Previous | Table of Contents | Next |
The SNMP standard, RFC 1157, and the SNMP Administrative Model, RFC 1351 [4-4], define a number of terms. Many of these definitions describe relationships between management entities:
Figure 4-3 illustrates some of the definitions described above.
Figure 4-3. Network management relationships
SMI managed object types have an object identifier (OID) that uniquely names them and locates their place on the object tree. An instance of an object type is an occurrence of that object type and has an assigned value. For example, the object sysDescr {1.3.6.1.2.1.1.1.0} might have a value of Retix Remote Bridge Model 2265M.
Suppose a network management station wishes to retrieve an instance of a specific object. The management station must use SNMP to communicate its question to the agent.
Now, suppose multiple instances (or occurrences) of that object are possible. For example, say a routers routing table contains a number of entries. How would the network management station retrieve just the value of the third entry in the table?
RFC 1157, pages 12-15 specifies these tasks. For these SNMP operations, a variable name uniquely identifies each instance of an object type. This name consists of two parts of the form x.y. The x portion is the object type defined in the MIB, and the y portion is an OID fragment that identifies the desired instance. The following examples should clarify this.
Consider a scalar object that has one instance. The objects contained in the System group are all scalar objects. For example, the sysServices object has an OID of {1.3.6.1.2.1.1.7} and occurs once. The x portion of the variable name is the OID, and the y portion has been assigned to 0. You can derive this by following the OID tree down to the object sysServices and adding the appropriate instance suffix (with the suffix, or y portion, shown in boldface type):
iso org dod internet mgmt mib-2 system sysServices Instance 1 3 6 1 2 1 1 7 0
Thus, the variable name for sysServices is {1.3.6.1.2.1.1.7.0}.
The variable name for a columnar object is more complicated because it must identify the location of an object within a two-dimensional data structure, such as a table having both rows and columns. (Within the RMON MIB, three-dimensional data structures are added, making the identification even more complex.) Using the familiar speadsheet as an example, the identification of a particular cell requires two coordinates, X and Y, which describe the horizontal and vertical positions, respectively. With columnar objects, an indexing scheme, specified in the INDEX clause in the ASN.1 definition for that object, provides a means for identifying the specific instance. The INDEX clause then further identifies the syntax to be used. And as one might expect, some of the indexing schemes are more complicated than others. RFC 1212, pages 8-10 lists many of the INDEX clauses that are found within the MIB-2. Lets look at some examples for further clarification.
Consider the IP Address Table object, ipAdEntBcastAddr, which specifies the value of the least significant bit (LSB) of the IP broadcast address (see Figures 4-4a and 4-4b). To begin, follow the OID tree down to ipAdEntBcastAddr:
iso org dod internet mgmt mib-2 ip 1 3 6 1 2 1 4 ipAddrTable ipAddrEntry ipAdEntBcastAddr 20 1 4
Figure 4-4a. The IP address table within the OID tree
Figure 4-4b. Object instance in the IP address tables
The OID is {1.3.6.1.2.1.4.20.1.4}, consisting of the IP Group {1.3.6.1.2.1.4}, the IP Address Table (20), the ipAddrEntry (1), and the object ipAdEntBcastAddr (4), shown in Figure 4-4a.
Consulting MIB-2, RFC 1157 page 31, the ASN.1 definition for the object ipAddrEntry includes an INDEX clause that specifies the object ipAdEntAddr. Moving a few lines down in RFC 1157, we see that the object ipAdEntAddr has a SYNTAX of IpAddress. Thus, we would expect the index for this object to be defined by an IP address and shown in dotted decimal notation: a.b.c.d. (More on IP addresses and dotted decimal notation can be found in Section 6.3.)
Previous | Table of Contents | Next |