Step-By-Step Networking: The Foundation

Network devices are useless if they do not have a standardized way of communicating with one another. If devices were unable to understand each other's encoding, how are they going to be able to process network traffic? Thankfully, those much smarter than I figured that out a long time ago with ethernet and cabling standards. Today, the Institute of Electrical and Electronics Engineers (IEEE) defines and manages these standards.

This article is part two of the Step-By-Step Networking series. Last week I went over the network we will be configuring and walked through the installation of Cisco Packet Tracer. In this article, we will go over our physical connections and define some of the standards used in our network.

If you have not read the previous article in this series, below is an index containing each post in sequential order.

Index

1. Step-By-Step Networking: A Project-Based Approach 
2. Step-By-Step Networking: The Foundation

The Foundation

Before we start configuration of each device, it will be easier to set up our physical connections first. In a real-world scenario, configuration may come before implementation in a production network or would be an automated process. You may also come across transporting heavy physical equipment to server racks instead of utilizing the less intense Cisco Packet Tracer interface.

It would definitely be helpful to pull up the topology and device list for reference throughout this article, so I have included it below as a reminder.

• 1 Cisco 2811 Integrated Services Router (RTR01)

• 2 Cisco Catalyst 3650 24 Port PoE Switches (SW01 & SW02)

• 2 Cisco 7960 SIP IP Phones (FINPH01 & ITPH01)

• 1 Server (DCSRV01)

• 2 Desktop PCs (FINPC01 & ITPC01)

Placing & Connecting Devices

Selecting and placing devices is a very straightforward process in Cisco Packet Tracer:

1. Open Packet Tracer and log in
2. On the bottom left of the screen, click on the "Network Devices" category and then click on the "Routers" subcategory
3. In the selection pane, move the scroll bar to the right and click on the 2811 router
4. Place the router in the white canvas by clicking the whitespace as shown below

That's pretty much all! From there, the same process goes for the switches and client devices. The only difference is their categories, naming conventions, and icons. Otherwise, placement is exactly the same.

You can also rename the device by clicking on the default name "Router0" and typing in a new name. This will not configure the device and is purely to simplify the topology view. For now, the choice to do so will be up to you.

Go ahead and repeat the steps above for each listed device until you have a similar topology to the one below. You will have to navigate the categories yourself, which will help you get comfortable with the interface:

Now that we have our devices, we'll need to connect them with cables in order for them to communicate. Keep in mind, a network will not function without a physical medium, often referred to as the Physical Layer of the Open Systems Interconnect (OSI) model, or Layer 1. To do so, follow the steps below:

1. Select the "Connections" category in the bottom left of the screen
2. Select the Copper Straight-Through cable indicated by the solid black line
3. Click on each device and connect the cable to an interface

Connecting cables is just as simple as placing devices. Connect each cable according to the interfaces in the picture below:

Notice the links from the PCs turn green indicating the links are functional, but the others don't. This is because of a few reasons. Starting from the router and working our way down, router interfaces by default are disabled. Next, all the switch interfaces are down because the 3650 PoE switches are turned off by default as well. The link from the IP phones to the PCs are down because the IP phones need power, which is provided by the PoE switches over copper cabling.

Let's turn on our switches. Follow the steps below:

1. Double click one of the 3650 switches
2. On the left-hand side of the window under "MODULES," click and drag "AC-POWER-SUPPLY" to one of the empty bays on the back of the switch as shown below

Repeat for the second switch, and you should see the links going to the IP phones turn green:

Why are the links to the router still down? Remember, our router interfaces are disabled by default as well, so these links will stay down until we enable those interfaces. Let's go ahead and enable them.

Enabling Router Interfaces

Enabling an interface requires us to get into the CLI prompt of the router. When configuring a device for the first time, you will usually use something called a console cable. A console cable is used to directly connect to a device for local configuration. The cable looks similar to a typical RJ45 cable, however it serves a different purpose.

To use a console cable you will need to plug in one end to your computer and the other to the device. Then, download and install a software client capable of handling those connections. PuTTY is a popular client for Windows machines, though there are several that could be used. A tutorial on how to use the PuTTY client is out of scope for this article, but I encourage you to research the tool further.

In Cisco Packet Tracer, we simply have to double click the device and click the "CLI" tab if the device offers us one. Once opened, you will be presented with a window similar to the one below.

At first this can seem rather daunting, but after repetition you will be proficient at the IOS command line in no time. Just enter the commands for now, and I'll explain what we did in the next section.

1. Enter the following commands on the router:

                              --- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]: no

Press RETURN to get started! 

Router>enable
Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z. 
Router(config)#int fa0/0
Router(config-if)#no shut
Router(config-if)#

%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

Router(config-if)#int fa0/1
Router(config-if)#no shut
Router(config-if)#

%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

Router(config-if)#

Now close the window and the links to the router should be green.

Congratulations, you now have a fully functional network!

Configuration Explanation 

What did I actually do in the previous section? What is a straight-through cable, and why should we care? Why do IP phones require PoE to function? These are valid questions when starting out and can be confusing to grasp for beginners, however I will attempt to answer them in the simplest way possible.

Straight-Through vs Crossover Cables

When we connected each device, I specifically stated to use straight-through cables for each link. To understand straight-through cables and why we use them, we first need to understand how cables are wired.

Standard ethernet cables are referred to as RJ45 cables, though some use the term 8P8C, which stands for 8 position, 8 contact. This is reflected by the 8 pin connectors used by RJ45 cables.

If you strip the casing off of an RJ45 cable, you'll see it consists of 8 separate wires. Each wire is color coded based on certain wiring standards. The two most well-known standards are T568A and T568B. Wires are placed in positions 1-8 at the ending connectors based on their color and the correlating standard. The standards are there to help us visualize which pins are being used to transmit and receive data. These standards are defined by the Telecommunications Industry Association (TIA).

The T568A standard is wired as follows:

1. White-Green
2. Green
3. White-Orange
4. Blue
5. White-Blue
6. Orange
7. White-Brown
8. Brown

The T568B standard is wired as follows:

1. White-Orange
2. Orange
3. White Green
4. Blue
5. White Blue
6. Green
7. White Brown
8. Brown

In a straight-through cable, both sides of the cable reflect the same color scheme. This type of cable is used to connect dissimilar devices together. For example, a switch to a router, or a PC to a switch.

However, a crossover cable is different. The crossover cable does not use the same standard on both ends and is used for a different purpose. This type of cable is used to connect similar devices such as router to a router or switch to a switch, and swaps the white-green and green cables with the white-orange and orange cables on the other end.

If you were to use the wrong cable, both devices would not agree on which wires are being used to send/receive data, thus bringing the link to a downed state. Today, most network devices autodetect which pins are being used for sending/receiving, so it is not as much of a problem. It is still important to be aware of the standards in the event you need to crimp your own cable or are managing older equipment.

Power over Ethernet (PoE)

Power over Ethernet is a standard defined by IEEE as 802.af. Simply put, this standard allows devices to be powered using an electrical current over an ethernet cable, allowing more flexibility to where devices can be placed in a physical location.

For example, consider a number of surveillance cameras that needs to be placed in high locations that are out of reach of potential threats. It would be very inconvenient and expensive to install a dedicated electrical circuit for each camera, and running extension cords is often unsafe due to the tripping and fire hazards they present. Another example would be a wireless access point, which needs to be installed in a location that offers maximum wireless coverage of an area that may not have direct access to power. Thus, PoE is a safer, scalable solution for providing power to networked devices.

The 802.3af standard offers a maximum of 15W per device but is relatively outdated and may not meet the demands of many modern day devices. So, IEEE developed the 802.3at standard known as PoE+ providing devices with 30W per port. At the time of this writing, the current standard is 802.3bt (PoE++) offering up to 60W per port.

Let's see this in action on our switches. By typing the "show power inline" command on our switch connected to our IP phones (SW02), we can see which device is connected to which port including the amount of watts they are drawing:

Here we see the max wattage supported by each interface is 30W, so we are using the 802.3at standard. The IP phones themselves are only drawing 10W. Other interfaces are currently off and not providing any power, which is what we want since they're not in use.

Note many businesses prefer to use PoE due to reasons already discussed. For IP phones, PoE may not be needed depending on the phone's location. It is perfectly fine to plug in its own power supply without using PoE.

Cisco IOS Commands

Let's go over some of the commands that I used on the Cisco 2811 router. When we first entered the command line, the router asked us if we wanted to go through a configuration dialog. The idea behind this dialog is to make initial setup easier, though I have never seen anyone actually use it. We told the router no, and there are some good reasons out there as to why. In many cases it is better to start from scratch instead of having an automated wizard configure everything for you. Personally, I enjoy the granular control the CLI gives me out-of-the-box, but feel free to give it a try when you have the chance.

Next, we needed to press enter to get to the User EXEC mode as indicated by the > at the end of the prompt. This is a barebones user mode with low privileges that cannot make any configuration changes to the router, so we had to go to the next level up to the Privileged EXEC mode. This is done by typing the "enable" command. We know the command worked as indicated by the # at the end of the prompt.

You can think of User EXEC as being a regular user account with no administrative privileges, and Privileged EXEC mode as the administrator (or root in Linux terms).

However, we were still unable to make global configuration changes, so we entered "configure terminal" to enter the global configuration mode. Here, we're able to make any change we want on a system wide level, though we are still unable to make interface configuration changes. So, we entered the shorthand command for "interface" (int) followed by the interface we wanted to configure.

We were then able to perform a "no shut" command (short for no shutdown) to bring the interface up. Finally, we switched to the second interface and did the same thing. Log messages were generated for each interface notifying us that the interfaces were now up.

That's it! No witchcraft, no secret code, just understanding the different levels of each prompt and enabling our interfaces.

Conclusion

Today we configured the physical layer of our network and went over the different standards at play. In the next post we will be discussing switches and VLANs.