I know what SNMP is and I know what ping is. What is meant by a 'SNMP ping'? SNMP can be used to see if devices on a network are still alive.. what does it use to do this? I wouldn't have thought an SNMP ping is the same as an ICMP ping?
What is a SNMP ping?
Asked Answered
As you correctly suspected SNMP "ping" is definitely not the same as ICMP ping. What it does is to try to retrieve some basic information through SNMP like DNS name, system name, location, system type, system description etc. and if successful the "ping" is deemed to be successful too.
But this is not any kind of standard the way ICMP Ping (echo) is. There is no special "ping" command in SNMP - it's just a name for a tool used to scan whether SNMP is alive at some target device (by retrieving some common MIB values). So as you would expect the implementation differs too as a consequence of it being a useful tool rather than a standard.
But that has little effect in practice as there is a set of 'mandatory' SNMP records so if a device does not respond to those, you can be pretty sure it doesn't run SNMP. For an SNMP "ping" to work SNMP MUST be enabled on the target device of course ... which isn't the case by default most of the time in general so that's a big difference to ICMP Ping which can be used almost universally.
I hope I answered your question
Ping was "Packetized Internet Node Groper", originally a tool that implemented an ICMP echo response. "Ping" is now commonly used to convey an abstraction of checking whether a device is online, available, responding.
There are MIB options to ask a device to ping or trace route something -- which seem to be asking a device to ICMP to a third party and indicate success/failure -- but the colloquial use of "snmp ping" is to check responsiveness by asking a simple SNMP question of a static data point and get a response.
Most devices offer some response to requests in the 1.3.6.1.2 subtree, but it's not a hard/fast requirement. For example, on a Unix (-like) command line, one may try "snmpget -v 1 -c public 192.168.0.1 1.3.6.1.2.1.1.1.0" to ask "192.168.0.1" what its name is. The device may respond; it typically will not NAK if the access (version 1, community string "public", in this case) is incorrect. The switch(es) in between may choose to alert the requestor "unreachable", but may not. In this case as others, if there is no response, the messages or hints aside from "no response" may be helpful.
It is typical for an SNMP ping to be equivalent to an ICMP echo. IBM states, for example, that it issues a single ICMP echo (to the SNMPD on the receiving end) and returns the minRTT. If a response times out, then it sends another raw echo packet after 1 second, and another again after 2 seconds. If there is yet no reponse, -1 is returned.
Cisco also has a sort of similar implementation (SNMP GET/SET/PING-MIB). It is definitely implementation-specific, however.
"ping" is just a generic term for sending a message for the purpose of seeing whether you get back a response.
ICMP is the most common method of "pinging" a host on an IP network. If you get an ICMP echo response from an IP address, then you know that something out there is able and willing to receive and reply to at least some network traffic for that address. Receiving a reply to an ICMP ping does not tell you anything about the state of any other network services. Not receiving a reply does not tell you anything at all: there are dozens of reasons why you might not receive a response to an ICMP ping at any particular moment.
The concept of a "ping" applies to any mechanism of communication. Some network protocols have specific standards for performing a "ping". For most, "pinging" just means trying to perform some basic operation to see if it works. For example, you could "ping" an HTTP server by just connecting to port 80 and doing "HEAD / HTTP/1.0". If you get back an HTTP response, then you know there's an HTTP server operational. The same idea applies to SNMP or any another network application.
In most contexts I have heard 'SNMP Ping' used, it was referring to a DISMAN PING. This method allows you to use SNMP to control a device or host remotely and tell it to 'ping' another device. Typically, this would be a traditional ICMP ping.
For example, imagine you are on your laptop, and you have a webserver which seems to be taking a long time to load a page. You can't quickly tell if it is a problem with the network between you and the webserver, the network between the webserver and the database, or something with one of the servers themselves. You want to eliminate network first, so you first ping the webserver from your laptop. No loss, and the latency looks reasonable. You ping the database server from your laptop, and it looks good too. The problem with the laptop-database ping test is it doesn't tell you what is going on between the webserver and the database. Ideally, you'd log into the webserver and ping the database, but you don't have a shell account. You do, however, have a read/write SNMP access. So, you decide to use 'snmpset' from your laptop to the webserver to create a table (for ping results) and specify a target (database server) to ping. The webserver snmpd initiates an ICMP ping from the webserver to the database and stores the results in a table. You then do a 'snmpget' from the laptop to pull the webserver-database ping results.
The other contexts you might see 'snmp ping' may be simply a snmpget from a SNMP client to a SNMP daemon to confirm SNMP is working.
In my understanding, an SNMP ping is the one defined in RFC 2925
https://www.rfc-editor.org/rfc/rfc2925
If the SNMP agent vendor indicates that a device supports this RFC, then you can use SNMP ping to monitor it. Otherwise, it won't work.
This is completely different from the "normal ping" (ICMP based).
Like ICMP ping, SNMP ping, just a form of a SNMP getrequest, is used to check the aliveness of any equipment that has standard SNMP agent running for monitoring. It is useful when the management systems that manage these equipments to query for the auto discovery. Any sysOid can be used to query the equipment as part of the request.
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