Introduction to Cisco routers and switches – CCNA Course

Cisco Routers and Switches

cisco router and switches

Up to this point, we have discussed general internetworking technologies and devices to provide a background of the material relevant to the CCNA exam in previous articles. If you want to read them, you can do that.

Here we have our previous articles under our CCNA Course.

Now the scope of the discussion widens to include Cisco technology. This article reviews internetworking devices (routers and switches) that were developed by Cisco for network implementations.

It also discusses the individual hardware components that make up a router and switch, which you must thoroughly understand before the configuration of these devices is explained.

This introduction to Cisco technology leads up to other articles, “Understanding the Cisco SDM” and another article, “Foundation Cisco IOS operations” where you will delve into hands-on device operations and configuration.

Interfaces and Modules

Networks can be connected to Cisco hardware in a variety of ways. Communication lines can be terminated to a Cisco device via hardware interfaces and modules. Interfaces provide a physical point of interaction between two networks. That hardware interface includes the cable, plug, socket, and signal that sync up together to communicate among devices.

Certain Cisco routers are built as fixed-port routers or fixed-configuration routers and do not allow for additional network module installations. Cisco also offers modular-port routers.

Modular-port routers allow for future system upgrades by mounting network modules in available spaces to accommodate changing network environments.

A module is a self-contained component. A Cisco network module has built-in hardware interfaces to add alternate connection options on a network. Because they are modular components, Cisco routers can be upgraded easily and with minimal expense to the company.

This article will review the variety of physical connection types on LAN and WAN devices.

LAN interfaces

Local Area Network (LAN) interfaces are used to provide a point of interconnection between Cisco switches and other network devices. Cisco provides a wide selection of switches that can be implemented on a LAN and offer end-user connectivity. We already have an article that describes Data Link Networking concepts and it introduced Layer 2 switches.

The Cisco 2950 series switch family includes various models with different interface options, such as the Cisco 1950-12 and the Cisco 2950-24. The 2950-12 has 12 built-in Ethernet ports, whereas the 2950-24 built-in Ethernet ports. Certain models also have Gigabit Ethernet slots.

If Gigabit Ethernet ports are included, the front panel of 2950 has 10/100 ethernet ports on the left side of the switch and 2 Gigabit Ethernet slots that accommodate LAN interface modules on the right.

The 10/100 ports allow for either a 10Mbps or 100Mbps connection speed. Media connects to an Ethernet switch port via an RJ-45 connector. As far as the switch is concerned, each Ethernet port is designated as a numbered interface for identification. The top left port is labeled 1 by the switch. Each interface begins with a 0/#, where the # sign equals the port number on the switch. The top left port is then named 0/1 on that switch. Given this formula, the bottom left port is labeled 0/2.

The Gigabit Ethernet slots are available for Gigabit interface converters (GBICs). A GBIC interface module can be inserted into the Gigabit Ethernet slot to allow for different media connections to that port. The physical media can range from copper to single mode fiber.

A GBIC is also hot-swappable, so you can remove and replace it without shutting off power to the switch. This helps to avoid interruption of service to that switch.

The back panel of 2950 includes power input and the switch’s console port. The console port has an RJ-45 connector and is connected to a terminal with a rollover cable for initial switch configuration.

WAN Interfaces

Wide Area Network (WAN) interfaces are also used to provide a point of interconnection between Cisco routers and other network devices. Types of WAN interfaces include

  • Basic Rate Interface (BRI)
  • Synchronous Serial
  • Asynchronous serial
  • High-speed Serial Interface
  • T1 Controller Card

Basic Rate Interface

BRI is an Integrated Services Digital Network (ISDN) service that consists of two 64Kbps bearers (B) channels and one 16Kbps data channel. Voice, video, and data traffic can be carried over the B-channels. Signals between telephone company switches use the D-channel. Cisco offers an 8-port ISDN-BRI with a built-in Network Termination Type 1 (NT-1) Network Module for router installation.

The NT-1 is a telephone company requirement for an ISDN line connection. This network module has BRI U interface, which means that the NT-1 is a built-in on the network module and does not require a separate NT-1 device.

Synchronous Serial

A synchronous serial interface synchronizes clocks for the bit stream of both the sending and the receiving end of a serial link. This allows for the data rate to be adjusted if necessary to ensure that both ends of a serial link are functioning at the same speed.

Asynchronous serial

An asynchronous serial interface does the opposite of a synchronous serial interface. It does not synchronize the clocks for the bit stream of the sending and receiving end of a serial link.

Cisco offers a 4-port asynchronous/synchronous serial network module.

With the asynchronous/synchronous serial network module, each port can be configured individually as either asynchronous or synchronous, depending on your network setup.


High-speed serial interfaces offer up to 52Mbps transmission rates to the WAN from a Cisco router. The higher speed capacity is relevant if the corporate backbone requires high-speed internet access and VPN connectivity. Cisco offers a 2-port HSSI port adapter.

T1 Controller Card

Also referred to as a digital signal level 1 (DS1) service, a T1 is a connecting line that offers a 1.544Mbps data transmission speed. A single T1 line consists of 24 digital signal level 0 (DS0) channels that are 64Kbps each and an additional 8Kbps that is reserved for management over-head. A T1 controller card can be installed in a router’s T1 slot to communicate with and control the 24 DS0 channels.

Data Communications Equipment (DCE)

Data Communications Equipment (DCE) or Data Circuit-Terminating Equipment (DCE) is the term used to identify a device that connects the Data Terminal Equipment (DTE) to a service provider’s communications line.

The DCE side of a connection sets the clock speed for a serial connection.

DCE equipment may consist of a

  • Modem
  • Channel Service Unit/Data Service Unit (CSU/DSU)
  • Basic Rate Interface Network Termination Type 1 (BRI NT-1)

Modems convert a digital signal into an analog signal for transmission over a telephone line. The signal is converted back into a digital format when it reaches the device on the other end of that telephone line.

A Channel Service Unit/Data Service Unit (CSU/DSU) serves as the intermediary between the service provider and the WAN router. In most cases, the CSU/DSU provides the clock speed for the router. A CSU/DSU may be a separate unit or it could be incorporated into a WAN interface card (WIC).

If it is not built in on a Cisco router via a BRI-U (Basic Rate Interface-User) interface, the service provider requires separate BRI NT-1 hardware as a termination point for the communications line. The BRI NT-1 then connects to the Cisco router.

Data Terminal Equipment (DTE)

Data Terminal Equipment is the term used to identify a device at the user end of a network and is connected to the service provider via the DCE device.

DTE equipment may consist of a

  • Router
  • PC
  • Server

In the following picture, the service provider, whom you will most likely hear called a telco, brings a communication line from its central office (CO) to the customer and terminates its line to the CSU/DSU. The CSU/DSU is then connected to the customer router. The point at which the telco terminates its line to the customer is called a demarcation point or demarc.

Customer-owned equipment, such as the router and typically the CSU/DSU, is referred to as customer premise equipment (CPE).

cisco routers and switches ccna course

Cabling between the CSU/DSU and router is decided by the type of CSU/DSU that is deployed on that network. If a WIC functions as a CSU/DSU, then the CPE is a telco jack and a category 5 or category 6 cable is used with an RJ-45 connector.

If a WIC does not function as the CSU/DSU, there are several types of connections possible between a CSU/DSU and the DTE device. With routers, typically a DB-60 connector is used to connect to the router while one of the following connectors is used to connect to the CSU/DSU:

  • EIA/TIA-232
  • EIA/TIA-449
  • EIA/TIA-530
  • V.35
  • X.21

The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) formed a standard body, which developed the 232, 449, and 530 cables. The V.35 and X.21 cables were developed by the International Telecommunication Union (ITU).

Your best bet is to position the CSU/DSU as close to the router as possible. This requires the shortest amount of cable and therefore ensures maximum speeds.

Cisco Memory Components

Four memory components are used by Cisco devices. Those components include ROM, Flash, RAM, and NVRAM.


Read-only memory (ROM) contains the basic code for booting a device and maintaining Power on Self Test (POST), ROM Monitor (ROMmon), bootstrap, and RXBOOT.

Because this type of memory is read-only, it cannot be changed by any configuration done at the networking device. ROM is nonvolatile, so data is not lost when the device is powered off.


Flash is installed on either an electrically erasable, programmable, read-only memory (EEP-ROM) or Personal Computer Memory Card International Association (PCMCIA) card.

Flash memory contains the Cisco Internetworking Operating System (IOS) image. The router uses Flash by default to locate the IOS when it is booted. Configuration files may also be stored on a Flash card like ROM, Flash is also nonvolatile memory.


Random-access Memory (RAM) is used for short-term storage of a machine’s running IOS and running configuration. The IOS is copied from Flash to RAM. This is the only type of system memory that is not permanent.

At times, you may hear RAM also referred to as dynamic random access memory (DRAM). Because this type of memory is volatile, it is lost whenever the machine is shut down.


Nonvolatile random access memory (NVRAM) stores the startup configuration. This is the configuration that is located when the machine is booted.

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