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SNMP is an IP application layer protocol that has become the standard for the exchange of management information between network devices. SNMP was initially described in RFC 1157. It is a simple solution that requires little code to implement, which allows vendors to build SNMP agents on their products.

SNMP runs over User Datagram Protocol (UDP) and thus does not inherently provide for sequencing and acknowledgment of packets, but it still reduces the amount of overhead used for management information.

SNMP Components

SNMP has three network-managed components:

• The managed device
• The agent that resides on the managed device
• The network management system (NMS)

Figure 16-1 shows the relationship of these components.

A managed device is a router or LAN switch or any other device that contains an SNMP agent. These devices collect and store management information and make this information available to the NMS. SNMP community strings (passwords) are configured on routers and switches to allow for SNMP management.

The agent is the network management software that resides in the managed device. The agent gathers the information and puts it in SNMP format. It responds to the manager's request for information and also generates traps.

The NMS has applications that are used to monitor and configure managed devices. It is also known as the manager. The NMS provides the bulk of the processing resources used for network management.

MIB

A Management Information Base (MIB) is a collection of information that is stored on the local agent of the managed device. MIBs are organized hierarchically and are accessed by the NMS. MIBs are organized in a treelike structure, with each branch containing similar objects. Each object has a unique object identifier (number) that uniquely identifies the managed object of the MIB hierarchy.

The top-level MIB object IDs belong to different standards organizations, and lower-level object IDs are allocated to associated organizations. Vendors define private branches that include managed objects for their products. Figure 16-2 shows a portion of the MIB tree structure. RFC 1213 describes the MIBs for TCP/IP. Cisco defines the MIBs under the Cisco head object. For example, a Cisco MIB can be uniquely identified by either the object name, iso.org.dod.private.enterprise.cisco, or the equivalent object descriptor, 1.3.6.1.4.1.9.

Each individual manageable feature in the MIB is called a MIB variable. The MIB module is a document that describes each manageable feature that is contained in an agent. The MIB module is written in Abstract Syntax Notation 1 (ASN.1). Three ASN.1 data types are required: name, syntax, and encoding. The name serves as the object identifier. The syntax defines the object's data type (integer or string). The encoding data describes how information associated with a managed object is formatted as a series of data items for transmission on the network. More specific information about Cisco MIBs can be found at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

SNMP Message Types

SNMPv1 was initially defined by RFC 1157. Since then, SNMP has evolved with a second and third version, each adding new message types. The CCDA should understand each message type and the version associated with each.

SNMPv1

SNMPv1 is defined by RFC 1157. It is a simple request-and-response protocol. The NMS manager issues a request, and managed devices return responses. The date types are limited to 32-bit values. SNMPv1 uses four protocol operations, with five message types to carry out the communication:

• Get Request
• GetNext Request
• Get Response
• Set Request
• Trap

Figure 16-3 shows the SNMPv1 message types.

The NMS manager uses the Get operation to retrieve the value-specific MIB variable from an agent. The GetNext operation is used to retrieve the next object instance in a table or list within an agent. The Get Response contains the value of the requested variable.

The NMS manager uses the Set operation to set values of the object instance within an agent. For example, the Set operation can be used to set an IP address on an interface or to bring an interface up or down. Agents use the Trap operation to inform the NMS manager of a significant alarm event. For example, a trap is generated when a WAN circuit goes down.

SNMPv2

SNMPv2 is an evolution of the initial SNMPv1 and is defined in RFCs 1901 and 1902. SNMPv2 offers improvements to SNMPv1, including additional protocol operations. The Get, GetNext, and Set operations used in SNMPv1 are exactly the same as those used in SNMPv1. The SNMP Trap operation serves the same function as in SNMPv1, but it uses a different message format.

SNMPv2 defines two new protocol operations:

• GetBulk
• Inform

The NMS manager uses the GetBulk operation to retrieve large blocks of data, such as multiple rows in a table. This is more efficient than repeating GetNext commands. If the agent responding to the GetBulk operation cannot provide values for all the variables in a list, it provides partial results. The Inform operation allows one NMS manager to send trap information to other NMS managers and to receive information. Another improvement is that data type values can be 64 bits.

SNMPv3

SNMPv3 was developed to correct several deficiencies in the earlier versions of SNMP, security being a primary reason. SNMPv3 is defined in RFCs 3410 through 3415. SNMPv3 provides authentication and privacy by using usernames and access control by using key management. Security levels are implemented to determine which devices a user can read, write, or create. SNMPv3 also verifies each message to ensure that it has not been modified during transmission.

SNMPv3 introduces three levels of security:

• noAuthNoPriv
• authNoPriv
• authPriv

The noAuthNoPriv level provides no authentication and no privacy (encryption). At the authNoPriv level, authentication is provided but not encryption. The authPriv level provides authentication and encryption.

Authentication for SNMPv3 is based on HMAC-MD5 or HMAC-SHA algorithms. The Cipher Block Chaining-Data Encryption Standard (CBC-DES) standard is used for encryption.