Questions to ask prospective SAN or LAN switch vendors.
Looking to buy a SAN or LAN switch? There are general similarities between LAN and SAN environments in that they both provide a communications path for data and storage movement. However, there are also differences that need to be understood in order to make balanced apples-to-apples comparisons. Here are a few things to keep in mind.
LANs run on Ethernet, which is IP-based and known for being low-cost and high-performance, using standard interfaces. Fibre Channel-based storage networks, on the other hand, are designed around the principal of guaranteed data integrity and deterministic performance, with price being secondary.
When evaluating any technology, it's important to have a good understanding of what your needs and requirements are. This would include figuring out what you have to have, what you need to have and what you would like to have. One question to ask yourself is, what type of storage network are you looking to deploy? If you are looking to deploy a NAS-based storage network using a file sharing protocol like NFS, CIFS or AppleTalk, you could use a traditional Ethernet-based switch. If you are looking to deploy an iSCSI-based storage network, you could also use an Ethernet switch, or a combination of Fibre Channel and iSCSI technology. Fibre Channel-based environments can support open systems as well as IBM S/390 (zSeries) based FICON servers.
Switches can be implemented using high speed cut-through routing or store-and-forward architectures. Many switch designs are based on the CLOS architecture, named after the AT&T Bell Labs researcher Dr. Charles Clos. Switches perform frame and packet moving from source to destination, based upon frame and packet addresses. Switch architectures can be full sp
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eed with no blocking (no congestion) at a higher cost, or over-subscribed with some level of blocking built-in and managed with Quality of Service (QoS) and traffic control management software. Traditional storage switches and networks are designed to be under-subscribed (non-blocking) for deterministic performance behavior and thus more expensive.
LAN switches tend to be Ethernet-based supporting speeds from 10 Mbps to 10 Gbps, using copper and optical interfaces ranging in size from a couple of ports to hundreds of ports. LAN switches typically support TCP/IP along with other protocols including TCP/UDP, IPX and AppleTalk. Other features found in many LAN switches include support for voice-over-IP (VoIP), Quality of Service (QoS), bandwidth management and reporting, multi-cast and jumbo frames. Some switch products include or support optional cards for routing and other network interfaces including SONET/SDH, SDH, and WiFi. LAN switches vendors include 3Com, Alcatel, Avaya, Cisco Systems, Inc., Dell, D-Link, Enterasys, Extreme, Foundry, Fujitsu, HP, Linksys, Marconi, NEC, SMC and Tasman Networks.
SAN switching products are primarily Fibre Channel-based, ranging in size from a couple of ports on embedded switches used on the back end (disk drive interface) of some storage controllers, to hundreds of ports on switching directors. Common protocols supported include SCSI Fibre Channel Protocol (SCSI_FCP a.k.a. FCP) for open systems and FICON for IBM mainframe environments. SAN switches are mainly 1 Gbps or 2 Gbps today, with many vendors making statements of direction about 4 Gbps and 10 Gbps, along with new functions for security, storage-over-IP, and advanced storage services, including virtualization. Some switches have also added support for Ethernet/IP blades and ports to support iSCSI, iFCP, or FCIP for storage over distance. SAN switch and intelligent switch vendors include Broadcom, Brocade, Cisco, CNT, Emulex, Maranti, Maxxan, McData and Qlogic.
In general, if you are looking to support Fibre Channel-based (FCP & FICON) storage networks, then deploy a Fibre Channel-based switch. If you are looking to deploy IP-based (iSCSI, iFCP, FCIP, NAS) storage networks you can use traditional LAN switches that support IP such as an Ethernet switch. Storage network routers like those available from Brocade, Cisco and McData are a very useful technology for interfacing Fibre Channel-based storage networks with LAN-based networks including protocol conversion and SAN segmentation.
The following are some general criteria and questions you should ask yourself, regardless of what type of network you will be deploying:
Avoid the temptation to simply evaluate switches on a price/port and apples-to-oranges comparison. You can learn more about evaluation criteria for storage networks and in particular Fibre Channel switching technology by accessing the free workbooks at www.evaluatorgroup.com/workbooks.
For more information:
Checklist: How to buy a switch
Tech Roundup: Switches
Advice: Which switches to use in an iSCSI SAN
About the author: Greg Schulz is a senior analyst with the independent storage analysis firm, The Evaluator Group Inc. Greg has 25 years of IT experience as a consultant, end user, storage and storage networking vendor, and industry analyst. Greg has worked with Unix, Windows, IBM Mainframe, OpenVMS and other hardware/software environments. In addition to being an analyst, Greg is also the author and illustrator of Resilient Storage Networks, Greg has contributed material to Storage Magazine. Greg holds both a computer science and software engineering degree from the University of St. Thomas. 
