When a switch receives a broadcast frame (that is, a frame destined for MAC address FFFF.FFFF.FFFF), the switch floods the frame out all switch ports other than the port on which the frame was received. Because a layer 2 frame does not have a TTL field, a broadcast frame endlessly circulates through the Layer 2 topology, consuming resources on both switches and attached devices (for example, user PCs).
Figure 11-14 and the following list illustrates how a broadcast storm can form in a Layer 2 topology when STP (Spanning Tree Protocol) is not functioning correctly:
Step 1.
PC1 sends a broadcast frame on to segment A, and the frame enters each switch on port Gig 0/1.
Step 2.
Both switches flood a copy of the broadcast frame out their Gig 0/2 ports (that is, on to segment B), causing PC2 to receive two copies of the broadcast frame.
Step 3.
Both switches receive a copy of the broadcast frame on their Gig 0/2 ports (that is, on to segment B) and flood the frame out their Gig 0/1 ports (that is, on to segment A), causing PC1 to receive two copies of the broadcast frame.
STP prevents Layer 2 loops - which might result in a broadcast storm or corruption of a switch's MAC address table - from occurring in a network.
STP Operation described on page 301 of Learn more about the CompTIA Network+ N10-008 Cert Guide at amazon.com
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
CompTIA Network+ N10-008 Cert Guide contains proven study features that allow you to succeed on the exam the first time. Expert instructor Anthony Sequeira shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills, essential for successful completion of the performance-based testing items on the exam. This complete, CompTIA-approved study package includes the following:
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Learn more about the CompTIA Network+ N10-008 Cert Guide at amazon.com
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