style="font-size:15px;"> The core layer is literally the core of the network. At the top of the hierarchy, 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. The traffic transported across the core is common to a majority of users. But remember that user data is processed at the distribution layer, which forwards the requests to the core if needed.
If there's a failure in the core, every single user can be affected! This is why fault tolerance at this layer is so important. The core is likely to see large volumes of traffic, so speed and latency are driving concerns here. Given the function of the core, we can now consider some design specifics. Let's start with some things we don't want to do:
■ We don't want 24/7 connectivity.
■ Never do anything to slow down traffic. This includes making sure you don't use access lists, perform routing between virtual local area networks, or implement packet filtering.
■ Don't support workgroup access here.
■ Avoid expanding the core (e.g., adding routers when the internetwork grows). If performance becomes an issue in the core, give preference to upgrades over expansion.
Here's a list of things that we want to achieve as we design the core:
■ Design the core for high reliability. Consider data-link technologies that facilitate both speed and redundancy, like Gigabit Ethernet with redundant links or even 10 Gigabit Ethernet.
■ Design with speed in mind. The core should have very little latency.
■ Select routing protocols with lower convergence times. Fast and redundant data-link connectivity is no help if your routing tables are shot!
The Distribution Layer
The distribution layer is sometimes referred to as the workgroup layer and is the communication point between the access layer and the core. The primary functions of the distribution layer are to provide routing, filtering, and WAN access and to determine how packets can access the core, if needed. The distribution layer must determine the fastest way that network service requests are handled – for example, how a file request is forwarded to a server. After the distribution layer determines the best path, it forwards the request to the core layer if necessary. The core layer then quickly transports the request to the correct service.
The distribution layer is where we want to implement policies for the network because we are allowed a lot of flexibility in defining network operation here. There are several things that should generally be handled 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
Key things to avoid at the distribution layer are those that are limited to 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 network resources most users need will be available locally because the distribution layer handles any traffic for remote services.
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 (microsegmentation/switches)
■ Workgroup connectivity into the distribution layer
■ Device connectivity
■ Resiliency and security services
■ Advanced technology capabilities (voice/video, etc.)
Technologies like Gigabit or Fast Ethernet switching are frequently seen in the access layer.
I can't stress this enough – just because there are three separate levels does not imply three separate devices! There could be fewer or there could be more. After all, this is a layered approach.
Collapsed Core
In the collapsed core approach the distribution layer and the core layer are combined into a single layer, thus the name collapsed core. When using this design it is critical that the devices operating as both distribution and core devices must exhibit the following characteristics:
■ High speed paths connecting to the network
■ Must be a Layer-2 aggregation point
■ Must enforce routing and network access policies
■ Must be capable of Intelligent network services such as QoS, and network virtualization.
The benefits are reduced cost in equipment, while the drawbacks can be slower performance and reduced network availability as compared to the three tier model.
Exam Essentials
Identify the layers in the Cisco three-layer model, and describe the ideal function of each layer. The three layers in the Cisco hierarchical model are the core (responsible for transporting large amounts of traffic both reliably and quickly), distribution (provides routing, filtering, and WAN access), and access (workgroup connectivity into the distribution layer).
Compare and contrast network topologies
Understand that every type of network has both a physical and a logical topology. The physical topology of a network refers to the physical layout of the devices, but mostly the cabling and cabling layout. The logical topology defines the logical path on which the signal will travel on the physical topology. Figure 1.11 shows the four types of topologies:
Figure 1.11 Physical vs. Logical Topolgies
Here are the topology types, although the most common, and pretty much only network we use today is a physical star, logical bus technology, which is considered a hybrid topology (think Ethernet):
■ Bus: In a bus topology, every workstation is connected to a single cable, meaning every host is directly connected to every other workstation in the network.
■ Ring: In a ring topology, computers and other network devices are cabled together in a way that the last device is connected to the first to form a circle or ring.
■ Star: The most common physical topology is a star topology, which is your Ethernet switching physical layout. A central cabling device (switch) connects the computers and other network
