 I worked with a customer recently where we needed to expand a fiber fabric to increase the port count. The customer's existing fabric consisted of two Cisco MDS 9222i switches, and the customer wanted to add two Cisco MDS 9124 switches. Each 9124 would connect to a 9222i via an ISL (<a href="http://en.wikipedia.org/wiki/Cisco_Inter-Switch_Link">Inter-switch Link</a>).

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 The Cisco MDS 9124 is a 24-Port Multilayer Fabric Switch capable of speeds of 4, 2, and 1 Gbps in a 1-rack-unit form factor. It can be expanded from 8 to 24 ports in 8-port increments. Up to 16 VSANs (<a href="http://en.wikipedia.org/wiki/VSAN">Virtual Storage-area Networks</a>) are supported per switch. If you need to expand a fabric by adding ports you can create an ISL between switches, enabling them to function as a single fabric.

PortChannel link aggregation can be used with ISLs between switches to reduce fabric disruptions. It allows the aggregation of up to 16 physical ISLs into a single logical bundle, providing optimized bandwidth use across all links. The bundle can consist of any port from the switch, providing aggregated bandwidth and helping ensure that the bundle remains active even in the event of a port failure. Hardware-based load balancing is enabled by default.

Follow this procedure to expand an existing fabric by adding a second switch in the MDS 9000 family with an ISL between them:
<ul>
	<li>Rack and initialize the second switch</li>
	<li>Manually create the VSAN ID's that will be shared with the existing fabric</li>
	<li>Connect a fiber cable between any two ports on the switches to form an ISL</li>
	<li>Use Cisco Device Manager to configure the ISL ports as type E-Port, 4 GB, Dedicated bandwidth, on VSAN 1</li>
	<li>On the production switch, click the Policy tab and specify which VSANs ID's will be shared</li>
	<li>The switches should automatically connect and begin sharing information</li>
</ul>
You should now be able to create zones between devices connected to both switches.

MDS 9124 product data sheets can be found <a href="http://www.cisco.com/en/US/prod/collateral/ps4159/ps6409/ps5987/ps8414/at_a_glance_c45-531026.pdf">here</a> and <a href="http://www.cisco.com/en/US/prod/collateral/ps4159/ps6409/ps5987/ps7079/product_data_sheet0900aecd8057451f.html">here</a>, and a Dell-specific 9124 FAQ <a href="http://www.dell.com/downloads/global/products/pvaul/en/cisco_mds9124_faq.pdf">here</a>. 

 Cisco has excellent white papers:
<ul>
	<li><a href="http://www.ciscocatalyst.info/en/US/prod/collateral/ps4159/ps6409/ps5987/ps7079/prod_white_paper0900aecd80575670.pdf">Storage Access Network Design Using the Cisco MDS 9124 Multilayer Fabric Switch</a> </li>
	<li><a href="http://www.cisco.com/en/US/netsol/ns340/ns394/ns259/ns261/networking_solutions_white_paper09186a0080114c21.shtml">Using VSANs and Zoning with the Cisco MDS 9000 Family</a></li>
</ul>