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LAN Switching Summary

Switches use Layer 2 logic, examining the Ethernet data-link header to choose how to process
frames. In particular, switches make decisions to forward and filter frames, learn MAC
addresses, and use STP to avoid loops, as follows:

Step 1. Switches forward frames based on the destination MAC address:
A. If the destination MAC address is a broadcast, multicast, or unknown destination
unicast (a unicast not listed in the MAC table), the switch floods the
frame.
B. If the destination MAC address is a known unicast address (a unicast
address found in the MAC table):
i. If the outgoing interface listed in the MAC address table is different
from the interface in which the frame was received, the switch forwards
the frame out the outgoing interface.
ii. If the outgoing interface is the same as the interface in which the
frame was received, the switch filters the frame, meaning that the
switch simply ignores the frame and does not forward it.

Step 2. Switches use the following logic to learn MAC address table entries:
A. For each received frame, examine the source MAC address and note the
interface from which the frame was received.
B. If it is not already in the table, add the MAC address and interface it was
learned on.

Step 3. Switches use STP to prevent loops by causing some interfaces to block, meaning
that they do not send or receive frames.

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