Redundant Topology Overview

© 2002, Cisco Systems, Inc. All rights reserved.
© 2002, Cisco Systems, Inc. All rights reserved.
2
Redundant Topology Overview
Objectives
Upon completing this lesson, you will be able to:
Describe the features of redundant switch and bridge topologies
Explain the problems associated with broadcast storms, multiple frame transmissions, and MAC address table instability
Describe loop avoidance and explain how it can solve redundant switch topology issues
Redundant topology eliminates single points of failure.
Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems.
Redundant Topology
Host X sends a broadcast.
Switches continue to propagate broadcast traffic over and over.
Broadcast Storms
Host X sends a unicast frame to router Y.
MAC address of router Y has not been learned by either switch yet.
Router Y will receive two copies of the same frame.
Multiple Frame Copies
Host X sends a unicast frame to router Y.
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
Summary
Bridged and switched networks are commonly designed with redundant links and devices, which can introduce problems, such as broadcast storms, multiple frame transmission, and MAC database instability.
A broadcast storm is when each switch on a redundant network floods broadcast frames endlessly.
In a redundant topology, multiple copies of the same frame can arrive at the intended host, potentially causing problems with the receiving protocol.
MAC database instability results when multiple copies of a frame arrive on different ports of a switch.