When setting up an Aruba CX VSX pair, which information does the Inter-Switch Link Protocol...

The reason is that the Inter-Switch Link Protocol (ISLP) is a protocol that enables VSX stack join and synchronization between two VSX peer switches. ISLP uses a hello interval to exchange control messages between the switches.

The hello interval is a parameter that specifies the time interval between sending hello messages. The default value of the hello interval is 1 second. The hello interval can be configured from 1 second to 10 seconds.

https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/index.html