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Monday, December 29, 2014

Learning the Cisco IOS

I've been absent from this blog for a while, because I'm working on completing two more books in the Accidental Administrator series. I'm hoping to finish both of them by the end of January and then I can get back to blogging.

In the meantime, I just received an email from a reader asking about using a Cisco 1720 router to learn IOS commands and thought I'd share my response here.

I used to own six 1720s and found them excellent for learning, experimenting, and teaching. Are you familiar with GNS3 ( It's a free router emulator platform which is used by millions of people to learn and test Cisco IOS commands. You have to provide the router software, but GNS3 itself is free. It supports the following platforms: 7200, 3600, 3620, 3640, 3660, 3700, 3725, 3745, 2600-series, and 1700-series. An easy way to get the software is to purchase one of the supported routers, making sure that the router you purchase has the most recent version of the software, such as 12.3 or 12.4. It's important to realize that on older platforms such as those supported on GNS3, the most recent software available could be three or four years old. Still, for practicing basic IOS commands, setting up access-control lists, and experimenting with routing protocols, the older versions of the software will be helpful.

Several of the screen captures from The Accidental Administrator Cisco Router Step-by-Step Configuration Guide were taken using emulated routers running in GNS3.

Hope that helps.

Sunday, February 23, 2014

What is a Default Gateway?

If you're new to networking, you've probably heard the term default gateway and wondered what it meant. You see it in the IP address configuration on hosts and routers, but what does it do?

The default gateway, also known as simply the gatewaythe default route, and on Cisco routers as the gateway of last resort, is the path packets use when there is no explicit route to a destination. Without a default gateway, each host would have to have a routing table containing explicit paths to every host on the Internet, an absurd and unworkable solution. When we configure a default gateway, we're saying to the host, "If you don't know what else to do with a packet, send it to the default gateway and let the gateway figure out what to do with the packet."

In order for a host's default gateway to work, there are certain criteria that must be met:
  • It must be an interface on a network already known to the host (If it's not known to the host, how will the host know how to get the packets to the gateway?)
  • It must be an interface on a device connected to the rest of the world, usually the public Internet.
Consider this diagram in which the three computers connected to the Cisco ASA use the ASA's inside interface as their gateway. The ASA then uses the ISP's router as its gateway.
In the above scenario, suppose the top laptop wants to visit a website, say It probably doesn't have an explicit route in its local routing table for's IP address, so it hands the request off to its default gateway, the ASA. The ASA also doesn't have an explicit route in its routing table for's IP address so it hands the packet off to its default gateway, which is the ISP's router. The process may be repeated through several routers before the packet finally arrives at its destination. You can see the entire process by using the command traceroute in a terminal window.

One word of caution, on hosts with multiple interfaces, you can sometimes end up with multiple default gateways which can produce unpredictable results. Generally, you want only one default gateway configured per device, regardless of the number of interfaces on the device.

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