Since the Aruba 5400R zl2 is a chassis based switch series...MTBF can't be referred to the whole family without specifying the Modules you're referring to...as example, the chassis with SKU J9850 has a MTBF of 700800 hours (see here)...Aruba 5400R zl2 Modules (Management Module and Interface Modules) clearly have different MTBF values.