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Cisco Certification: The Cisco Three-Layered Hierarchical Model

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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.

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.

Low latency period: The core layer typically uses high-speed low latency circuits which only forward packets and do not enforcing policy.

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 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.

About The Author 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. For more information visit: Making Cisco cetification easy!

Cisco Certification: Selecting Cisco Products

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You must understand the business requirements of the network before buying a product. Based on the requirements, you can select the appropriate Cisco products.

Today, WAN technologies most often take advantage of high-speed connections, such as T-1 and E-1. Additional WAN technologies exist, including Frame Relay, ISDN, and dial-up asynchronous connections. ISDN or dial-up is used if infrequent connection is made. If a connection is used for more than two to four hours, you should use a Frame Relay or a leased line.Based on the different services available through the service provider, you can select Cisco products that can fulfill your office requirements.

Selecting Hubs

Cisco has various hub products. You can select hubs according to the type of connection required. Higher-end hubs offer network management port and console connections, middle-end hubs offer both 10 Mbps and 100 Mbps auto-sensing connections, and the lower-end hubs offer only 10 Mbps connections. Examples of fast hub series of Cisco are Cisco Fh100, Cisco Fh200, Cisco Fh300, and Cisco Fh400. Examples of micro hub series are Cisco MH1500 and Cisco MH1528.

Selecting Routers

Routers are the most popular Cisco products. To select a router, you must first know the port density and the interface speed required based on the LAN or WAN technologies used. In addition, you must identify the capacity and performance requirements of your business. The cost of the router varies according to the features you select.

Selecting Switches

A switch can be used instead of a hub in newer networks. Most switches are configured to limit collision domains, which often provides a limited security feature because users will not be able to sniff traffic as easily as in a standard hub-based network. This feature can cause confusion when using a network sniffer to analyze traffic. It is possible, however, to analyze packets on a switched network by enabling mirroring on the switch, or by using a dedicated port that allows authorized users to view all network traffic. It is important to consider whether you need 10, 100, or 1000 Mbps for each desktop or to connect between switches. To select a switch, you must consider various issues, such as business requirements, requirements for inter-switch links and trunking, port density, and type of user interface.

Product Selection Criteria

When selecting the right products for any network environment, keep the following considerations in mind:

Will the device be used as a core device, distribution device, or access device?

Does the device provide the functions that your business requires?

Does the device have adequate number of ports and the right mix of interfaces as required by your business?

Do you have to pay a lot for installing the device or can it be easily installed?

Can the device be centrally managed from an SNMP Management station or do you have to physically access the device to manage it?

Is the migration path easy?

How well does the device handle redundancy requirements?

Do you have to change your existing infrastructure, such as cabling or existing devices, to install the new device?

You may use the Cisco website to gather more information to select products based on your requirements.

About The Author 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. For more information visit: : Making Cisco cetification easy!

Cisco Certification: Cabling Cisco Devices

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Unshielded Twisted Pair (UTP) is a cable that has four pairs of wires twisted inside it to eliminate electrical interference. UTP cables are connected using RJ-45 connectors that have eight connector pins. The data equipment that is used for internetworking is classified as Data Terminal Equipment (DTE e.g. router interfaces) and Data Communication Equipment (DCE e.g. A WAN connects DTEs through DCE network).

UTP cables are used in many configurations and for different purposes, to form an internetwork including:




Applications of UTP include:

Serial transmission

Integrated Service Digital Network (ISDN) connections

Console connections

Modular and fixed interfaces

These configurations and applications are discussed below.

Straight-Through Cable

A straight-through cable is the standard network cable connection and is used to connect the source and destination computers through an internetworking device. Specifically, you can use it to connect a host to a hub or switch

Crossover Cable

In a crossover cable, the standard RJ-45 cable between the source and destination computers is cross-connected. A crossover cable can be used to connect:

Two computers

Two hubs

A hub to a switch

A cable modem to a router

Two router interfaces

Rollover Cable

These Cisco proprietary cables used to connect to a router or switch console port. In a rollover cable (8 pins), RJ-45 connectors are usually present at each ends and are used to connect router and computer ports. Pin 1 on one end of cable connects to Pin 8 at the other end of the cable, similarly, Pin 2 connects Pin 7, and so on.

Serial Transmission

All WANs use serial transmission, which can pass one bit at a time over a single channel. Cisco provides a 60-pin serial connector for one end of the cable while other end of the cable may be EIA/TIA-232, V.35, EIA/TIA-449, X.21 or others;

ISDN Connections

ISDN refers to a collection of standards that define a digital architecture that can carry integrated voice and data, using the public-switched network. ISDN BRI (Basic Rate Interface) is a two 64 kbps Bearer channels (2B) plus one 16 kbps Data channel (D) service. ISDN BRI terminates at the customer premise with the U interface.

Console Connections

A console is a type of DTE through which commands are entered into the host device. Most Cisco devices support the RJ-45 console connection. A rollover cable with an RJ-45 connector is used to connect the PC or terminal to the console port of the Cisco device.

Modular and Fixed Interfaces

Unlike a fixed interface router, a modular interface router allows you to add new interface cards without buying a new router.

In the fixed interface series, you refer to the interfaces as:

interface_type interface_number

For example, the first serial interface on a Cisco 2503 router would be referred to as serial 0. Higher routers have modular interfaces that allow you to change the interfaces as and when required. Here, the interfaces are referred to as:

interface_type module_number/interface_number

To practice such commands you can download a free CCNA router simulator from and proceed to the troubleshooting 1 exercise that configures ethernet 0 interface.

About The Author 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. For more information visit: : Making Cisco cetification easy!