Continue the series of Abstract CCNA study guide book .
Data Encapsulation
To communicate and exchange information, each layer uses Protocol Data Units (PDUs). These hold the control information attached to the data at each layer of the model. They are usually attached to the header in front of the data field but can also be in the trailer.
PORT NUMBER
The Transport layer uses port numbers to define both the virtual circuit and the upper-layer process, as you can see from Figure 1.30.
If you’re using TCP, the virtual circuit is defined by the source port number. Remember, the host just makes this up starting at port number 1024 (0 through 1023 are reserved for well-known port numbers). The destination port number defines the upper-layer process (application) that the data stream is handed to when the data stream is reliably rebuilt on the receiving host.
The Cisco Three-Layer Hierarchical Model
The Cisco hierarchical model can help you design, implement, and maintain a scalable, reliable, cost-effective hierarchical internetwork. Cisco defines three layers of hierarchy, as shown in next Figure , each with specific functions.
The following are the three layers and their typical functions:
- The core layer: backbone
- The distribution layer: routing
- The access layer: switching
Remember, however, that the three layers are logical and are not necessarily physical devices.
when we build physical implementations of hierarchical networks, we may have many devices in a single layer, or we might have a single device performing functions at two layers.
Now, let’s take a closer look at each of the layers.
The Core Layer
the core layer is responsible for transporting large amounts of traffic both reliably and quickly.
The only purpose of the network’s core layer is to switch traffic as fast as possible.
There is some things we don’t want to do in core layer:
- Don’t do anything to slow down traffic as access lists, VLANs.
- Don’t support workgroup access here.
- Avoid expanding the core (i.e., adding routers).
Now, there are a few things that we want to do as we design the core:
- high reliability.
- Design with speed in mind.
- Select routing protocols with lower convergence times.
The Distribution Layer
The distribution layer is sometimes referred to as the workgroup layer
The primary functions of the distribution layer are to provide routing, filtering, and WAN access and to determine how packets can access the core.
There are several actions that generally should be done at the distribution layer:
- Routing
- Implementing tools (such as access lists), packet filtering, and queuing
- Implementing security and network policies, including address translation and firewalls
- Redistributing between routing protocols, including static routing
- Routing between VLANs and other workgroup support functions
- Defining broadcast and multicast domains
Things to avoid at the distribution layer are limited to those functions that exclusively belong to one of the other layers.
The Access Layer
The access layer controls user and workgroup access to internetwork resources. The access
layer is sometimes referred to as the desktop layer.
The following are some of the functions to be included at the access layer:
- Continued (from distribution layer) use of access control and policies
- Creation of separate collision domains (segmentation)
- Workgroup connectivity into the distribution layer
Technologies such as DDR and Ethernet switching are frequently seen in the access layer. Static routing (instead of dynamic routing protocols) is seen here as well.
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