High Capacity Counters
Since the advent of faster and faster network interfaces, SNMP's original use of 32 bits for counters has become somewhat outdated. As a counter of network throughput in bytes, a 32bit counter can only count up to ~4GB before rolling back to zero. On a very fast interface, the counter may reset before it can be read again.
High Capacity Counters are designed to solve this issue by increasing counters from 32-bit (maximum of 4,294,967,295 bytes) to 64-bit (maximum of 9,223,372,036,854,775,807 bytes)
For more information on High Capacity Counters, see the 2nd question of this FAQ: SNMP Counters: Frequently Asked Questions [cisco.com]