11-22-2011 12:21 PM
when I need another ISL? --> if you need more redundancy on your ISL/trunks.
As you reach the limits? --> throughput limit of a single link at aprox 320mb in one direction. But I assume that you have more an IO problem than a throughput issue.
If you have discards class3 on a ISL port than you should think about additional ISLs or increase the buffer depending on the longdistance solution between your sites.
As I can measure the bottleneck? --> Bottleneckmon / toptalker on the device side and FabricWatch of ISL throughput alerts.
I hope this helps,