variable instance number in snmp traps

Question

Suppose I'm writing an SNMP v1/2 agent.

Is it mandatory to append the instance number to the variable OIDs in a trap PDU?

In SNMP V1 for instance a trap is defined in RFC 1157, with these fields:

• Enterprise
• Agent address
• Generic trap type
• Specific trap code
• Time stamp
• Variable bindings

The "variable bindings" section consist of a list of variable names and values. According to RFC 1157 :

In general, the name of an SNMP variable is an OBJECT IDENTIFIER of the form x.y, where x is the name of a non-aggregate object type defined in the MIB and y is an OBJECT IDENTIFIER fragment that, in a way specific to the named object type, identifies the desired instance.

What's bothering me is the "In general", is the .y mandatory?

Answer 1

Yes, it's mandatory to include instance subidentifiers for any variable OIDs in the variable bindings. The paragraph is describing the general (i.e., generic) form for a variable OID, not using "in general" to mean a non-committal "well, it's usually like this, but not always". Later RFCs use more concrete wording:

A variable-binding list is a simple list of variable bindings.

The name of a variable is an OBJECT IDENTIFIER which is the concatenation of the OBJECT IDENTIFIER of the corresponding object- type together with an OBJECT IDENTIFIER fragment identifying the instance. The OBJECT IDENTIFIER of the corresponding object-type is called the OBJECT IDENTIFIER prefix of the variable.

[RFC 3416, section 2.1]