Overview of SNMP - IMG


SNMP (Simple Network Management Protocol) was designed as the standard language to be used by all computers on a network. The SNMP protocol is used by network management systems to communicate with network elements to monitor and control different aspects of the equipment. For SNMP to function in a network there needs to be at least two elements: The SNMP Manager and the SNMP Agent. See descriptions below:


SNMP Manager: An SNMP Manager is an application that performs the operational roles of generating requests to modify and retrieve management information and receiving the requested information and trap-event reports that are generated by the SNMP agent. SNMP management applications such as Castlerock (SNMPC), and AdRem can be used and have been verified to operate on the IMG. The IMG is not limited to the SNMP manager applications mentioned. Other applications can be utilized, but the applications mentioned above have been verified.


SNMP Agent: An SNMP Agent is an application that performs the operational role of receiving and processing requests, sending responses to the manager, and sending traps when an event occurs. The IMG acts as an SNMP Agent. The SNMP agent software must have three sets of hooks into the SNMP Manager:


In an SNMP operation, network information is exchanged through messages sent between the Manager and Agent. The Manager uses messages to request operations to be performed on the SNMP agent. The messages sent are GetRequest, GetNextRequest, SetRequest, and GetBulk. The SNMP Agent responds with a GetResponse message. If a specific condition or event occurs the SNMP Agent sends a Trap message to the Manager for viewing. Below are descriptions of messages sent between the Manager and Agent


SNMP Trap: Message which is initiated by an SNMP Agent and sent to the SNMP Manager. For example, the IMG (SNMP Agent) could send a message to the SNMP Manager that one of its spans has failed. This is termed an SNMP Trap. Basically, it is an alarm telling the Manager that a span is out of service


SNMP Get: Message which is initiated by the SNMP Manager when it wants to retrieve some data from an SNMP Agent. For example, the SNMP Manager might query the IMG for span status every 2 minutes. From the data retrieved via the IMG, the SNMP manager could create a chart of Out of Service versus In Service, which would give the operator an idea of network quality.


SNMP SET: (Not Supported on the IMG) Message which is initiated by the SNMP Manager that is sent when the operator wants to change data on a network element. For example, the operator could change a system (SNMP Agent) operating parameter.


Each message sent between the SNMP Agent and the SNMP Manager makes reference to a specific object. For example, requesting span status is requesting the status of the object which is a span. These objects have an address or number associated with them. The number is referred to as an OID (Object Identifier). A series of OIDs in a specific area are considered the MIB (Management Information Base). Below is a description of what a MIB is. For more information on how a MIB is identified, see the Topic What are MIBs ?


MIB: A Management Information Base (MIB) is a scheme that contains the hierarchical order of all of the managed objects. Each managed object in an MIB has a unique identifier. The identifier includes the type (such as counter, string, gauge, or address), access level (such as read/write), size restrictions, and range information of the object. The MIB is extensible, which means the manufacturer can append information to an object to extend the tree as much as needed. These new MIB definitions must be added both to the SNMP Agent and to the SNMP Manager (NMS).


Note: (An IMG does not support company specific or Enterprise MIBS)




Notable Terms:

NMS: A Network Management Station is a console that monitors and controls SNMP Agents by executing SNMP Manager Applications. To perform NMS functions, high-performance workstation computers that have fast CPUs and extensive disk space are used. A managed network requires at least one NMS workstation to perform SNMP operations.


Enterprise MIB: A proprietary MIB developed by a private company. Users of the MIB will have to design their managers around it. The IMG does not support proprietary MIBS


Entity MIB: A well-defined MIB laid out in RFCs. Users of this MIB will likely already have software in place to handle it.


Get: A type of SNMP message sent from a manager to an agent which requests to know the value of a particular variable on the agent.


NMS: Network Management System. A collection of applications used to monitor and maintain a network. An SNMP manager would be an example of one such application.


Notify: A message sent from an SNMP agent (server) to the set of SNMP managers (clients) that have registered to receive it. The agent expects an acknowledgement from the manager. If it does not receive the acknowledgement within the specified interval, it will resend a number of times.


Community String: The most basic form of SNMP security is the Community String. SNMP Community Strings are like passwords for network elements. Generally, there is one community string which is used for read-only access to an SNMP Agent. The default value for this community string is often "public". Using this community string like a password, the NMS (Network Management System) can retrieve data from network elements. Less often, there is also a read-write community string. The default value for this is often "private". Using this community string, the NMS can actually change MIB variables on a network element.


Object Identifier (OID): An Object Identifier is the identification value of an object that is defined in a MIB. Object identifiers are arranged in a hierarchical tree structure that is compliant with Internet standard and that consists of roots and branches. An object identifier is written as a sequence of sub-identifiers, starting with the tree root, in dotted decimal notation. For example, the IMG branch of the MIB naming tree is expressed as (Cantata IMG) / (Dialogic) This identifies the IMG and would be the identifier for enterprise MIBS. For more information on OID values see http://www.alvestrand.no/objectid/top.html



SNMP Message Format

Each SNMP message contains the following:


An SNMP message also contains the following: (All messages other than traps)


A Trap (alarm) contains the following:



SNMP Diagram of Basic Implementation

The diagram below shows a basic SNMP Implementation, in which there is a packet-switched network that is monitored and maintained by a network administrator. The portal into the network is through an NMS - network management station (or network management system). The NMS is a collection of applications which fall into the following broad categories:



SNMP Implementation on  the IMG


SNMPv2c: SNMPv2 with community-based security allows for configuration of an agent for authentication, encryption, data access, and trap management. To utilize the SNMPv2c features, one must have an SNMP Manager Application loaded on a PC which supports SNMPv2c. SNMPv2c will monitor DS1, DS3 and Ethernet Interfaces using industry standards MIBs


Get and Trap Commands:


IMG Supports:




Miscellaneous Information:


Related Topics:

Supported MIBs

Supported Traps

Configuring SNMP

Configure SNMP Agent

Configuring SNMP Managers

Configure SNMP Manager