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Cisco IOS Software Releases 12.0 S

Sampled NetFlow

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Sampled NetFlow

Table Of Contents

Sampled NetFlow

Feature Overview

Benefits

Restrictions

Supported Platforms

Supported Standards, MIBs, and RFCs

Prerequisites

Configuration Tasks

Configuring Sampled NetFlow Mode on an Interface

Configuring the Sampling Mode and the Sampling Interval

Monitoring and Maintaining Sampled NetFlow

Configuration Examples

Configuring Sampled NetFlow on an Interface Example

Configuring the Sampling Mode and Packet Interval Example

Command Reference

ip route-cache flow

ip flow-sampling-mode

show ip flow sampling

Debug Commands

debug ip flow sampling


Sampled NetFlow


This feature module describes the Sampled NetFlow feature. It includes information on the benefits of the feature, supported platforms, supported standards, and the commands necessary to configure the Sampled NetFlow feature.

Feature History

Release
Modification

12.0(11)S

This feature was introduced on the Cisco 12000 series Internet router.

12.0(14)ST

This feature was integrated into Cisco IOS Release 12.0(14)ST.

12.0(18)S

This feature was introduced on Cisco 12000 series Engine 2 Packet-over-SONET (POS) line cards.

12.0(21)S

This feature was introduced on Cisco 12000 series (Engine 2) 3-Port Gigabit Ethernet line cards.

12.0(24)S

This feature was introduced on Cisco 12000 series IP Service Engine (ISE) line cards as an output feature called Output Sampled NetFlow.

12.0(26)S

This feature was introduced on Cisco 12000 series 8-Port OC-3 STM-1 ATM line cards.


This document includes the following sections:

Feature Overview

Supported Platforms

Finding Support Information for Platforms and Cisco IOS Software Images

Prerequisites

Configuration Tasks

Configuration Examples

Command Reference

Debug Commands

Feature Overview

On an interface, Sampled NetFlow allows you to collect NetFlow statistics for a subset of incoming (ingress) IPv4 traffic on the interface, selecting only one out of "N" sequential packets, where "N" is a configurable parameter.

These sampling packets will substantially decrease the CPU utilization needed to account for NetFlow packets by allowing the majority of the packets to be switched faster because they will not need to go through additional NetFlow processing.

Benefits

The Sampled NetFlow feature was developed to alleviate the performance penalty incurred by turning on NetFlow on Cisco 12000 series Internet routers. In order to scale to higher forwarding rates, NetFlow will now allow the user to sample one out of every "x" IP packets being forwarded. These sample packets will be accounted for in the NetFlow cache on the router. The user can configure the "x" interval. This feature will substantially decrease the CPU utilization needed to account for NetFlow packets by allowing a majority of the packets to be switched faster because they will not need to go through additional NetFlow processing.

Restrictions

The Sampled NetFlow feature will be available only on the Cisco 12000 series Internet routers. The Cisco 12000 series Internet router has only one image, and no special packaging is necessary.

In IOS Release 12.0(24)S and later releases, the Sampled NetFlow feature is supported on Cisco 12000 series IP Service Engine (ISE) line cards as an output feature, called Output Sampled NetFlow, providing NetFlow statistics for outgoing traffic. The following restrictions apply to the implementation of Output Sampled NetFlow on Cisco 12000 series ISE line cards:

The input interface is reported as Null in all flows.

Starting in IOS Release 12.0(26)S, the input interface is reported as the lowest interface number on the ingress line card where a network flow arrived. For example, if the input subinterface of the current flow is POS3/2.10, Output Sampled Netflow will report POS3/0.

For more information about how to configure and use the Output Sampled NetFlow feature, refer to http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120limit/120s/120s24/12soutfl.htm.

Supported Platforms

This feature is supported on the following platforms:

Cisco 12000 series Engine 0 line cards

Cisco 12000 series Engine 1 line cards

Cisco 12000 series Engine 2 line cards

Cisco 12000 series IP Service Engine (ISE) line cards

Cisco 12000 series Engine 4 Plus (E4+) line cards

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Supported Standards, MIBs, and RFCs

Standards

None

MIBs

None

For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

RFCs

None

Prerequisites

The Sampled NetFlow feature must be enabled explicitly.

Configuration Tasks

See the following sections for configuration tasks for the Sampled NetFlow feature.

Configuring Sampled NetFlow Mode on an Interface

Configuring the Sampling Mode and the Sampling Interval

Configuring Sampled NetFlow Mode on an Interface

Command
Purpose

Router(config-if)# ip route-cache flow sampled

Enables NetFlow switching in the sampled mode.



Note When the Sampled NetFlow feature is disabled on an interface, normal NetFlow also becomes disabled. This restriction was made to prevent the interface from being overwhelmed by the sudden transition from sampled NetFlow to normal Netflow. You need to explicitly re-enable NetFlow if so desired. The default value for the sampling interval is 4 billion. This default packet interval was designed to protect the router from being choked by a misconfiguration. You need to explicitly configure a usable packet interval for your case.


Configuring the Sampling Mode and the Sampling Interval

To configure the sampling mode (that is, the type of sampling) and the sampling interval value, use the following command in global configuration mode:

Command
Purpose

Router(config)# ip flow-sampling-mode packet-interval value

Enables sampling mode and the sampling interval for NetFlow sampling.


Monitoring and Maintaining Sampled NetFlow

To monitor sampled NetFlow, use the following command in EXEC mode:

Command
Purpose

Router# show ip flow sampling

Displays the sampling mode and sampling interval settings.


Configuration Examples

This section provides the following configuration examples:

Configuring Sampled NetFlow on an Interface Example

Configuring the Sampling Mode and Packet Interval Example

Configuring Sampled NetFlow on an Interface Example

interface FastEthernet9/0/0
mac-address 4000.0209.0000
ip address 150.1.2.2 255.255.255.0
no ip directed-broadcast
ip route-cache flow sampled
ip route-cache distributed
no ip mroute-cache
no keepalive
!\

Configuring the Sampling Mode and Packet Interval Example

ip classless
ip route 20.1.1.1 255.255.255.255 Null0
ip route 171.69.1.129 255.255.255.255 10.0.101.1
ip flow-export version 5
ip flow-export destination 10.42.42.1 9991
ip flow-sampling-mode packet-interval 100
!

Command Reference

This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.

The following is a modified Cisco IOS command:

ip route-cache flow

The following is a list of new Cisco IOS commands:

ip flow-sampling-mode

show ip flow sampling

ip route-cache flow

To enable NetFlow switching for IP routing, use the ip route-cache flow command in interface configuration mode. To disable NetFlow switching, use the no form of this command.

ip route-cache flow [sampled [input | output]]

no ip route-cache flow [sampled [input | output]]

Syntax Description

sampled

(Optional) Enables NetFlow cache in sampled mode.

input

(Optional) Enables NetFlow sampling for incoming IPv4 traffic on a Cisco 12000 series ISE interface.

output

(Optional) Enables NetFlow sampling for outgoing IPv4 traffic on a Cisco 12000 series ISE interface.


Defaults

This command is not enabled by default.

Command Modes

Interface configuration

Command History

Release
Modification

11.1

This command was introduced.

12.0(11)S

The sampled keyword was added.

12.0(24)S

The input and output keywords were added for use on Cisco 12000 series ISE line cards.


Usage Guidelines

NetFlow is an accounting and acceleration mechanism that captures a rich set of traffic statistics. These traffic statistics include user, protocol, port, and type of service information that can be used for a wide variety of purposes such as network analysis and planning, accounting, and billing. To export NetFlow data, use the ip flow-export global configuration command.


Caution Entering the ip flow-export or no ip flow-export command on the Cisco 12000 Series Internet Routers and specifying any version format other than version 1 (in other words, entering the ip flow-export or no ip flow-export command and specifying either the version 5 or version 9 keyword) causes packet forwarding to stop for a few seconds while NetFlow reloads the route processor and line card CEF tables. To avoid interruption of service to a live network, apply this command during a change window, or include it in the startup-config file to be executed during a router reboot.

NetFlow is supported on IP and IP encapsulated traffic over all interface types and encapsulations except for Inter-Switch Link/VLAN, ATM, and Frame Relay interfaces when more than one input access control list is used on the interface, and ATM local area network emulation (LANE).

In conventional switching at the network layer, each incoming packet is handled on an individual basis with a series of functions to perform access list checks, capture accounting data, and switch the packet. With NetFlow, after a flow has been identified and access list processing of the first packet in the flow has been performed, all subsequent packets are handled on a "connection-oriented" basis as part of the flow, where access list checks are bypassed and statistics capture are performed in tandem.

A network flow is identified as a unidirectional stream of packets between a source and destination—both defined by a network-layer IP address and transport-layer port number. Specifically, a flow is identified as the combination of the following fields:

Source IP address

Destination IP address

Source port number

Destination port number

Protocol type

Type of service

Input interface

NetFlow operates by creating a flow cache that contains the information needed to perform access list check for all active flows. The NetFlow cache is built by processing the first packet of a flow through the standard fast switching path. As a result, each flow is associated with an incoming and outgoing interface port number and with a specific security access permission and encryption policy. The cache also includes entries for traffic statistics that are updated in tandem with the switching of subsequent packets. After the NetFlow cache is created, packets identified as belonging to an existing flow have their traffic statistic counters increased and security access list checks bypassed. Flow information is maintained within the NetFlow cache for all active flows.

NetFlow is not one of the available switching modes. When you configure NetFlow on an interface, you must have some other switching method to actually switch the packet. Also, with NetFlow you can export data (traffic statistics) to a remote workstation for further processing.

NetFlow accounting is based on identifying packet flows and maintaining statistics and access list processing within a router. It does not involve any connection-setup protocol either between routers or to any other networking device or end station and does not require any change externally—either to the traffic or packets themselves or to any other networking device. Thus, NetFlow is completely transparent to the existing network, including end stations and application software and network devices like LAN switches. Also, because NetFlow is performed independently on each internetworking device, it does not need to be operational on each router in the network. Network planners can selectively invoke NetFlow accounting (and NetFlow data export) on a router/interface basis to gain traffic performance, control, or accounting benefits in specific network locations.


Note When sampled NetFlow is disabled on an interface, normal NetFlow also becomes disabled. This restriction was made to prevent the interface from being overwhelmed by the sudden transition from sampled NetFlow to normal NetFlow. You need to explicitly re-enable NetFlow if so desired. The default value for the sampling interval is 4 billion. This default packet interval was designed to protect the router from being choked by a misconfiguration. You need to explicitly configure a usable packet interval for your case.



Note NetFlow does consume additional memory and CPU resources in comparison with other switching modes; therefore, it is important to understand the resources required on your router before enabling NetFlow.


Starting in IOS Release 12.0(24)S, Sampled NetFlow is supported on Cisco 12000 series ISE line cards as an output feature, called Output Sampled NetFlow, providing NetFlow statistics for outgoing traffic. Output Sampled NetFlow uses the output interface on an ISE line card as a key flow field instead of the input interface. The input interface is reported as Null in all flows. Starting in IOS Release 12.0(26)S, the input interface is reported as the lowest interface of the ingress line card on which a flow arrives. For example, if the current flow arrives on ISE input subinterface POS3/2.10, it is reported as POS3/0.

To enable Output Sampled NetFlow on a Cisco 12000 series ISE interface, use the ip route-cache flow sampled [input | output] command in interface configuration mode.

Examples

The following example enables NetFlow switching on the interface:

interface ethernet 0/5/0
 ip address 17.252.245.2 255.255.255.0
 ip route-cache flow

Related Commands

Command
Description

ip flow-export

Enables the exporting of information in NetFlow cache entries.


ip flow-sampling-mode

To enable sampling mode in the NetFlow cache, use the ip flow-sampling-mode global configuration command. To disable sampling mode, use the no form of this command.

ip flow-sampling-mode packet-interval value

no ip flow-sampling-mode packet-interval

Syntax Description

packet-interval

Samples a single packet out of a user-defined interval.

value

Number of intervals to the next sampled packet in the NetFlow cache. The valid range is from 10 to 16382. The default is 4 billion.


Defaults

This command is not enabled by default.

Command Modes

Global configuration

Command History

Release
Modification

12.0(11)S

This command was introduced.


Examples

The following example shows how to enable sampling mode and to set the sampling interval to 100:

ip flow-sampling-mode packet-interval 100

show ip flow sampling

To display the settings of the sampling type and sampling interval, use the show ip flow sampling EXEC command.

show ip flow sampling

Syntax Description

This command has no keywords and arguments.

Defaults

No default behavior or values.

Command Modes

EXEC

Command History

Release
Modification

12.0(11)S

This command was introduced.


Examples

The following example displays NetFlow sampling by using the show ip flow sampling command:

show ip flow sampling

Flow sampling is enabled
'Packet Interval' sampling mode is configured.
1 out of every 100 packets is being sampled.

Debug Commands

This section documents the new debug ip flow sampling command. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.

debug ip flow sampling

To help debug sampling-related activities, use the debug ip flow sampling command in EXEC mode.

debug ip flow sampling

Syntax Description

This command has no keywords and arguments.

Defaults

No default behavior or values.

Command Modes

EXEC

Command History

Release
Modification

12.0(11)S

This command was introduced.


Examples

The following example displays NetFlow sampling by using the debug ip flow sampling command:

debug ip flow sampling


The following example displays a configuration of a sampled mode and sampling interval:

ip flow-sampling-mode packet-interval 100
debug ip flow sampling

01:58:18: Flow sampling is enabled
01:58:18:   Sampling parameters: mode = 'Packet Interval', interval = 100
01:58:18: IPFLOW: Sending sampling config message to all LC

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