includes the following activities:

  • Examine the functionality of the MAC Address Table
  • Configure parameters of the MAC Address Table and observe their results
  • Examine frame switching methods and understand how they function
  • Observe frame flooding and understand its purpose and its vulnerabilities

Exercise 1 - Examining the Functionality of the MAC Address Table

The MAC Address Table, also known as the Content Addressable Memory table, or CAM table, is a data structure that is created and maintained by a switch to keep track of which end-device MAC addresses are associated with which switch port. The purpose of the MAC table is to allow the switch to perform the most fundamental function that distinguishes it from a hub: to create one collision domain per interface.

In this exercise, you will examine how the MAC table functions in order to understand its purpose more fully. Before continuing, make sure the PLABCSCO01 server is also powered on.

Exercise 2 - Examining MAC Address Aging

Dynamically learned MAC addresses remain in the MAC address table for a specific period of time. If there is no activity from the specific MAC address after this specified period of time, the address is removed from the switch. It is possible to adjust this aging time according to your network needs.

Exercise 3 - Frame Switching Methods and Frame Flooding

Frame switching refers to the method by which a frame is switched, or transferred from the ingress interface to the egress interface. There are two main methods by which this is accomplished: Store-and-Forward and Cut-Through Ethernet switching.

All frames have a certain size in bytes. The maximum size is usually around 1500, although this can be adjusted. It takes a finite amount of time for a frame to enter a switch port. Depending on the method of switching, this amount of time can add latency to network communications.

Store-and-Forward switching does what its name suggests. It receives a frame and stores it in its entirety in the switch buffer before it begins sending it out of its egress port. This allows the switch to read and calculate the Frame Check Sequence which is in the trailer of the frame, to verify that there were no errors in transmission before it sends it out of the egress port. In this scenario, both latency and reliability are increased.

Cut-Through switching begins sending a frame out the egress port before it has been received in its entirety. Switching begins once the destination MAC address has been read from the header of the frame and the egress port has been determined. Here, latency is decreased as is reliability.

The default method of switching for the switches used in these labs is Store-and Forward. In fact, it is the only method of switching that is supported by these switches. Cisco Nexus switches support Cut-Through switching by default and can be configured for either switching method.

In this exercise, you will examine the buffers that are used to store frames as they are switched from ingress to egress ports on a switch.

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