Subject : Telecom Network Management

Topic : OSI Network Management

Difficulty : medium

NMP traps are used in the management of a data network.

• SNMP traps enable an agent to notify the management station of significant events by way of an unsolicited SNMP message.

• In this diagram, the setup on the left shows a network management system that polls information and gets a response. The setup on the right shows an agent that sends an unsolicited or asynchronous trap to the network management system (NMS).

• SNMPv1 (Simple Network Management Protocol) and SNMPv2c, along with the associated Management Information Base (MIB), encourage trap-directed notification.

• The idea behind trap-directed notification is that if a manager is responsible for a large number of devices, and each device has a large number of objects, it is impractical for the manager to poll or request information from every object on every device. The solution is for each agent on the managed device to notify the manager without solicitation. It does this by sending a message known as a trap of the event.

• After the manager receives the event, the manager displays it and can choose to take an action based on the event. For instance, the manager can poll the agent directly, or poll other associated device agents to get a better understanding of the event.

• Trap-directed notification can result in substantial savings of network and agent resources by eliminating the need for frivolous SNMP requests. However, it is not possible to totally eliminate SNMP polling.

• SNMP requests are required for discovery and topology changes. In addition, a managed device agent can not send a trap, if the device has had a catastrophic outage.

• SNMPv1 traps are defined in RFC 1157, with these fields:

Enterprise—Identifies the type of managed object that generates the trap.

Agent address—Provides the address of the managed object that generates the trap.

Generic trap type—Indicates one of a number of generic trap types.

Specific trap code—Indicates one of a number of specific trap codes.

Time stamp—Provides the amount of time that has elapsed between the last network reinitialization and generation of the trap.

Variable bindings—The data field of the trap that contains PDU. Each variable binding associates a particular MIB object instance with its current value.

• Standard generic traps are: coldStart, warmStart, linkDown, linkUp, authenticationFailure, egpNeighborLoss

• In order for a management system to understand a trap sent to it by an agent, the management system must know what the object identifier (OID) defines.

• Therefore, it must have the MIB for that trap loaded. This provides the correct OID information so that the network management system can understand the traps sent to it.

• A device does not send a trap to a network management system unless it is configured to do so.

• A device must know that it should send a trap.

• The trap destination is usually defined by an IP address, but can be a host name, if the device is set up to query a Domain Name System (DNS) server.

• Device administrators can choose which traps they would like send.