Many SNMP MIB
definitions define arbitrary 32-bit indices for their object tables. MIB
implementations often do a mapping from the MIB indices to some internal data
structure that is keyed by some other set of data. In these MIB tables the data
contained in the table are often other identifiers of the element being
modelled. For example, in the ENTITY-MIB, entries in the entPhysicalTable are
indexed by the 31-bit value, entPhysicalIndex, but the entities could also be
identified by the entPhysicalName or a combination of the other objects in the
table.
Because of the size
of some MIB tables, significant processing is required to discover all the
mappings from the 32-bit MIB indices to the other data which the network
management station identifies the entry. For this reason, it may be necessary
for some MIB indices to be persistent across process restarts, switchovers, or
device reloads. The ENTITY-MIB entPhysicalTable and CISCO-CLASS-BASED-QOS-MIB
are two such MIBs that often require index values to be persistent.
Also, because of
query response times and CPU utilization during CISCO-CLASS-BASED-QOS-MIB
statistics queries, it is desirable to cache service policy statistics.