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Re: [nv-l] question on snmpCollect

To: nv-l@lists.us.ibm.com
Subject: Re: [nv-l] question on snmpCollect
From: James Shanks <jshanks@us.ibm.com>
Date: Tue, 27 Dec 2005 09:43:41 -0500
Delivery-date: Tue, 27 Dec 2005 14:44:12 +0000
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You are laboring under something of a misconception.  Your assumption of
how the process works (" it will check, somewhere, what are the instances
of the device and issue the snmpget against every instance")  is incorrect.
The actual process involves doing an snmpget for the first value in the
table, and an snmpnext for every subsequent value, until the agent responds
with data that is not part of what you asked for, which indicates that
there is no more data in the table originally specified.  The process is
most similar to the snmpwalk command.  You tell it where to start and it
keeps walking until the end.  The only difference is that snmpCollect stops
when the data returned is no longer relevant to what was specified in his

When you ask snmpCollect to collect all instances, he does not know in
advance whether there is data available or not, nor how many interfaces the
device has, nor even whether it will respond at all.  He does not first
query agent  on the device and ask how many interfaces are active.  He asks
the agent for just two things, it's sysUpTime, and  the first instance MIB
variable you specified, and he does SNMP getnext requests until the data
returned indicates that there is no more data to be had concerning that

As for whether there will be data for down interfaces, try using the MIB
Browser against the same device you want to collect from, for the same MIB
variable,  and you should easily see how this works.  If you are querying a
table and don't specify what instance of the table you want, then you will
see all that the agent cares to show you.   SNMP recognizes two kinds of
tables, sparse and "full" for want of a better term.  If it is a sparse
table, then you will get data only for those instances which have data;
while a "full" regular  table would probably contain zeros as placeholders
for nonexistent values.

Finally, the number of down interfaces does not impact snmpCollect.  What
impacts him is the size of the table you've asked him to get, how
frequently he has to get it,  and how quickly the remote agent responds
when queried.  Some agents put a low priority on SNMP requests if they are
busy with other things.  The thing to remember that snmpCollect is not
talking to individual interfaces.  He's talking to the SNMP agent on the
box, which keeps track of them.  Whether they are all up or all down, all
that matters to snmpCollect is that the agent responds.


James Shanks
Level 3 Support  for Tivoli NetView for UNIX and Windows
Tivoli Software / IBM Software Group

             Gang Cheng                                                    
             m>                                                         To 
             Sent by:                  nv-l@lists.us.ibm.com               
             owner-nv-l@lists.                                          cc 
                                       [nv-l] question on snmpCollect      
             12/27/2005 09:09                                              
             Please respond to                                             

Hi, list,

        Got a question on snmpCollect's behavior. When I configure through
xnmsnmpconf to collect data from
        all instances of a network device, what does snmpCollect do? I
assume it will check, somewhere, what
        are the instances of the device and issue the snmpget against every

        If the instance (interface) is already down, will snmpCollect
detect the down status and stop query
        that interface? Or it will do the snmpget regardless and return
with an error or defer the query to
        some later time.

        If the above assumption is true and if there are a lot of down
interfaces on a router, will it impact
        the performance of snmpCollect?


Best Regards

Geoffrey (Gang) Cheng
Team Leader, Tivoli Lab Based Services
IBM Certified Professional - ITS/ITA
Tivoli Certified Instructor/ITIL Manager Certified
IBM China Software Development Lab
Tel:   (86-10) 65391188 Ext 4159
Mobile:(86-0) 13911867597
Fax:   (86-10) 65391688
E-mail: gcheng@cn.ibm.com

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