Can someone please explain SNMP for me?

From what I can surmise SNMP looks like a neat way to collect information from networked devices, and it would be really nice to get working but everytime I go to look for information I end up confused.

How does it work, what do all these words mean, communities traps etc...

Can someone just explain in idiot terms

SNMP is simple network management protocol

A community is all what all the devices must be a part of. An SNMP server can't query a device not in the same community.

A trap is an "alert" or "event" that happens. If you say I want to generate an alert if my servers CPU gets above 95% that is a trap.

SNMP uses what are called MIB's. They look something similar to this.
1.3.2.5.4.1.42
Each manufacturer uses different MIB's for each thing that can be monitored. Combining MIB's and preforming expressions on them give you things like total disk space, etc.
Taking the MIB for total hard drive capacity, then minus the MIB for amount of disk space used gives you "free space"

That's it in a nutshell.
There are different versions of SNMP. The latest is SNMPv3 which supports authentication. Most people use SNMPv2.

I'll let others go into the whole ReadOnly ReadWrite stuff if you want to get really in depth.

So traps are events that happen; are these processed on the device or on the monitoring server? ie: Do the devices catch the metric and send them off to the monitoring server themselves?

I'm an snmp dope. I've used it for some very light network monitoring ( and i've hacked together a printer toner check.. ), but I really don't understand the tech.

How is it monitored then, I understand that traps are sent? to a monitoring server, I'm guessing software must be installed on the server to display that information?

SNMP clients can send traps to the SNMP server (to alert of issues) such as hardware failures, or custom alerts based on predefined thresholds. Most people just do SNMP polling, the server polls the client MIBs for things like disk free, RAM usage, bytes sent/received, etc.
SNMP can also write to a client, and change options, but thats used less frequently (I've never really done it myself TBH)

Here is another way of looking at it.

SNMP Polling means the Network management server (Nagios, Cacti, PRTG, etc), will poll the managed device (switch, server, etc) at specific intervals. It's like scheduling a ping test every 30 seconds. But instead of sending and ICMP packet, it will do a SNMP GET request to the managed Device IP and get a response. The response is usually a numeric value (CPU Utilization, network utilization, available ram, etc).

SNMP Traps are sent from the managed device to a Network management server. The parameters of these traps are configured on the managed device. Usually when a Network management server receive these traps, it usually logs and/or notifies by SMS/Email.

Since there are all these new devices coming out, the managed device manufacturer allows you to monitor other things using SNMP, they will provide MIBs that can be imported into your network management server.

Not the most exciting facet of IT, but an important one. It helps tremendously when looking at trends or to answer the question "what just happened".

Subscribed.

why not just read the RFC ?

SNMP events are monitored via software such as

Solarwinds Orion (expensive)
Cacti (free)
Nagios (free)
Spiceworks (free)
PRTG

and lots lots more.

As stated, most people do not utilize the ReadWrite of SNMP. most people also only "poll". Traps can be setup on the devices, but that requires you to set them up manually on each device usually. It's far easier and simpler to just setup notifications (pager, email, text message, etc) on the monitoring server when certain things happen (or don't happen)

The SNMP monitoring software will create graphs and historic information for you to refer to so you can watch for trends and be proactive about disk's filling up, using too much bandwidth, cpu's running too high, too much memory being used, etc.

SNMP is one of the pillars of modern network management.

<3 SNMP

One of the first things I set up on my servers and my clients' servers is SNMP service/daemon. I prefer to use MRTG. Easy to compile and install. Easy configuration file.