On an IB switch, you can have up to 144 ports (196?), each capable of doing 40Gb in QDR mode across the backbone.  To cascade, you are going to want to take one, or some number of ports and dedicate them for back end traffic.   Right off the top, unless the number of ports used to link the switches  are equal to, or greater than, the number of connected servers, there will be a bottle neck between switches. If I already have to double the number of ports just to cascade, then clearly the switch size is no longer an issue.  Another issue is that, in order to do this, you would need to move to a managed switch, as opposed to a simple flat topology. Once you move in that direction, you would need to consider management and administration as part of both installation and support, a level of complexity that doesn’t seem beneficial in a time when we are trying to keep costs down.
Hello Team,  Can someone provide a short explanation why we don't cascade infiniband switches?   I know this is not supported but curious why.
Cascading IB switches