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7.1 | |
Redundant Topologies | |
| 7.1.6 | |
Media access control database instability |
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 see 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 will 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.
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