So you’re going along with your work and you need to add a new server to your fiber channel switches. However you don’t have any more ports left on the switch. You’ve got two options, buy two new switches and link each pair of switches (first existing switch to one new switch, second existing switch to the other new switch). So logic tells you that you’ve got 4 Gig ports on the switch, and 4 gig cables, so if you plug one cable between the two switches you’ll get 4 Gigs of bandwidth between the switches right?
Not so much. When you ISL the switches together (that’s the fancy technical term for connecting two fiber channel switches in a single fabric) you only get 50% of the bandwidth through the cable. So in order to get the full 4 Gigs of bandwidth you’ll need to string two cables between each pair of switches. You’ll see more clearly in the diagram below.
You’ll see in the diagram that Existing Switch 1 connects to both sides of the storage array, and it connects to New Switch 1 as well. Existing Switch 2 also connects to both sides of the storage array as well as to New Switch 2. You’ll see that in the diagram you don’t connect Existing Switch 1 to New Switch 2 or visa verse.
Now if you have a sever which will need more than 4 Gigs of bandwidth that will be plugged into one of the new switches you’ll want to connect more than two cables between the switches so that you can get the full 8 Gigs of possible bandwidth between the server and the storage array.
Like Ethernet switches you can bond these ports between the switches together into a single virtual switch via the built in trunking options within the switch. As each switch vendor has a different way to do this (and those methods vary depending on the OS running on the switch) I won’t do into the specifics here, but your support desk with your vendor should be able to provide you with support in doing this.
One last thing to keep in mind when ISLing these switching together is that some vendors has a license fee to enable the ISL features so keep that in mind when doing your planning.
P.S. This diagram is based on an EMC CX3 Storage Array, but it is perfectly valid for any dual head fiber channel storage array.