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The Cisco Three-Layered Hierarchical Model

By SemSim.com

Cisco has defined a hierarchical model known as the hierarchical internetworking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical internetwork because rather than focusing on packet construction, it focuses on the three functional areas, or layers, of your network:

Core layer: This layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets.

Distribution layer: This layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. This layer is also called the Workgroup layer.

Access layer: This layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers.

Figure INT.2.1 displays the three layers of the Cisco hierarchical model.

Cisco hierarchical models

When you implement these layers, each layer might comprise more than two devices or a single device might function across multiple layers.The benefits of the Cisco hierarchical model include:

High Performance: You can design high performance networks, where only certain layers are susceptible to congestion.
Efficient management & troubleshooting: Allows you to efficiently organize network management and isolate causes of network trouble.
Policy creation: You can easily create policies and specify filters and rules.
Scalability: You can grow the network easily by dividing your network into functional areas.

Behavior prediction: When planning or managing a network, the model allows you determine what will happen to the network when new stresses are placed on it.

Core Layer
The core layer is responsible for fast and reliable transportation of data across a network. The core layer is often known as the backbone or foundation network because all other layers rely upon it. Its purpose is to reduce the latency time in the delivery of packets. The factors to be considered while designing devices to be used in the core layer are:

High data transfer rate: Speed is important at the core layer. One way that core networks enable high data transfer rates is through load sharing, where traffic can travel through multiple network connections.

High reliability: Multiple data paths ensure high network fault tolerance; if one path experiences a problem, then the device can quickly discover a new route.

At the core layer, efficiency is the key term. Fewer and faster systems create a more efficient backbone. There are various equipments available for the core layer. Examples of core layer Cisco equipment include:

Cisco switches such as 7000, 7200, 7500, and 12000 (for WAN use)

Catalyst switches such as 6000, 5000, and 4000 (for LAN use)

T-1 and E-1 lines, Frame relay connections, ATM networks, Switched Multimegabit Data Service (SMDS)

Distribution Layer
The distribution layer is responsible for routing. It also provides policy-based network connectivity, including:

Packet filtering (firewalling): Processes packets and regulates the transmission of packets based on its source and destination information to create network borders.
QoS: The router or layer 3 switches can read packets and prioritize delivery, based on policies you set.
Access Layer Aggregation Point: The layer serves the aggregation point for the desktop layer switches.
Control Broadcast and Multicast: The layer serves as the boundary for broadcast and multicast domains.
Application Gateways: The layer allows you to create protocol gateways to and from different network architectures.
The distribution layer also performs queuing and provides packet manipulation of the network traffic.

It is at this layer where you begin to exert control over network transmissions, including what comes in and what goes out of the network. You will also limit and create broadcast domains, create virtual LANs, if necessary, and conduct various management tasks, including obtaining route summaries. In a route summary, you consolidate traffic from many subnets into a core network connection. In Cisco routers, the command to obtain a routing summary is:

show ip route summary

You can practice viewing routing information using a free CCNA exam router simulator available from SemSim.com. You can also determine how routers update each other�s routing tables by choosing specific routing protocols.

Examples of Cisco-specific distribution layer equipment include 2600,4000, 4500 series routers

Access Layer
The access layer contains devices that allow workgroups and users to use the services provided by the distribution and core layers. In the access layer, you have the ability to expand or contract collision domains using a repeater, hub, or standard switch. In regards to the access layer, a switch is not a high-powered device, such as those found at the core layer.

Rather, a switch is an advanced version of a hub.

A collision domain describes a portion of an Ethernet network at layer 1 of the OSI model where any communication sent by a node can be sensed by any other node on the network. This is different from a broadcast domain which describes any part of a network at layer 2 or 3 of the OSI model where a node can broadcast to any node on the network.

At the access layer, you can:

Enable MAC address filtering: It is possible to program a switch to allow only certain systems to access the connected LANs.
Create separate collision domains: A switch can create separate collision domains for each connected node to improve performance.
Share bandwidth: You can allow the same network connection to handle all data.
Handle switch bandwidth: You can move data from one network to another to perform load balancing.

SemSim.com provides training resources for Cisco certification exams: CCIE, CCNP, CCNA, CCDP, CCDA. It offers FREE learning resources to students such as study guides and router simulation labs at it's online learning center located at: http://www.semsim.com/ccna/learn.html

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