SNMP probe parameters

  • Release version: Xanadu
  • Updated August 1, 2024
  • 4 minutes to read
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    Summary of SNMP probe parameters

    SNMP probe parameters in ServiceNow enable precise configuration of SNMP data collection from network devices. These parameters control how SNMP queries are performed, including which OIDs to retrieve, timeout settings, request intervals, and the use of efficient SNMP operations like GETBULK and GETSCALAR. Proper configuration helps optimize performance, accuracy, and resource usage during network discovery and monitoring.

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    Key Parameters and Their Practical Use

    • oidspeclist: Defines the list of OIDs to query, supporting walking entire OIDs or specific tables with optional filters. Critical for targeting exact SNMP data points.
    • source: Specifies the IP address or hostname of the target SNMP device. Required to direct the probe correctly.
    • index: Used mainly for Cisco-style community string indexing (e.g., VLAN interrogation), defaulting to 0.
    • timeout and establishedsessiontimeout: Control how long the probe waits for responses, adjustable per probe to improve accuracy or adapt to device responsiveness.
    • requestinterval and requestdelay: Manage the timing between SNMP requests and responses to balance query speed and network load.
    • debug: Enables detailed logging for troubleshooting when set to true.
    • resultformat: Returns SNMP data in JSON format for specific probes to avoid memory issues; should not be altered.
    • usegetbulk: Enables efficient retrieval of tabular SNMP data using GETBULK requests instead of multiple GETNEXT requests. Improves performance for large tables but disables some timing parameters.
    • usegetscalar: Simplifies retrieval of scalar SNMP values, also overriding some timing parameters and enabling retries.
    • retries: Specifies additional attempts for GETBULK or GETNEXT requests to improve data completeness in case of intermittent device responsiveness.

    Why It Matters

    Configuring these parameters allows ServiceNow customers to tailor SNMP probes for their specific network environments, improving data accuracy and efficiency. Using GETBULK and GETSCALAR appropriately can reduce network load and speed up discovery processes. Proper timeout and retry settings ensure probes handle network delays and device responsiveness gracefully.

    What to Expect

    • Targeted and filtered SNMP data collection via customizable OID lists.
    • Improved probe reliability with timeout, retry, and interval tuning.
    • More efficient retrieval of large tables with GETBULK, reducing overhead.
    • Debug mode to assist with diagnosing SNMP probe issues.
    • JSON formatted results for specific probes to prevent sensor failures.

    This list of parameters may be passed to the SNMP probes.

    For instructions on configuring probe parameters, see Set probe parameters.

    Parameter Description Default Value
    oid_spec_list

    A list of OID specifications, one per line. Each specification must be in one of the following two forms:

    • walk {OID}: Walks the OID and all its children
    • table {OID} {OID Children}: Walks all entries in the table, returning only the given children (for example, "iso.org.dod.internet")

    {OID Children} refers to a comma-delimited list of child nodes within the entries for the given table. For example, "ifEntry.ifIndex,ifEntry.ifDescr,ifEntry.ifType" are OID children of the table "iso.org.dod.internet.mgmt.mib-2.interfaces.ifTable". As a convenience, the table entry prefix may be left off. (The preceding children could be specified as "ifIndex,ifDescr,ifType".)

    Any child may include a filter qualifier in parentheses. For example, the child "entPhysicalContainedIn(=0)" specifies returning table entries only if the value of "entPhysicalContainedIn" equals 0. The operators supported in the expression are:
    • = : equals
    • != : does not equal
    • # : contains

    If more than one child has a filter expression, a match on any one of the children causes that entry to be read.

    Any content including and after a "//" is ignored (comments). Any OID that does not start with "1.3.6.1" or "iso.org.dod.internet" automatically prefaces with "1.3.6.1." as a convenience.

    		 required
    source The IP address or host name of the device to query SNMP on. required
    index The index to apply after the community string, for Cisco-style community string indexing (for VLAN interrogation). 0
    credential_id The sys_id of a specific credential that is preferred for use above the rest. This parameter is for internal use only and is not supported. none
    credential_tag The credential tag that must be used. This parameter is for internal use only and is not supported. none
    timeout

    The timeout value (in milliseconds) to wait for a response, instead of the default. You can use this parameter to override the mid.snmp.request.timeout SNMP MID Server configuration parameter.

    Note:
    When use_getbulk is set to true, the timeout value is for an individual GETBULK request.
    		 1500
    established_session_timeout The interval (in milliseconds) to wait for a response after at least one response has been received. Longer values can be useful for collecting complete and accurate data. You can use this parameter to override the mid.snmp.session.timeout SNMP MID Server configuration parameter. 500
    debug Enables debug logging. Set to true for debug mode. false
    request_interval The interval (in milliseconds) between successive requests for an OID when a response has not been received, until the timeout (or established_session_timeout) value is reached. If this value is set to at least as long as the timeout (or established_session_timeout) value, then only a single request is sent for any particular OID. If you change the timeout (or established_session_timeout) value, adjust the request_interval at the same time to avoid sending too many or too few requests for the same OID, as appropriate for a given environment. 400
    request_delay The interval (in milliseconds) between the receipt of a response and the transmission of the next request. The default is 0 (no delay). This value may be set to slow the overall rate of an SNMP query. 0
    result_format Returns JSON formatted payloads for these probes:
    • SNMP - F5 BIG-IP - System
    • SNMP - Netscaler - System
    • SNMP - Routing
    This parameter returns data in a more compact format to prevent sensor failure or memory problems on a node when the payload becomes large. Do not change this value or delete this parameter.
    Warning:
    Use of this parameter with any other probes causes the sensor to fail.
    		 JSON
    use_getbulk
    Enables the use of SNMP GETBULK requests to retrieve tabular data from SNMP devices instead of using multiple SNMP GETNEXT requests. For tabular data, GETBULK is more efficient. Regardless of the request type, certain devices may not return any results when they are busy with other tasks. This parameter is used to configure at the probe level. GETBULK can also be set for an individual MID Server or globally for all MID servers. Settings are listed in the order of precedence:

    The established_session_timeout, request_interval, and request_delay parameters are ignored when use_getbulk is set to true. Instead, the retries parameter is available. The timeout configuration is the same one used by use_getscalar.

    By default, the following probes use GETBULK requests (the parameter value is true).
    • SNMP - Switch - Vlan
    • SNMP - Switch - BridgePortTable
    • SNMP - Switch - ForwardingTable
    • SNMP - Switch - SpanningTreeTable
    • SNMP - Network - ArpTable
    • SNMP - Layer 2 Protocol Caches
    • SNMP - F5 BIG IP - System (only for Service Mapping customers)
    Note:
    These probes have a timeout value of 5000.
    		 false
    use_getscalar Enables the use of simplified retrieval and handling of scalar values from SNMP devices.

    The established_session_timeout, request_interval, and request_delay parameters are ignored when use_getscalar is set to true. Instead, the retries and timeout parameters are available. The timeout configuration is the same one used by use_getbulk.

    		 false
    retries The number of additional attempts Discovery makes to complete an individual GETBULK request (see use_getbulk) or a GETNEXT request when the use_getscalar parameter is set to true. 2