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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 devices together. This category includes star and extended star topologies. Physical connection is commonly made using twisted-pair wiring.

      ■ Mesh: In a mesh topology, every network device is cabled together with a connection to each other. Redundant links increase reliability and self-healing. The physical connection is commonly made using fiber or twisted-pair wiring.

      ■ Hybrid: Ethernet uses a physical star layout (cables come from all directions), and the signal travels end-to-end, like a bus route.

      Exam Essentials

      Describe the major physical topologies in use. Identify the differences between a physical and logical topology. List the distinguishing features of the star, ring, bus, mesh, and hybrid topologies

      Select the appropriate cabling type based on implementation requirements

      The EIA/TIA (Electronic Industries Alliance and the newer Telecommunications Industry Association) is the standards body that creates the Physical layer specifications for Ethernet. The EIA/TIA specifies that Ethernet use a registered jack (RJ) connector on unshielded twisted-pair (UTP) cabling (RJ45). But the industry is moving toward simply calling this an 8-pin modular connector.

      Every Ethernet cable type that's specified by the EIA/TIA has inherent attenuation, which is defined as the loss of signal strength as it travels the length of a cable and is measured in decibels (dB). The cabling used in corporate and home markets is measured in categories. A higher-quality cable will have a higher-rated category and lower attenuation. For example, category 5 is better than category 3 because category 5 cables have more wire twists per foot and therefore less crosstalk. Crosstalk is the unwanted signal interference from adjacent pairs in the cable.

      Here is a list of some of the most common IEEE Ethernet standards, starting with 10 Mbps Ethernet:

      10Base-T (IEEE 802.3) 10 Mbps using category 3 unshielded twisted pair (UTP) wiring for runs up to 100 meters. Unlike with the 10Base-2 and 10Base-5 networks, each device must connect into a hub or switch, and you can have only one host per segment or wire. It uses an RJ45 connector (8-pin modular connector) with a physical star topology and a logical bus.

      100Base-TX (IEEE 802.3u) 100Base-TX, most commonly known as Fast Ethernet, uses EIA/TIA category 5, 5E, or 6 UTP two-pair wiring. One user per segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and

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