20.3 Routing Overlapping MIBs

In a master agent application, routing is performed depending on the OIDs requested. SNMP MIB agents are registered for branches of the global OID tree, and varbinds are dispatched to subagents according to the requested OIDs. The SnmpV3AdaptorServer supports registration of overlapping MIBs. Overlapping MIBs are MIBs that are registered below the OID of a previously registered MIB. The actual value returned to the manager is the value implemented by the deepest MIB registered for that value. When a get or set request is received, for each varbind in the request, the SnmpV3AdaptorServer identifies the deepest MIB responsible for handling the OID contained in the varbind. All varbinds targeted to that MIB are grouped in an SnmpMibRequest, and that request is passed to the identified SnmpMibAgent.

20.3.1 Shadowing Overlapping MIBs

MIB overlapping enables you to shadow MIBs that are implemented in subagents. For instance, you might want to implement the system group and snmp from MIB-II locally in your master agent, and delegate all other requests concerning MIB-II to a legacy SNMP agent implementing MIB-II. You do this as described in To Shadow Overlapping MIBs.

To Shadow Overlapping MIBs

1. Run the mibgen tool to generate an implementation of MIB-II

2. Instrument the system group and snmp group as shown in the basic SNMP agent example in “Creating an SNMP Agent”

3. Register your generated MIB with the OIDs of both the system group, 1.3.6.1.2.1, and snmp group, 1.3.6.1.2.1.1.1

4. Create an SnmpProxy object proxying your legacy MIB-II agent, and register that proxy for the root OID of MIB-II

20.3.2 Delegation and Precedence of Overlapping MIBs

MIB overlapping also enables you to delegate all unhandled OIDs to a legacy master agent, such as the Solaris agent snmpdx. If you register an SnmpProxy at the root of the OID tree, then all varbinds for whose OIDs no SnmpMibAgent was registered are passed to that SnmpProxy.

In addition, MIB overlapping enables you to give precedence to a specific agent for a part of a MIB, by registering it deeper in the OID tree.

For a get-next or get-bulk request, the incoming OID does not make it possible to identify directly the subagent that will contain the next OID. Indeed, the next OID might not be a sub-OID of the requested OID, but might be in a different part of the global OID tree. The get-next and get-bulk requests are thus forwarded to all subagents, and sorted a posteriori to identify the deepest matching subagent.