A unique identifier for a particular protocol. Standard identifiers will be defined in a manner such that they can often be used as specifications for new protocols - i.e. a tree-structured assignment mechanism that matches the protocol encapsulation `tree' and which has algorithmic assignment mechanisms for certain subtrees. See RFC XXX for more details.

Despite the algorithmic mechanism, the probe will only place entries in here for those protocols it chooses to collect. In other words, it need not populate this table with all of the possible ethernet protocol types, nor need it create them on the fly when it sees them. Whether or not it does these things is a matter of product definition (cost/benefit, usability), and is up to the designer of the product.

If an entry is written to this table with a protocolDirID that the agent doesn't understand, either directly or algorithmically, the SET request will be rejected with an inconsistentName or badValue (for SNMPv1) error.