Tag: packet

Packet capture in Cisco IOS

Packet capture on IOSThis will be a brief article but a good one. It will save you some walking time to server room. I have the need to capture traffic on the switch or on the router several times every week. That action needed from me to be physically near the switch and to configure SPAN port so that I can connect to the switch with my machine and capture some packets with wireshark. Okay, I could use RSPAN to get captured packets to the closest switch but this altogether is not good enough. It’s too time consuming for short packets captures in troubleshooting sessions.

Recently in my CCIE study I came across the info that Cisco IOS is able to capture packets on the device itself and on more interfaces in once. You can later export that capture to your PC and analyze it with wireshark.

You can do it like this

Data Packet – IP Packet – What is this packet story all about?

From the computer networking point of view, a packet is referred to a planned data unit. But certain computer communication links such as point to point communication don’t support data packets. In such cases, data is simply transmitted as an array of bytes or characters etc over the medium. Anyway, transformation of data into packets is a good way to achieve better bitrate of a communication medium in the course of sharing data amongst the users.

How We Open a Web Page and What is Going on Behind?

When you open a web page, all sorts of things will need to be done in the background before you get your shiny website on your screen.

We will see now what is happening in the networking system to make that possible.

TCP/IP protocol is what makes sending and receiving most of the data across the Internet possible. But how data packets know how to find us and how we know how to find the IP addresses of the web servers where these pages are saved?

Data will maybe not even take the same route in each direction. It can happen that when we send something, a request to the server, the packets will flow through one route and the server response towards our computer will take some other route.

The Internet is the biggest computer networking system. It knows in every moment how to find the best route to some device connected anywhere in-between all his nodes.

But how is this data transferred across the wires, fibres and air?

Data is divided into small packets. Every time we send a request towards a server, our request must firstly be divided into packets, most of the same size. Each of those packets needs an IP address of the destination to be written on it so that he can be routed through the network.

In order to find out what is the destination IP address of the server – (remember that we are typing an URL into the browser, usually we are not typing IP address into it) –  your computer, before sending out all those packets, will contact public DNS serverdomain name server, that will have the information about IP address to which packets must be forwarded in order to get to your URL-linked page.

Public DNS servers are set up into a hierarchical system that keeps the knowledge of IP addresses for all URLs (domain names) that are registered on the Internet. With this database, DNS is able to translate our request for the web page URL into the IP address of the server on which the web page is stored.