Router - Transmitting Packets

In a previous article we discussed the basics of what a router did. We're now going to get into a more detailed, and yes technical, explanation of how packets are transmitted as well as a few other tech specs of how routers work. So put on your learning caps because you're in for a real mind bender.

Internet data, whether it be in the form of a web page, a downloaded file or an email message, travels over what is called a packet switching network. Basically what happens is that the data is broken up into individual packets because there is only so much data that can be transmitted at one time. Each packet is about 1500 bytes long. Each packet contains quite a bit of information including the sender's address, the receiver's address and of course the information being sent which includes the order of each packet how it should be put back together so that the end user can make sense of the data. The packet is sent off to its destination based on what the router believes to be the best route to follow, which is usually the route with the least amount of traffic and if possible, the shortest route. Each packet may actually given a different route depending on conditions at the time, which in a high traffic network can change every second. By doing this, the router can balance the load across the network so that no one segment gets overloaded. Also, if there is a problem with one piece of equipment in the network, the router can bypass this piece of equipment and send the packet along another route. This way if there is a problem, the entire message will still arrive intact.

In conducting this process, routers have to speak to each other. They tell each other about any problems on the network and make recommendations on routes to take. This way, paths can be reconfigured if they have to be. However, not all routers do all jobs as routers come in different sizes and have different functions.

There are what we call simple routers. A simple router is usually used in a simple small network. Simple routers simply look to see where the data packet needs to go and sends it there. It doesn't do much else.

Slightly larger routers, which are used for slightly larger networks, do a little bit more. These routers will also enforce security for the network, protecting the network from outside attacks. They are able to do a good enough job of this that additional security software is not needed.

The largest routers are used to handle data at major points on the Internet. These routers handle millions of packets of information per second. They work very hard to configure the network as efficiently as possible. These are stand alone systems and actually have more in common with supercomputers than with a simple server one might have in a small office.

In our next instalment we'll look at how to actually trace the path that a message has taken and some examples of transmitting packets.

Passing Cisco’s CCNA and CCNP Exams: Traceroute

In preparation for your CCNA and CCNP exam success, you've got to learn to troubleshoot Cisco routers. And while ping is a great basic IP connectivity tool, it doesn't give you all the information you need to diagnose network connectivity issues.

Let's say you have six routers between CityA and CityB. You send a ping from A to B, and get this return:

R1#ping 172.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.1.1.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

The five periods indicate that there is no IP connectivity to CityB. Problem is, that's about all ping tells you. You can have 5 or 50 routers between the two points, so how can you tell which downstream router has the problem?

That's where traceroute comes in. Traceroute sends three datagrams with a Time To Live (TTL) of 1. Those datagrams will timeout once they hit the first router in the path, and that router will respond with an ICMP Time Exceeded message.

In response, the sending router sends three more datagrams, but these have a TTL of 2. This means that the next router in line will send back ICMP Time Exceeded messages. This process continues until the final destination (CItyB) is reached the output of the command shows us the path the data took:

Router1#traceroute 271.1.1.1


Type escape sequence to abort.

Tracing the route to 271.1.1.1


1 20.1.1.1 4 msec 4 msec 4 msec

2 30.1.1.1 20 msec 16 msec 16 msec

3 271.1.1.1 16 msec * 16 msec

How does this help troubleshoot a problem? Let's say that the second router in this path, 30.1.1.1, doesn't know how to get to 271.1.1.1. The output would look like this:

Router1#traceroute 271.1.1.1


Type escape sequence to abort.

Tracing the route to 271.1.1.1


1 20.1.1.1 4 msec 4 msec 4 msec

2 30.1.1.1 20 msec 16 msec 16 msec

3 * * *

This indicates that the router at 30.1.1.1 doesn't know how to get to the final destination. Now you have a better idea of which router has an issue!
Now here's the bad part: you're going to get 30 lines of three asterisks, and until you abort this traceroute, you're going to just watch those asterisks go across the screen. There's an abort sequence that the router mentions in the first line of the console output, but the router doesn't tell you what it is! So I will - this top-secret sequence is TWICE, one right after the other.

That keystroke takes a little getting used to, but a CCNA or CCNP can do it! Add this command to your Cisco skill set, and it will serve you well both on the CCNA and CCNP exams and your real-world networks. And you'll impress your friends by knowing how to stop a traceroute!