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Overview of Switching Logic

Ultimately , the role of a LAN switch is to forward Ethernet frames. LANs exist as a set of
user devices, servers, and other devices that connect to switches, with the switches connected
to each other. The LAN switch has one primary job: to forward frames to the correct
destination (MAC) address. And to achieve that goal, switches use logic—logic based on the
source and destination MAC address in each frame’s Ethernet header.

LAN switches receive Ethernet frames and then make a switching decision: either forward
the frame out some other ports or ignore the frame. To accomplish this primary mission,
switches perform three actions:
1. Deciding when to forward a frame or when to filter (not forward) a frame, based on
the destination MAC address
2. Preparing to forward frames by learning MAC addresses by examining the source
MAC address of each frame received by the switch
3. Preparing to forward only one copy of the frame to the destination by creating
a (Layer 2) loop-free environment with other switches by using Spanning Tree
Protocol (STP)

The first action is the switch’s primary job, whereas the other two items are overhead

NOTE Throughout this book’s discussion of LAN switches, the terms switch port and
switch interface are synonymous.

Although Chapter 2’s section titled “Ethernet Data-Link Protocols” already discussed the
frame format, this discussion of Ethernet switching is pretty important, so reviewing the
Ethernet frame at this point might be helpful. Figure 7-2 shows one popular format for an
Ethernet frame. Basically, a switch would take the frame shown in the figure, make a decision
of where to forward the frame, and send the frame out that other interface.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 7-2 IEEE 802.3 Ethernet Frame (One Variation)

Most of the upcoming discussions and figures about Ethernet switching focuses on the use
of the destination and source MAC address fields in the header. All Ethernet frames have both
a destination and source MAC address. Both are 6-bytes long (represented as 12 hex digits in
the book), and are a key part of the switching logic discussed in this section. Refer back to
Chapter 2’s discussion of the header in detail for more info on the rest of the Ethernet frame.

NOTE The companion DVD and website include a video that explains the basics of
Ethernet switching.

Now on to the details of how Ethernet switching works!

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