Using Flexible NetFlow Flow Sampling

Last Updated: November 7, 2011

This document contains information about and instructions for configuring sampling to reduce the CPU overhead of analyzing traffic with Flexible NetFlow.

NetFlow is a Cisco IOS technology that provides statistics on packets flowing through a router. NetFlow is the standard for acquiring IP operational data from IP networks. NetFlow provides data to support network and security monitoring, network planning, traffic analysis, and IP accounting.

Flexible NetFlow improves on original NetFlow by adding the capability to customize the traffic analysis parameters for your specific requirements. Flexible NetFlow faciltates the creation of more complex configurations for traffic analysis and data export through the use of reusable configuration components.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Using Flow Sampling

  • You are familiar with the information in the " Cisco IOS Flexible NetFlow Overview " module.
  • The networking device must be running a Cisco IOS release that supports Flexible NetFlow.

IPv4 Traffic

  • The networking device must be configured for IPv4 routing.
  • One of the following must be enabled on your router and on any interfaces on which you want to enable Flexible NetFlow: Cisco Express Forwarding or distributed Cisco Express Forwarding.

IPv6 Traffic

  • The networking device must be configured for IPv6 routing.
  • One of the following must be enabled on your router and on any interfaces on which you want to enable Flexible NetFlow: Cisco Express Forwarding IPv6 or distributed Cisco Express Forwarding IPv6.

Restrictions for Using Flow Sampling

Cisco IOS Release 12.2(50)SY

  • Deterministic sampling is not supported.

Information About Flexible NetFlow Samplers

Flow Samplers

Flow samplers are created as separate components in a router's configuration. Flow samplers are used to reduce the load on the device that is running Flexible NetFlow by limiting the number of packets that are selected for analysis. Samplers use either random or deterministic sampling techniques (modes):

  • Deterministic--The same sampling position is used each time a sample is taken.
  • Random--A randomly selected sampling position is used each time a sample is taken.

Flow sampling exchanges monitoring accuracy for router performance. When you apply a sampler to a flow monitor, the overhead load on the router of running the flow monitor is reduced because the number of packets that the flow monitor must analyze is reduced. The reduction in the number of packets that are analyzed by the flow monitor causes a corresponding reduction in the accuracy of the information stored in the flow monitor's cache.

Samplers are combined with flow monitors when they are applied to an interface with the ip flow monitor command.

How to Configure Flexible NetFlow Flow Sampling

Flow sampling reduces the CPU overhead of analyzing traffic with Flexible NetFlow by reducing the number of packets that are analyzed.


Note
Only the keywords and arguments required for the Flexible NetFlow commands used in these tasks are explained in these tasks. For information about the other keywords and arguments available for these Flexible NetFlow commands, refer to the Cisco IOS Flexible NetFlow Command Reference .

Configuring a Flow Monitor

Samplers are applied to an interface in conjunction with a flow monitor. You must create a flow monitor to configure the types of traffic that you want to analyze before you can enable sampling. To configure a flow monitor, perform the following required task.

Each flow monitor has a separate cache assigned to it. Each flow monitor requires a record to define the contents and layout of its cache entries. The record format can be one of the predefined record formats, or an advanced user may create his or her own record format using the collect and matchcommands in Flexible NetFlow flow record configuration mode.


Note

You must use the no ip flow monitorcommand to remove a flow monitor from all of the interfaces to which you have applied it before you can modify the parameters for the record command on the flow monitor.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    flow monitor monitor-name

4.    description description

5.    record {record-name | netflow-original | netflow {ipv4 | ipv6} record [peer]}

6.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
flow monitor monitor-name


Example:

Router(config)# flow monitor FLOW-MONITOR-1

 

Creates a flow monitor and enters Flexible NetFlow flow monitor configuration mode.

  • This command also allows you to modify an existing flow monitor.
 
Step 4
description description


Example:

Router(config-flow-monitor)# description Used for basic traffic analysis

 

(Optional) Creates a description for the flow monitor.

 
Step 5
record {record-name | netflow-original | netflow {ipv4 | ipv6} record [peer]}


Example:

Router(config-flow-monitor)# record netflow ipv4 original-input

 

Specifies the record for the flow monitor.

 
Step 6
end


Example:

Router(config-flow-monitor)# end

 

Exits Flexible NetFlow flow monitor configuration mode and returns to privileged EXEC mode.

 

Configuring and Enabling Flow Sampling

To configure and enable a flow sampler, perform the following required task.


Note

When you specify the "NetFlow original," or the "NetFlow IPv4 original input," or the "NetFlow IPv6 original input" predefined record for the flow monitor to emulate original NetFlow, the flow monitor can be used only for analyzing input (ingress) traffic.

When you specify the "NetFlow IPv4 original output" or the "NetFlow IPv6 original output" predefined record for the flow monitor to emulate the Egress NetFlow Accounting feature, the flow monitor can be used only for analyzing output (egress) traffic.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    sampler sampler-name

4.    description description

5.    mode {deterministic | random} 1 out-of window-size

6.    exit

7.    interface type number

8.    {ip | ipv6} flow monitor monitor-name [[sampler] sampler-name] {input | output}

9.    end

10.    show sampler sampler-name


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
sampler sampler-name


Example:

Router(config)# sampler SAMPLER-1

 

Creates a sampler and enters sampler configuration mode.

  • This command also allows you to modify an existing sampler.
 
Step 4
description description


Example:

Router(config-sampler)# description Sample at 50%

 

(Optional) Creates a description for the flow sampler.

 
Step 5
mode {deterministic | random} 1 out-of window-size


Example:

Router(config-sampler)# mode random 1 out-of 2

 

Specifies the sampler mode and the flow sampler window size.

  • The range for the window-size argument is from 2 to 32,768.
 
Step 6
exit


Example:

Router(config-sampler)# exit

 

Exits sampler configuration mode and returns to global configuration mode.

 
Step 7
interface type number


Example:

Router(config)# interface ethernet 0/0

 

Specifies an interface and enters interface configuration mode.

 
Step 8
{ip | ipv6} flow monitor monitor-name [[sampler] sampler-name] {input | output}

Example:

Router(config-if)# ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-1 input

 

Assigns the flow monitor and the flow sampler that you created to the interface to enable sampling.

 
Step 9
end


Example:

Router(config-if)# end

 

Exits interface configuration mode and returns to privileged EXEC mode.

 
Step 10
show sampler sampler-name


Example:

Router# dhoe sampler SAMPLER-1

 

Displays the status and statistics of the flow sampler that you configured and enabled,.

 

Configuration Examples for Using Flexible NetFlow Flow Sampling

Example Configuring and Enabling a Deterministic Sampler for IPv4 Traffic

The following example shows how to configure and enable deterministic sampling for IPv4 output traffic.

This sample starts in global configuration mode:

!
flow monitor FLOW-MONITOR-1
 record netflow ipv4 original-output
 exit
!
sampler SAMPLER-1
 mode deterministic 1 out-of 2
 exit
!
ip cef
!
interface Ethernet 0/0
 ip address 172.16.6.2 255.255.255.0
 ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-1 output
!

The following example shows how to configure and enable deterministic sampling for IPv4 input traffic.

This sample starts in global configuration mode:

!
flow monitor FLOW-MONITOR-1
 record netflow ipv4 original-input
 exit
!
sampler SAMPLER-1
 mode deterministic 1 out-of 2
 exit
!
ip cef
!
interface Ethernet 0/0
 ip address 172.16.6.2 255.255.255.0
 ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-1 input
!

Example Configuring and Enabling a Deterministic Sampler for IPv6 Traffic

The following example shows how to configure and enable deterministic sampling for IPv6 output traffic.

This sample starts in global configuration mode:

!
flow monitor FLOW-MONITOR-2
 record netflow ipv6 original-output
 exit
!
sampler SAMPLER-1
 mode deterministic 1 out-of 2
 exit
!
ip cef
ipv6 cef
!
interface Ethernet 0/0
 ipv6 address 2001:DB8:2:ABCD::2/48
 ipv6 flow monitor FLOW-MONITOR-2 sampler SAMPLER-1 output
!

The following example shows how to configure and enable deterministic sampling for IPv6 input traffic.

This sample starts in global configuration mode:

!
flow monitor FLOW-MONITOR-2
 record netflow ipv6 original-input
 exit
!
sampler SAMPLER-1
 mode deterministic 1 out-of 2
 exit
!
ip cef
ipv6 cef
!
interface Ethernet 0/0
 ipv6 address 2001:DB8:2:ABCD::2/48
 ipv6 flow monitor FLOW-MONITOR-1 sampler SAMPLER-1 input
!

Example Adding a Sampler to a Flow Monitor When a Flow Monitor Is Already Enabled

The following example shows what happens when you try to add a sampler to a flow monitor that has already been enabled on an interface without a sampler:

Router(config)# interface Ethernet 
0/0
Router(config-if)# ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-2 input
% Flow Monitor: Flow Monitor 'FLOW-MONITOR-1' is already on in full mode and cannot be enabled with a sampler.

The following example shows how to remove the flow monitor from the interface so that it can be enabled with the sampler:

Router(config)# interface Ethernet 
0/0
Router(config-if)# no ip flow monitor FLOW-MONITOR-1 input
Router(config-if)# ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-2 input

Example Removing a Sampler from a Flow Monitor

The following example shows what happens when you try to remove a sampler from a flow monitor on an interface by entering the flow monitor command again without the sampler keyword and argument:

Router(config)# interface Ethernet 
0/0
Router(config-if)# ip flow monitor FLOW-MONITOR-1 input
% Flow Monitor: Flow Monitor 'FLOW-MONITOR-1' is already on in sampled mode and cannot be enabled in full mode.

The following example shows how to remove the flow monitor that was enabled with a sampler from the interface so that it can be enabled without the sampler:

Router(config)# interface Ethernet 
0/0
Router(config-if)# no ip flow monitor FLOW-MONITOR-1 sampler SAMPLER-2 input
Router(config-if)# ip flow monitor FLOW-MONITOR-1 input

Where to Go Next

For information on advanced Flexible NetFlow configurations for specific purposes such as quality of service (QoS) and bandwidth monitoring, application and user flow monitoring and profiling, and security analysis, refer to the "Customizing Cisco IOS Flexible NetFlow Flow Records and Flow Monitors" module.

If you want to configure any of the predefined records for Flexible NetFlow, refer to the "Configuring Cisco IOS Flexible NetFlow with Predefined Records" module.

If you want to configure data export for Flexible NetFlow, refer to the "Configuring Data Export for Cisco IOS Flexible NetFlow with Flow Exporters" module.

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Overview of Flexible NetFlow

"Cisco IOS Flexible NetFlow Overview"

Emulating original NetFlow with Flexible NetFlow

"Getting Started with Configuring Cisco IOS Flexible NetFlow"

Configuring flow exporters to export Flexible NetFlow data

"Configuring Data Export for Cisco IOS Flexible NetFlow with Flow Exporters"

Customizing Flexible NetFlow

"Customizing Cisco IOS Flexible NetFlow Flow Records and Flow Monitors"

Configuring Flexible NetFlow using predefined records

"Configuring Cisco IOS Flexible NetFlow with Predefined Records"

Using Flexible NetFlow Top N Talkers to analyze network traffic

"Using Cisco IOS Flexible NetFlow Top N Talkers to Analyze Network Traffic"

Configuring IPv4 multicast statistics support for Flexible NetFlow

"Configuring IPv4 Multicast Statistics Support for Cisco IOS Flexible NetFlow"

Configuration commands for Flexible NetFlow

Cisco IOS Flexible NetFlow Command Reference

Standards

Standard

Title

None

--

MIBs

MIB

MIBs Link

None

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

RFC 3954

Cisco Systems NetFlow Services Export Version 9

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Flexible NetFlow

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for Flexible NetFlow

Feature Name

Releases

Feature Information

Flexible NetFlow

12.2(33)SRC

12.2(50)SY

12.4(9)T

15.0(1)SY

Flexible NetFlow is introduced.

Support for this feature was added for Cisco 7200 series routers in Cisco IOS Release 12.2(33)SRC.

The following commands were introduced or modified: cache (Flexible NetFlow), clear flow exporter, clear flow monitor, clear sampler, collect counter, collect flow, collect interface, collect ipv4, collect ipv4 destination, collect ipv4 fragmentation, collect ipv4 section, collect ipv4 source, collect ipv4 total-length, collect ipv4 ttl, collect routing, collect timestamp sys-uptime, collect transport, collect transport icmp ipv4, collect transport tcp, collect transport udp, debug flow exporter, debug flow monitor, debug flow record, debug sampler, description (Flexible NetFlow), destination, dscp (Flexible NetFlow), exporter, flow exporter, flow monitor, flow record, ip flow monitor, match flow, match interface (Flexible NetFlow), match ipv4, match ipv4 destination, match ipv4 fragmentation, match ipv4 section, match ipv4 source, match ipv4 total-length, match ipv4 ttl, match routing, match transport, match transport icmp ipv4, match transport tcp, match transport udp, mode (Flexible NetFlow), option (Flexible NetFlow), record, sampler, show flow exporter, show flow interface, show flow monitor, show flow record, show sampler, source (Flexible NetFlow), statistics packet, template data timeout, transport (Flexible NetFlow).

Flexible NetFlow--Random Sampling

12.2(50)SY

12.4(20)T

15.0(1)SY

Flow samplers are created as separate components in a router's configuration. Flow samplers are used to reduce the load on the device that is running Flexible NetFlow by limiting the number of packets that are selected for analysis. Samplers use either random or deterministic sampling techniques (modes).

The following commands were introduced or modified: clear sampler, debug sampler, mode, record, sampler show sampler.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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