How does switch fabric network work

A network engineer can list a number of issues you can potentially run when using STP protocol  in your switch network. Over the years the network industry has created successor protocols like RSTP or MSTP. Both are improvements and offer much better convergence time and respond much quicker to switch topology changes. One of the major disadvantages for networks that relay on STP is the fact that they don't support multipathing. It means once network topology converges there will be blocked path between switches that are elected and managed by STP. This often redundant links can't be used because of a loop risk.

But there are better solutions today on the market to design better layer 2 Ethernet networks (more scalable, with higher throughput and with active link redundancy as an example). The 2 most popular are based on SPB and TRILL protocols. Both of them are used as a foundation in switch fabrics products. To better understand both of them the pictures below provide a side by side comparison. This was taken from Avaya document: Compare and Contrast SPB and TRILL.

Avaya is a SPB promoted so the comparison is a bit waited towards SPB but nevertheless it gives some inside view into both protocols.

References

http://cciethebeginning.wordpress.com/2008/11/20/differences-between-stp-and-rstp/
http://etherealmind.com/spb-attention/
http://en.wikipedia.org/wiki/IEEE_802.1aq
http://en.wikipedia.org/wiki/TRILL_(computing)
http://www.avaya.com/uk/resource/assets/whitepapers/SPB-TRILL_Compare_Contrast-DN4634.pdf
http://nanog.org/meetings/nanog50/presentations/Monday/NANOG50.Talk63.NANOG50_TRILL-SPB-Debate-Roisman.pdf
http://www.ebrahma.com/2012/06/trill-vs-spb-similarities-differences/
http://wikibon.org/wiki/v/Network_Fabrics,_L2_Multipath_and_L3

How to design a network for Openstack or cloud deployment

Designing a network is a topic within itself and there is no way we can cover all of it in this single post. Cisco has its own certification path path CCDE for these who want to know more.

Cisco design and implementation guide - old best practices

In a very simplistic view back in the old days a network used to be design similar to the picture below (although it is hard to say when the new era started ;)). Every big network had to have a core, distribution(sometimes called aggregation) and access layer. The network was engineered mainly to help with North to South traffic in the data center or in another words to help get the data out and into the data center.

New cloud friendly data center design best practice

With the advent and popularization of new networking devises that support layer 2 routing, commonly know as networks fabrics (more info about TRILL and fabric) the network design has shifted in data centers. The way we design the networks today is to maximize the East to West traffic instead of the North to South ( old design above). The purpose of the new network is to allow more efficiently exchange data between the servers within the rack or data center.

They say that a picture is worth more than a thousand words. To help us to visualize how a new data center network/cloud network is designed these demonstration pictures (taken from Cisco document: Cisco Massively Salable Data Center) will shed some more light on it. Please note that we no longer use the core, distribution or access keyword but instead: spine, leaf or superspine to describe the different network layers :).