Month: April 2015

Why numbering should start at zero?

Please note:

This has nothing to do with networking in particular! Not if you look from only one perspective. If you look from totally different perspective, with Cisco ACI and all other SDN solutions, you will probably meet with Python programming language (because you will) and then, somewhere in beginning of Python exploration this is the first question that will cross your mind. Of course, if you think like me!

Although not directly related to networking, the question bothered me for some time now and the answer is not only really logical when you read it but it is also given by a cool guy named prof.dr. Edsger W. Dijkstra

For almost every networking geek it will be enough to read through it.

Intro

After getting the CCIE I found the next thing I wanted to get my hands around. Python. As it seems, Cisco ACI fabric will be the next big thing in Cisco world. Other vendors all have something about SDN to offer these days. Most important part for us, networking engineers, is that they want us to give up the CLI and get us to use Python and REST calls to speak with our network equipment. Python was the logical next step.

I signed and passed Dr. Chuck’s Coursera Python class:

And got some books:

  • Python for Informatics by Charles Severance (Coursera Professor)
  • Learning Python, Fifth Edition by Mark Lutz
  • Python Pocket Reference, Fifth Edition by Mark Lutz
  • Fluent Python by Luciano Ramalho

After first few great lectures on Coursera by Dr.Chuck the question was born!

The Question

So my questions arose: “Why the hell in Python slices and range exclude the last item?”, “Why?”, “It’s not logical to me!”

The Answer

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What is a non-blocking switch?

It is fairly common to hear about switch being non-blocking. It’s because almost all switches today are non-blocking. But what that means? When I asked people around me on what exactly non-blocking switch means, they were unable to get to the same conclusion.

I was going through a lot of different internet places and vendor documents before I wrote this here, but, do not hesitate to add something in comments if you have different view on the subject.

Line-rate switch means the same as if you would said wire-speed switch. It basically means that this switch has the forwarding capacity that supports concurrently all ports at full port capacity. It should be true for minimum packet sizes to. Non-blocking switch means the same thing. Non-blocking Switch internal bandwidth can handle all the port bandwidths, at the same time, at full capacity. Sometimes for high end switches non-blocking is also refereed to switch architecture ability to significantly reduce head-of-line blocking (HOL blocking).

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HOL Head-of-line blocking

Head-of-line blocking (HOL blocking) in networking is a performance issue that occurs when a bunch of packets is blocked by the first packet in line. It can happen specially in input buffered network switches where out-of-order delivery of packets can occur. A switch can be composed of input buffered ports, output buffered ports and switch fabric.

When first-in first-out input buffers are used, only the first received packet is prepared to be forwarded. All packets received afterwards are not forwarded if the first one cannot be forwarded. That is basically what HOL blocking really is.

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