Tag: tw

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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TCAM and CAM memory usage inside networking devices

As this is networking blog I will focus mostly on the usage of CAM and TCAM memory in routers and switches. I will explain TCAM role in router prefix lookup process and switch mac address table lookup.

However, when we talk about this specific topic, most of you will ask: how is this memory made from architectural aspect?

How is it made in order to have the capability of making lookups faster than any other hardware or software solution? That is the reason for the second part of the article where I will try to explain in short how are the most usual TCAM memory build to have the capabilities they have.

CAM and TCAM memory

When using TCAM – Ternary Content Addressable Memory inside routers it’s used for faster address lookup that enables fast routing.

In switches CAM – Content Addressable Memory is used for building and lookup of mac address table that enables L2 forwarding decisions. By implementing router prefix lookup in TCAM, we are moving process of Forwarding Information Base lookup from software to hardware.

When we implement TCAM we enable the address search process not to depend on the number of prefix entries because TCAM main characteristic is that it is able to search all its entries in parallel. It means that no matter how many address prefixes are stored in TCAM, router will find the longest prefix match in one iteration. It’s magic, right?

CEF Lookup

Image 1 shows how FIB lookup functions and points to an entry in the adjacency table. Search process goes through all entries in TCAM table in one iteration.


Router

In routers, like High-End Cisco ones, TCAM is used to enable CEF – Cisco Express Forwarding in hardware. CEF is building FIB table from RIB table (Routing table) and Adjacency table from ARP table for building pre-prepared L2 headers for every next-hop neighbour.

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How can router decide so fast?

Cisco created all sorts of different magic inside their boxes that optimize forwarding processing of packets.

CEFMain router function is fairly self-explanatory. Router performs IP forwarding more often called IP routing. IP routing is process of deciding where to send the packet after it was received.

 

 

 

IP Routing explained in detail

Logic behind IP forwarding is listed in steps here with the assumption it will be an IPv4 packet that was received. This is process switching explained in 11 steps:

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