How SNMP MIBs communicate with the managed hardware? 1

[parazhang]

Does an SNMP MIB contain only the object ID and datatype,but does not contains the actual value of an Managed Object? For example,the MIB has an OID of one interface speed,then is there a value 100Mbps in the MIB also,or does the MIB must communicate with the interface when the NMS sends a request signal every time?

 

[Castor66]

The MIB does not contain any values. The NMS must maintain a database of responses from SNMP requests.

 

[parazhang]

Castor66 has said that "The NMS must maintain a database of responses from SNMP requests." But I feel that a managed device should maintian a database but not the NMS,is it right?

 

[parazhang]

Castor66 has said that "The NMS must maintain a database of responses from SNMP requests." Thanks a lot,but I feel that a managed device should maintian a database but not the NMS,is it right?

 

[Castor66]

The managed device keeps a 'point in time' value for each OID. When the NMS polls the OID it retrieves that value. Therefore you will only ever get a snapshot of the value. If you need to keep historical data then you will need to store the OID and Values retrieved in a database on the NMS.

 

[parazhang]

thx,castor66,but i can't understand what 'point in time' is,could you give me a lively detail of your answer?

 

[InDenial]

What Castor means is:

A managed device has a database with values. These values might change every second. (Like InOctets on a router interface). so if you do an snmp query on the InOctets OID for a certain interface it might have changed a second later. The value you got using an anmp query is the value at that point in time.

Even with values that do not change much you still talk about a point in time. Like with software versions. You might have done an snmp query to get the software version but maybe a day later someone might have upgraded the managed device.

Hope this is a good explanation from a snmp Newb

InDenial

 

[parazhang]

Thx,now i have understand it,so i think: A managed device will creat a database in memory when running,depends on the MIB in it. Is it right?

 

[lgarner]

Managed devices don't maintain a database relating to SNMP. They check the value from its source each time a query is made. The source may be from a database, a performance counter, a text file, whatever.

Manager: "How long have you been running?"
Agent: "Let me check... Here it is: SNMPv2-MIB::sysUpTime.0 = Timeticks: (240830536) 27 days, 20:58:25.36"

And, when the next check occurs:
Manager: "How long have you been running?"
Agent: "Let me check... Here it is: SNMPv2-MIB::sysUpTime.0 = Timeticks: (240838136) 27 days, 20:59:41.36"

This conversation occurs with every poll by the management station.

 

[IMC]

a MIB only gives the definition of the objects that you can query in the managed device. It´s only a tree-like structure of the possible variables. Every vendor can get it own leaf in the tree and adds the specific variables of its devices under it. If you have a mananging station and wish to query some propietary value of, let say, a Cisco device you should add the corresponding MIB to the station (usually a text file) so you can browse all the possible objects of the device.

Juan Manuel Garcia Carral

 

[parazhang]

Thx all,and lgarner gived me a particular answer.
Now I want to know:
In your viewpoint of "SNMPv2-MIB::sysUpTime.0 = Timeticks: (240838136) 27 days 20:59:41.36", Where does the value(240838136) of the time reside? is the value in the managed device memory? or somewhere else? Will the value of the time change after I reboot the device?

 

[Castor66]

The value is a counter which is sotred on the computer (

http://www.freesoft.org/CIE/RFC/1902/36.htm).

sysUpTime will reset to 0 on reboot.

 

[lgarner]

In the case of sysUpTime, I believe that the startup time is stored by the managed device. When the uptime request is made, it is calculated as (current time - boot time).