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Configuring Catalyst Switch Operations

Identifying Problems ThatOccur in RedundantSwitchedTopologies

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Outline

Overview

Redundant Switched and Bridged Topologies

Broadcast Storms

Multiple Frame Transmissions

MAC Database Instability

Summary

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Redundant topology eliminates single points of failure.

Redundant topology causes broadcast storms, multiple frame

copies, and MAC address table instability problems.

Redundant Topology

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Host X sends a broadcast.

Switches continue to propagate broadcast traffic

over and over.

Broadcast Storms

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Host X sends a unicast frame to router Y.

The MAC address of router Y has not been learned by

either switch.

Router Y will receive two copies of the same frame.

Multiple Frame Copies

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Host X sends a unicast frame to router Y.

The MAC address of router Y has not been learned by either switch.

Switches A and B learn the MAC address of host X on port 0.

The frame to router Y is flooded.

Switches A and B incorrectly learn the MAC address of host X on port 1.

MAC Database Instability

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Summary

Bridged and switched networks are commonly designedwith redundant links and devices, which can introduceproblems, such as broadcast storms, multiple frametransmission, and MAC database instability.

A broadcast storm is created when each switch on aredundant network floods broadcast frames endlessly.

Multiple frame transmissions occur when multiplecopies of the same frame arrive at the intended host,potentially causing problems with the receiving protocol.

MAC database instability occurs when multiple copies ofa frame arrive on different ports of a switch.

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Module Summary

LAN switches and bridges have the ability to listento traffic and make intelligent forwarding decisionsat Layer 2.

Most complex networks include redundant devices

to avoid single points of failure. Although aredundant topology eliminates some problems, itcan introduce others.

STP provides path redundancy while preventingundesirable loops in switched or bridged networks.

A Cisco Catalyst switch comes with factory defaultsettings, which set up the switch to function as atransparent bridge.

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