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How frequently should a device be polled?

by Juniper Employee on ‎02-01-2016 12:41 PM - edited on ‎10-09-2017 05:57 AM by Administrator Administrator (829 Views)

Question

How frequently should a device be polled?

Answer

It is difficult to give an absolute number for the rate of SNMP polls per second since the rate depends on the following two factors:

 

  • The number of variable bindings in a protocol data unit (PDU)
  • The response time for an interface from the Packet Forwarding Engine

In a normal scenario where no delay is being introduced by the Packet Forwarding Engine and there is one variable per PDU (a Get request), the response time is 130+ responses per second. However, with multiple variables in an SNMP request PDU (30 to 40 for GetBulk requests), the number of responses per second is much less. Because the Packet Forwarding Engine load can vary for each system, there is greater variation in how frequently a device should be polled.

 

Frequent polling of a large number of counters, especially statistics, can impact the device. We recommend the following optimization on the SNMP managers:

 

  • Use the row-by-row polling method, not the column-by-column method
  • Reduce the number of variable bindings per PDU
  • Increase timeout values in polling and discovery intervals
  • Reduce the incoming packet rate at the SNMP process (snmpd)

For better SNMP response on the device, Junos OS does the following:

 

  • Filters out duplicate SNMP requests
  • Excludes interfaces that are slow in response from SNMP queries

One way to determine a rate limit is to note an increase in the Currently Active count from the show snmp statistics extensive command.

 

The following is a sample output of the show snmp statistics extensive command:

 

user@host> show snmp statistics extensive

SNMP statistics:

Input:

   Packets: 226656, Bad versions: 0, Bad community names: 0,

   Bad community uses: 0, ASN parse errors: 0,

   Too bigs: 0, No such names: 0, Bad values: 0,

   Read onlys: 0, General errors: 0,

   Total request varbinds: 1967606, Total set varbinds: 0,

   Get requests: 18478, Get nexts: 75794, Set requests: 0,

   Get responses: 0, Traps: 0,

   Silent drops: 0, Proxy drops: 0, Commit pending drops: 0,

   Throttle drops: 27084, Duplicate request drops: 0

V3 Input:

   Unknown security models: 0, Invalid messages: 0

   Unknown pdu handlers: 0, Unavailable contexts: 0

   Unknown contexts: 0, Unsupported security levels: 0

   Not in time windows: 0, Unknown user names: 0

   Unknown engine ids: 0, Wrong digests: 0, Decryption errors: 0

Output:

   Packets: 226537, Too bigs: 0, No such names: 0,

   Bad values: 0, General errors: 0,

   Get requests: 0, Get nexts: 0, Set requests: 0,

   Get responses: 226155, Traps: 382

SA Control Blocks:

   Total: 222984, Currently Active: 501, Max Active: 501,

   Not found: 0, Timed Out: 0, Max Latency: 25

SA Registration:

   Registers: 0, Deregisters: 0, Removes: 0

Trap Queue Stats:

   Current queued: 0, Total queued: 0, Discards: 0, Overflows: 0

Trap Throttle Stats:

Current throttled: 0, Throttles needed: 0

Snmp Set Stats:

   Commit pending failures: 0, Config lock failures: 0

   Rpc failures: 0, Journal write failures: 0

   Mgd connect failures: 0, General commit failures: 0

 

For more information, click Understanding SNMP implementation in Junos OS.