SNMP is a standardized protocol used to collect and manage devices. Although the standard allows you to use SNMP for device management, in my experience, most network administrators prefer to keep SNMP as an information collection mechanism only. Since SNMP operates on UDP that is connectionless and considering the relatively weak security mechanism in versions 1 and 2, making device changes via SNMP tend to make network operators a bit uneasy. SNMP Version 3 has added cryptographic security and new concepts and terminologies to the protocol, but the way it's adapted varies among network device vendors.

SNMP is widely used in network monitoring and has been around since 1988 and was part of RFC 1065. The operations are straightforward with the network manager sending GET and SET requests toward the device and the device with the SNMP agent responding with the information per request. The most widely adapted standard is SNMPv2c, which is defined in RFC 1901 - RFC 1908. It uses a simple community-based security scheme for security. It has also introduced new features, such as the ability to get bulk information. The diagram below display the high-level operation for SNMP.

The information residing in the device is structured in the Management Information Base (MIB). The MIB uses a hierarchical namespace containing an Object Identifier (OID), which represents the information that can be read and fed back to the requester. When we talk about using SNMP to query device information, we are really talking about using the management station to query the specific OID that represents the information we are after. You're required to put some efforts for consolidating basic common information into a common OID structure; however, the output of the effort varies in terms of how successful it is. At least in my experience, I typically need to consult with vendor documentation to find the OID that I need.

Some of the main points to take away from the operation are:

• The implementation heavily relies on the amount of information the device agent can provide. This, in turn, relies on how the vendor treats SNMP: as a core feature or an added feature.
• SNMP agents generally require CPU cycles from the control plane to return a value. Not only is this inefficient for devices with, say, large BGP tables, it is also not feasible to use SNMP to constantly query the data.
• The user needs to know the OID in order to query the data.

Since SNMP has been around for a while, my assumption is that you have some experience with it already. Let's jump directly into our first example.