CCNA Study Guide Chapter 06 – Page 2

Console Port on a Cisco Router

In order to perform the initial configuration of a Cisco router, you will need to create a terminal session with the router via its console port. On a Cisco 2501, the console port uses an RJ-45 connection and is an asynchronous serial port that uses the common EIA/TIA-232 communications standard. Asynchronous serial differs from synchronous in that it doesn’t require synchronized clocking between the two systems. Instead, systems use what are known as start and stop bits to mark the beginning and end of a character.

While the console port may look like it should be used for a standard Ethernet connection, do not make this mistake – it is not an Ethernet port, and a straight-through or crossover cable will not provide a proper connection. Instead, connections to this port are made using what is referred to as a rollover cable. This cable is usually included when you purchase the router, but we’ll look at how to create our own rollover cable shortly.

The physical connection to the console port is made via a standard PC COM port. Since the COM port on a PC is usually accessed via a DB-9 or DB-25 serial interface, a small RJ-45 to DB-9 (or DB-25) adapter is required. Again, this adapter is usually included with the router, and is labeled “TERMINAL”. Once the rollover cable is connected to both the PC COM port and the console port on the router, the router can be configured using a terminal emulation program like Windows HyperTerminal. The communications settings are the same as those looked at in Chapter 3 for connecting to the console port on a Cisco 1900 switch. As a reminder, the settings remain:

Bits per second – 9600
Data bits – 8
Parity – None
Stop bits – 1
Flow Control – None

We’ll discuss the initial connection to and configuration of a Cisco 2500 router in Chapter 7.

Introduction to Cisco Routers

In Chapters 1 through 5 we mainly concentrated on what I would consider essential networking concepts. Those concepts represent a huge portion of the information that you need to be familiar with to be successful on your CCNA and CCDA exams. However, doing well on the exams will also require knowledge of Cisco equipment, commands, and configuration. It’s tempting to want to just jump right in and start configuring a router. Before we do that, we first need to take a closer look at router hardware and internals, and also learn a little bit about the Cisco IOS. Topics that we’ll cover in this chapter include:

Understanding a router’s ports, LEDs, connections, and cables
Understanding router memory and storage
Investigating IOS versions
An overview of Cisco router models

Because the Cisco 2500 series of routers are relatively inexpensive to obtain and still widely deployed, they represent a great platform to use when studying for the CCNA exam. This chapter focuses on the Cisco 2500 series models to explain concepts, but these concepts generally apply to almost all models universally.

Serial Ports on Cisco Routers

A Cisco 2501 includes two synchronous DB-60 serial ports, which are used for the purpose of WAN connections. Synchronous communication relies on the use of timing in order to control (or synchronize) the transmission. Before synchronous systems communicate, they agree on parameters such as the time interval that will be used between sending bits of data.

A router is referred to as Data Terminal Equipment, or DTE. When DTE devices connect to a data network such as a service provider WAN link, they usually require an external device to handle their serial transmission timing, also referred to as clocking. In order to accomplish this, they connect to devices referred to as Data Communications Equipment (DCE). The function of the DCE is to provide access to the actual data network and provide clock synchronization. Two common examples of DCE equipment are modems and Channel Service Units / Data Service Units (CSU/DSU).

A CSU/DSU looks somewhat similar to an external modem, and it many ways, performs similar functions. The exception is that a modem converts digital signals to analog, while a CSU/DSU uses digital signals throughout. The DSU portion is responsible for timing, and actually connects to the DTE (in this case the router) via its serial port. The CSU portion is responsible for terminating the service provider’s link, and handles transmitting and receiving data over the WAN link. The figure below shows the connection between a Cisco 2501, CSU/DSU (which is assumed to have an EIA/TIA-232 interface), and a service provider’s data network.

Figure: CSU/DSU connection to a serial port on a Cisco 2501.

A variety of Physical Layer standards are supported over synchronous serial interfaces to connect to different types of DCE equipment. Some of the different signaling standards and connectors that might be found on DCE equipment include EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA-530. Cisco and a variety of other vendors manufacture “transition” cables capable of connecting a router’s DB-60 DTE port to DCE equipment using these different standards. The list below outlines each standard, and for reference purposes provides information on the associated connector and connection properties.

Standard: EIA/TIA-232
Connector: DB-25
Properties: Formerly known as RS-232, this standard supports transmissions of up to 64kbps

Standard: EIA/TIA-449
Connector: DB-37
Properties: A faster version of the EIA/TIA-232 standard, not widely adopted. Speeds up to 2Mbps.

Standard: V.35
Connector: 34-pin
Properties: Speeds between 48kbps and 4 Mbps

Standard: X.21
Connector: 15-pin
Properties: Commonly used in the UK to connect to the public data network. Common speed of 64kbps, up to 2Mbps

Standard: EIA-530
Connector: DB-25
Properties: Similar to EIA/TIA-449, but uses a smaller DB-25 connector. Speeds up to 4Mbps possible.

Ethernet Ports on a Cisco Router

A Cisco 2501 includes a single 10Mb Ethernet port. While many Cisco router models now include an integrated 10/100 RJ-45 port, the 2500 series uses what is referred to as a generic attachment unit interface (AUI) DB-15 port instead. The name for this connector (DB-15) comes from the fact that it is physically shaped like the letter ‘D’ and uses a 15-pin connector.

The purpose of providing an AUI port instead of a fixed RJ-45 port is flexibility – AUI ports use an external transceiver, which allow different media types to be connected, according to your network needs. Different transceivers can be attached that allow twisted pair, coaxial, or fiber connections. In that way, a transceiver connects to the DB-15 port, and then provides a port to which an RJ-45 or BNC connection could be made, for example.
But what is a transceiver? Well, its name comes from what it does – transmitting and receiving data. In most NICs that you’ll come across today, the transceiver is built directly into the network card. With older network cards (like those found on Ethernet 10Base5 networks) the transceiver was usually an external device.

When connecting a Cisco 2501’s Ethernet connection to a common RJ-45-based network, a transceiver attaches to the AUI port, and then a patch cable connects the transceiver to a switch or hub. Remember, a router connecting to a switch or hub always uses a straight-through cable, as shown in the figure below.

Figure: Connection between a Cisco 2501 and a switch or hub via an Ethernet transceiver.

Cisco Router Externals

The first thing that you’ll notice when you pull a Cisco 2500 series router out of the box is obviously its physical elements. A Cisco 2501 includes not only Ethernet and serial ports, but also console and auxiliary ports. In this section we’ll look at the purpose of each, their physical characteristics, and how devices are attached and cabled. The figure below outlines the location of the various ports on a Cisco 2501. Note that hardware ports are numbered nominally starting at 0. Therefore, on a system with only one Ethernet port, that port is referred to as Ethernet 0. On a Cisco 2501, the leftmost serial port is referred to as Serial 0, and the rightmost as Serial 1.

Figure: Cisco 2501 rear view.

The port numbering arrangement for newer Cisco router models (well, newer than the old workhorse 2500s) is somewhat different, and you should be aware of it. For example, a Cisco 2600 router follows the convention interface slot/port when referring to an interface. Fixed interfaces (those not part of add-in modules) are generally referenced using slot 0. So, if you were attempting to access the first fixed Fast Ethernet port, it would be port FastEthernet 0/0. If the model had a second such port, it would be FastEthernet 0/1. By the same token, if you added a four-port serial module to your 2600 router, those ports would be known as Serial 1/0 through 1/3 respectively. On the CCNA exam, it is very important that you reference ports correctly. If you’re not sure of the port numbering method used on a particular model (especially during router simulation questions) you can always use the show interfaces command to confirm. This command and many others will be explained in detail in Chapter 7.