Consistent Behavior over All Links Using the Ethernet Data Link Layer
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Although Ethernet includes many physical layer standards, Ethernet acts like a single LAN
technology because it uses the same data link layer standard over all types of Ethernet
physical links. That standard defines a common Ethernet header and trailer. (As a reminder,
the header and trailer are bytes of overhead data that Ethernet uses to do its job of sending
data over a LAN.) No matter whether the data flows over a UTP cable or any kind of fiber
cable, and no matter the speed, the data-link header and trailer use the same format.
While the physical layer standards focus on sending bits over a cable, the Ethernet data-link
protocols focus on sending an Ethernet frame from source to destination Ethernet node.
From a data-link perspective, nodes build and forward frames. As first defined in Chapter 1,
“Introduction to TCP/IP Networking,” the term frame specifically refers to the header and
trailer of a data-link protocol, plus the data encapsulated inside that header and trailer. The
various Ethernet nodes simply forward the frame, over all the required links, to deliver the
frame to the correct destination.
Figure 2-4 shows an example of the process. In this case, PC1 sends an Ethernet frame to
PC3. The frame travels over a UTP link to Ethernet switch SW1, then over fiber links to
Ethernet switches SW2 and SW3, and finally over another UTP link to PC3. Note that the
bits actually travel at four different speeds in this example: 10 Mbps, 1 Gbps, 10 Gbps, and
100 Mbps, respectively.
Figure 2-4 Ethernet LAN Forwards a Data-Link Frame over Many Types of Links
So, what is an Ethernet LAN? It is a combination of user devices, LAN switches, and different
kinds of cabling. Each link can use different types of cables, at different speeds.
However, they all work together to deliver Ethernet frames from the one device on the
LAN to some other device.
The rest of this chapter takes these concepts a little deeper, first looking at the details
of building the physical Ethernet network, followed by some discussion of the rules for
forwarding frames through an Ethernet LAN.