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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Figure 5-12. The Protocol Stack
Between two protocol stacks, members of the same layer are known as peers and communicate by well-
known (open and published) protocols. Within a protocol stack, adjacent layers communicate by an
internal interface. This interface is usually not publicly documented and is frequently proprietary. It
has some of the same characteristics of a protocol and two stacks from the same software vendor may
communicate in the same way. Two stacks from different software vendors (or different products from
the same vendor) may communicate in completely different ways. As long as peers can communicate
and interoperate, this has no impact on the functioning of the network.
The communication between layers within a given protocol stack can be both different from a second
stack and proprietary, but communication between peers on the same OSI layer is open and consistent.
A brief description of the most commonly used functional layers is helpful to understand the scope of
how protocol layering works.
Layer 1
This is referred to as the physical layer. It handles the electrical connections and signaling required to
make a physical link from one point in the network to another. It is on this layer that the unique Media
Access Control (MAC) address is defined.
Layer 2
This layer, commonly called the switching layer, allows end station addressing and the establishment of
connections between them.
Layer 2 switching forwards packets based on the unique MAC address of each end station and offers
high-performance, dedicated-bandwidth of Fast or Gigibit Ethernet within the network.
Layer 2 does not ordinarily extend beyond the intranet. To connect to the Internet usually requires a
router and a modem or other device to connect to an Internet Service Provider’s WAN. These are Layer
3 functions.
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