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Media access control database instability

Media access control database instability
7.1.6 This page will explain how incorrect information can be forwarded in a redundant switched network.
In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port. In this example the MAC address of Router Y is not in the MAC address table of either switch.
Host X sends a frame directed to Router Y. Switches A and B learn the MAC address of Host X on port 0.
The frame to Router Y is flooded on port 1 of both switches. Switches A and B receive this information on port 1 and incorrectly learn the MAC address of Host X on port 1. When Router Y sends a frame to Host X, Switch A and Switch B also receive the frame and will send it out port 1. This is unnecessary, but the switches have incorrectly learned that Host X is on port 1.
In this example the unicast frame from Router Y to Host X will be caught in a loop.
This page concludes this lesson. The next lesson will describe the Spanning-Tree Protocol (STP). The first page will discuss physical and logical loops in a redundant network




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