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SNMP stands for Simple Network Management Protocol.
SNMP is a standardized protocol used to collect and organise device information on a network. It does this over UDP port 161 (Traps are received on port 162).
There are three versions of SNMP.
- SNMPv1 which is the original version of SNMP
- SNMPv2c - This is the revised version which introduced some enhancements. Most notably the use of community strings for security. You can think of a community string like a password.
- SNMPv3 - Which is another improvement adding authentication and encryption. This version is much more secure than the previous versions.
A device that is SNMP enabled is known as an agent. An SNMP agent has several objects that can be interacted with. Some objects are part of the industry standard, and some will be vendor-specific for this device.
For example, this router will have objects for, name, uptime, interfaces, and the routing table to name just a few.
Each object is assigned an object identifier or OID. The OID is a sequence of numbers, which at first glance can look like an IP address. This is used to identify the object in a file called a MIB which stands for Management Information Base.
To interact with these object, we need something called a network management system or NMS. The NMS is a piece of software that can communicate with the SNMP agent.
There are a few ways the NMS can speak to the agent. The first way is by using GET requests. Get message include get, getNext and getBulk. They are used to actively request information from the agent.
The second way we can talk to an agent is by using SET requests. A set request is used to change a value on the agent.
The last method I want to talk about is Traps and Informs. Agents use traps and informs to actively communicate with the NMS. The agent can send a trap or inform to the NMS when an event occurs on the device.
Traps and informs are two different types of massage but they both do the same thing. The difference is that informs are reliable. An inform message will wait for an acknowledgement from the NMS. If it doesn’t receive one, it will resend the inform. A trap on the other hand is sent and then forgotten. Even though informs are consider reliable, both still use UDP.
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