Cisco networking 101: Five more things you should know

by David "Davis CCIE, MCSE+I, SCSA" | Jan 26, 2006 8:00:00 AM

Tags: Routers & switches, Network technology, NETWORKING, David Davis CCIE, MCSE+I, SCSA, IP, router, Cisco Systems Inc., Cisco Routers and Management Newsletter

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Takeaway: Last time�because it never hurts to review the basics every now and then�David Davis offered a list of five things every administrator needs to know about Cisco networking. This time, he adds five more things to the list.

No matter how long you've been working on networks, it never hurts to review the basics every now and then�even seasoned administrators sometimes need a refresher. Taking a break from your well-honed routine of shortcuts and going over the finer points of networking can even open your eyes to new best practices and troubleshooting methods.

Last time, we discussed five things everyone should know about Cisco networking and working with routers, including the difference between a router and a switch, the basics of IP addressing, and more. Now, let's look at five more things you should know about Cisco networking.

Comprehend the difference between routing and routed protocols

It always surprises me how many people don't really know what a routing protocol is or why it's necessary. And there are those who can't explain the difference between a routing protocol and a routed protocol.

Open Shortest Path First (OSPF), Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP), and BGP are all routing protocols. They distribute routing information throughout all routers on a network. Each router must know which networks all others routers have connected to, which enables the router to determine the best path to take to deliver traffic.

The Internet Protocol (IP) is a routed protocol. Conversely, NetBIOS Extended User Interface (NetBEUI) is an example of a non-routed protocol.

For more information, see "Cisco administration 101: What you need to know about EIGRP" and "Take a closer look at routing redistribution."

Be familiar with Cisco's administrative modes

The Cisco IOS isn't always user-friendly or intuitive. That's why it's important for admins to understand the varying levels of privilege on a router. Understanding User EXEC Mode, Privileged EXEC Mode, and Global Configuration Mode is a definite "must-know."

User EXEC Mode: Designated by router>, this is the default mode when you first log in to a Cisco router. However, you can't do very much in this mode; you have access to some information, but you can't make any changes. To move to Privileged EXEC Mode, type enable.

Privileged EXEC Mode: Designated by router#, this mode usually has full administrative access on the router to view pretty much anything. For example, you can use the show command when in this mode. However, you still can't make any changes; for that, you need to be in Global Configuration Mode. To do so, type config terminal.

Global Configuration Mode: Designated by router(config)#, this mode offers full access to make changes to the router's configuration. To move back to Privileged EXEC Mode, type exit.

For more information, see "Understand the levels of privilege in the Cisco IOS."

Know the various uses of the show command

Cisco's show command offers a wide variety of uses. While it's important that you have a general understanding of all these uses, some are more useful than others. In my opinion, here are the three most helpful uses of the show command.

Show ip route

This command displays the available routes on the router. These routes can be static or dynamic. If the destination that your traffic is going to isn't on the list and there's no default route, the router will drop (throw away) that traffic.

Show running-configuration

This command displays the current configuration of the router. If you make changes to the configuration, make sure you save those changes by using the copy run start command.

Show ip interface brief

This command displays a summary of the current status of all interfaces on the router. You can use this output to see how many interfaces are on the router, their types, the IP address of the interfaces, and if the interface is up or down.

For more information, check out "Effectively filter Cisco router command output" and "Take advantage of Cisco's show version command."

Understand how private IP addressing and NAT work together

Based on RFC 1918, private IP addresses are not routable on the Internet. Instead, the purpose of these addresses is for use on internal networks only.Internet routers just throw away any traffic received from these networks, which include:

10.0.0.0 /8 or 255.0.0.0
172.16.0.0 /12 or 255.240.0.0
192.168.0.0 /16 or 255.255.0.0

Most companies and internal home networks currently use private IP addressing. In addition, there's no need for these networks to be unique when using private IP addressing.

However, if you use private IP addressing and if Internet routers throw away traffic from your IP address range, how can you communicate on the Internet? The answer is network address translation (NAT). NAT translates private IP addresses into public IP addresses.

For example, let's say your ISP gives you one public Internet IP address on your home network. On your internal LAN, you're using the 192.168.0.0 /16 network. Your router then uses NAT to translate the internal IP addresses into your one external IP address. When responses come back, NAT converts it back into an internal private IP address.

For more information, see "Set up NAT using the Cisco IOS" and "Cisco IP subnetting 101: Five more things you should know."

Know how to troubleshoot a network problem using the OSI model

Troubleshooting a problem is how most network admins earn their keep. Effective troubleshooting in a jam can save the day and prove your worth.

While there are several approaches to network troubleshooting, I recommend starting at the physical layer (Layer 1) of the OSI model and working your way through each layer until you find the problem. I've found this to be the safest and most straightforward way to solve any network problem.

For more information, check out "Choose a network troubleshooting methodology" and "Get real-world examples for applying network troubleshooting methodologies."

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David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.

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