MIB: CISCO-MEMORY-POOL-MIB
Object name: ciscoMemoryPoolFree
Indicates the number of bytes from the memory pool that are currently unused on the managed device. Note that the sum of ciscoMemoryPoolUsed and ciscoMemoryPoolFree is the total amount of memory in the pool
Reference: http://www.cisco.com/c/en/us/support/docs/ip/simple-network-management-protocol-snmp/15216-contiguous-memory.html