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Cisco IOS Flexible NetFlow Overview

First Published: June 19th, 2006Last Updated: June 19th, 2006

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

Flexible NetFlow improves on original NetFlow by adding the capability to customize the traffic analysisparameters for your specific requirements. Flexible NetFlow makes it easier to create more complexconfigurations for traffic analysis and data export through the use of reusable configuration components.

This module provides an overview of Flexible NetFlow and the advanced Flexible NetFlow features andservices.

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Contents• Information About Flexible NetFlow, page 1

• Where to Go Next, page 12

• Additional References, page 12

Information About Flexible NetFlowThis section contains information about Flexible NetFlow.

• Typical Uses for NetFlow, page 2

• Benefits of Flexible NetFlow, page 3

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Cisco IOS Flexible NetFlow Overview Information About Flexible NetFlow

2Cisco IOS Flexible NetFlow Configuration Guide

• Flexible NetFlow Components, page 4

• Benefits of Flexible NetFlow Components, page 11

• Feature Comparison of Original NetFlow and Flexible NetFlow, page 11

Typical Uses for NetFlowNetFlow is typically used for several key customer applications, including the following:

• Network monitoring

NetFlow data enables extensive near-real-time network monitoring capabilities. Flow-basedanalysis techniques are utilized to visualize traffic patterns associated with individual routers andswitches and network-wide traffic patterns (providing aggregate traffic or application-based views)to provide proactive problem detection, efficient troubleshooting, and rapid problem resolution.

• Application monitoring and profiling

NetFlow data enables network managers to gain a detailed, time-based, view of application usageover the network. This information is used to plan, understand new services, and allocate networkand application resources (for example, Web server sizing and voice over IP (VoIP) deployment) toresponsively meet customer demands.

• User monitoring and profiling

NetFlow data enables network engineers to gain detailed understanding of customer/user utilizationof network and application resources. This information may then be used to efficiently plan andallocate access, backbone and application resources as well as to detect and resolve potentialsecurity and policy violations.

• Network planning

NetFlow can be used to capture data over a long period of time, affording the opportunity to trackand anticipate network growth and plan upgrades to increase the number of routing devices, ports,and higher-bandwidth interfaces. NetFlow services data optimizes network planning for peering,backbone upgrades, and routing policy. NetFlow helps to minimize the total cost of networkoperations while maximizing network performance, capacity, and reliability. NetFlow detectsunwanted WAN traffic, validates bandwidth and quality of service (QoS) and allows the analysis ofnew network applications. NetFlow will give you valuable information to reduce the cost ofoperating your network.

• Security analysis

NetFlow identifies and classifies distributed denial of service (DDoS) attacks, viruses and worms inreal-time. Changes in network behavior indicate anomalies that are clearly demonstrated in NetFlowdata. The data is also a valuable forensic tool to understand and replay the history of securityincidents.

• Accounting and billing

NetFlow data provides fine-grained metering (for instance, flow data includes details such as IPaddresses, packet and byte counts, time-stamps, type of service (ToS) and application ports) forhighly flexible and detailed resource utilization accounting. Service providers may use theinformation for billing based on time of day, bandwidth usage, application usage, quality of service,and so on. Enterprise customers may utilize the information for departmental charge-back or costallocation for resource utilization.



• NetFlow data warehousing and data mining

NetFlow data (or derived information) can be warehoused for later retrieval and analysis in supportof proactive marketing and customer service programs (for example, figuring out which applicationsand services are being used by internal and external users and targeting them for improved service,advertising, and so on). In addition, NetFlow data gives Market Researchers access to the “who,”“what,” “where,” and “how long” information relevant to enterprises and service providers.

Benefits of Flexible NetFlowNetFlow is now a standard method to perform billing and accounting, capacity planning, securitymonitoring, application/user profiling, and usage-based billing in IP networks.

Flow information allows Cisco customers to understand the details of network behavior and can be usedto classify and identify traffic and usage patterns in a network. Flexible Netflow is a new concept that isa superset of most IP accounting technologies available in Cisco IOS.

Traditionally NetFlow has used a fixed seven tupple of IP information to identify a flow and the newflexible concept allows the flow to be user defined. The immediate benefits from flexible flow include:

• High-capacity flow recognition, including scalability and aggregation of flow information.

• Enhance flow infrastructure for security monitoring and DDoS detection and identification.

• New information from packets to adapt flow information to a particular service or operation in thenetwork. The flow information available will be customizable by NetFlow users.

• Extensive use of Cisco’s flexible and extensible NetFlow Version 9 export format

• A comprehensive IP accounting feature that can be used to replace many accounting features suchas IP accounting, BGP Policy Accounting, and persistent caches.

NetFlow allows you to understand what the network is doing and to optimize network design and reduceoperational costs. Flexible NetFlow allows you to understand network behavior with more efficiency,with specific flow information tailored for various services used in the network. The following are someexample applications for a Flexible NetFlow feature.

• Flexible NetFlow enhances Cisco NetFlow as a security monitoring tool. For instance, new flowkeys can be defined for packet length or MAC address, allowing users to search for a specific typeof attack in the network.

• Flexible NetFlow allows you to quickly identify how much application traffic is being sent betweenhosts by specifically tracking TCP or UDP applications by the class of service (CoS) in the packets.

• The accounting of traffic entering a MPLS or IP core network and its destination for each next hopper class of service. This capability will allow the building of a edge to edge traffic matrix.

Security Detection with Flexible NetFlow

Flexible NetFlow is an excellent attack detection tool with capabilities to track all parts of the IPv4header and even packet sections and characterize this information into flows. It is expected that securitydetection systems will listen to NetFlow data and upon finding an issue in the network create a virtualbucket or virtual cache that will be configured to track specific information and pinpoint details aboutthe attack pattern or worm propagation. The capability to create caches on the fly with specificinformation combined with input filtering (for example, filtering all flows to a specific destination)allows Flexible NetFlow to be a better security detection tool than current flow technologies. It isexpected common attacks such as port scans for worm target discovery and worm propagation will betracked in Flexible NetFlow.



One common type of attack is when TCP flags are used to flood open TCP requests to a destination server(for example, a SYN flood attack). The attacking device will send a stream TCP SYN’s to a givendestination address but never send the ACK in response to the servers SYN-ACK as part of the TCP3-way handshake. The flow information needed for security monitor requires the tracking of three keyfields: destination address or subnet, TCP flags and packet count. The security device may be monitoringgeneral NetFlow information and this data may trigger a detailed view of this particular attack. Thedetailed flow monitor might include input filtering to limit what traffic is visible in the NetFlow cachealong with the tracking of the specific information to diagnose the TCP based attack. In this case the usermay want to filter all flow information to the server destination address or subnet to limit the amount ofinformation the security server needs to evaluate. If the security detection server decided it understoodthis attack it might then program another virtual cache or bucket to export payload information orsections of packets to take a deeper look at a signature within the packet. This example is just one ofmany possible examples of how Flexible NetFlow can be used to detect security incidents.

Typical Example for Deploying Flexible NetFlow

Figure 1 is an example of how Flexible NetFlow might be deployed in a network.

Figure 1 Typical Deployment for Flexible NetFlow

Flexible NetFlow ComponentsFlexible NetFlow consists of components that can be used together in several different variations toperform traffic analysis and data export:

• Records, page 5

• Benefits of Flexible NetFlow Predefined Records, page 5

• Benefits of User-Defined Records, page 6

• Flow Monitors, page 7

• Flow Exporters, page 9

• Flow Samplers, page 10



Records

Flexible NetFlow uses key and non-key fields just as original NetFlow does to create and populate flowsin a cache. In Flexible NetFlow a combination of key and non-key fields is called arecord. OriginalNetFlow and Flexible NetFlow both use the values in key fields in IP datagrams, such as the IP sourceor destination address and the source or destination transport protocol port, as the criteria fordetermining when a new flow must be created in the cache while network traffic is being monitored. Aflow is defined as a stream of packets between a given source and a given destination. New flows arecreated whenever NetFlow analyzes a packet that has a unique value in one of the key fields.

Figure 2 is an example of the process for inspecting packets and creating flow records in the cache. Inthis example, two unique flows are created in the cache because there are different values in the sourceand destination IP address key fields.

Figure 2 Packet Inspection

Benefits of Flexible NetFlow Predefined Records

Flexible NetFlow includes several predefined records that you can use right away to start monitoringtraffic in your network. The predefined records are available to help you quickly deploy FlexibleNetFlow and are easier to use than user defined flow records. You can choose from a list of alreadydefined records that may meet the needs for network monitoring. As Flexible NetFlow evolves popularuser-defined flow records will be made available as predefined records to make then easier to implement.

The predefined records ensure backward compatibility with your existing NetFlow Collectorconfigurations for the data that is exported. Each of the predefined records has a unique combination ofkey and non-keys fields that offer you the built-in ability to monitor various types of traffic in yournetwork without customizing Flexible Netflow on your router.

Two of the predefined records (netflow-original 1 andnetflow ipv4 original-output ) emulate original(ingress) NetFlow and the Egress NetFlow Accounting feature in original NetFlow, respectively. Someof the other predefined records are based on the aggregation caches available in original NetFlow. Thepredefined records that are based on the aggregation caches available in original NetFlow do not performaggregation. Instead each flow is tracked separately by the predefined records. If you want to learn more

1. Thenetflow-original andnetflow ipv4 original-input predefined records are functionally equivilent.



about the Flexible NetFlow predefined records, refer to the “Getting Started with Configuring Cisco IOSFlexible NetFlow” module or the “Configuring Cisco IOS Flexible NetFlow with Predefined Records”module.

Benefits of User-Defined Records

Flexible NetFlow enables you to define your own records by specifying the key and non-key fields tocustomize the data collection to your specific requirements.

Each packet that is forwarded within a router or switch is examined for a set of IP packet attributes.These attributes are the IP packet identity or key fields for the flow and determine if the packetinformation is unique or similar to other packets.

Traditionally, an IP Flow is based on a set of seven IP packet attributes. This set of key fields is trackedand if the set of values for these fields are unique a new flow record is created in the NetFlow cache.

All packets with the same source/destination IP address, source/destination ports, protocol, interface andclass of service are grouped into a flow and then packets and bytes tallied. This methodology of flowcharacterization or determining a flow is scalable because a large amount of network information iscondensed into a database of NetFlow information called the NetFlow cache. The NetFlow cache isassociated with a Flexible Flow Monitor capability that will be discussed in more detail within thisdocument.Figure 3 shows the basic components of NetFlow including the flow key fields, and theNetFlow cache.

The values in non-key fields are added to flows to provide additional information about the traffic in theflows. A change in the value of a non-key field does not create a new flow. In most cases the values fornon-key fields are taken from only the first packet in the flow. Flexible NetFlow enables you to capturecounter values such as the number of bytes and packets in a flow as non-key fields.

You can create your own custom records for applications such as QoS and bandwidth monitoring,application and end user traffic profiling, and security monitoring for denial of service (DoS) attacks.Flexible NetFlow also includes several predefined records that emulate original NetFlow.

Sections from Packets for Key and Non-Key Flow Fields

Flexible NetFlow records provide the capability to monitor a contiguous section of a packet of auser-configurable size, and use it in a flow record as a key or a non-key field along with other fields andattributes of the packet. The section may potentially include any Layer 3 data from the packet.

The packet section fields will allow the user to monitor any packet fields that are not covered by theFlexible NetFlow predefined keys. The ability to analyze packet fields that are not collected with thepredefined keys enables more detailed traffic monitoring, facilitates the investigation of distributeddenial of service (dDoS) attacks, and enables implementation of other security applications such as URLmonitoring.

Flexible NetFlow provides the following predefined types of packet sections of a user configurable size:

• ipv4 section header sizeheader-size—Starts capturing the number of bytes specified by theheader-size argument from the beginning of the IPv4 header of each packet.

• ipv4 section payload sizepayload-size—Starts capturing bytes immediately after the IPv4 headerfrom each packet. The number of bytes captured is specified by thepayload-size argument.

Theheader-sizeor payload-sizeis the size in bytes of the field in the flow record. If the correspondingfragment of the packet is smaller than the requested section size, Flexible NetFlow will fill the rest ofthe section field in the flow record with zeros. If the packet type does not match the requested sectiontype, Flexible NetFlow will fill the entire section field in the flow record with zeros.



Flexible NetFlow adds a new Version 9 export format field type for the IPv4 header and IPv4 packetsection types. Flexible NetFlow will communicate to the NetFlow collector the configured section sizesin the corresponding Version 9 export template fields.

The payload sections will have a corresponding length field that can be used to collect the actual size ofthe collected section.

Flow Monitors

Flow monitors are the Flexible NetFlow component that is applied to interfaces to perform networktraffic monitoring. Flow monitors consist of a record that you add to the flow monitor after you createthe flow monitor, and a cache that is automatically created at the time the flow monitor is applied to thefirst interface. Flow data is collected from the network traffic during the monitoring process based onthe key and non-key fields in the record which is configured for the flow monitor and stored in the flowmonitor cache.

Flexible NetFlow makes it very easy to perform different types of analysis on the same traffic. InFigure 3, packet 1 is analyzed using a record designed for standard traffic analysis on the input interfaceand a record designed for security analysis on the output interface.

Figure 3 Example of Using Two Flow Monitors to Analyze the Same Traffic



Figure 4 shows a more complex example of how you can apply different types of flow monitors withcustom records.

Figure 4 Complex Example of Using Multiple Types of Flow Monitors with Custom Records

There are three types of flow monitor caches. You change the type of cache used by the flow monitorafter you create the flow monitor. The three types of flow monitor caches are as follows:

• Normal, page 8

• Immediate, page 8

• Permanent, page 9

Normal

The default cache type is “normal”. In this mode, the entries in the cache are aged out according to the“timeout active” and “timeout inactive” settings. When a cache entry is aged out it is removed from thecache and exported via any exporters configured.

Immediate

A cache of type “immediate” ages out every record as soon as it is created. As a result every flowcontains just one packet. The commands that display the cache contents will provide a history of thepackets seen.

This mode is desirable when you expect only very small flows and you want a minimum amount oflatency between seeing a packet and exporting a report.

Caution This command may result in a large amount of export data that can overload low-speed links andoverwhelm any systems that you are exporting to. We recommended that you configure sampling toreduce the number of packets seen.

Note The timeout settings have no effect in this mode.



Permanent

A cache of type “permanent” never ages out any flows. This behavior is useful when the number of flowsyou expect to see is low and there is a need to keep long-term statistics on the router. For example, if theonly key field is IP ToS there is a limit of 256 different flows to be seen so to monitor the long term usageof the IP ToS field then a permanent cache can be used. Permanent caches are useful for billingapplications, and for an edge to edge traffic matrix for a fixed set of flows are being tracked. Updatemessages will be sent periodically to any Flow Exporters configured according to the “timeout update”setting.

Note When a cache becomes full in permanent mode, new flows will not be monitored. If this occurs, a “Flowsnot added” message will appear in the cache statistics.

Note A permanent cache uses update counters rather than delta counters. This means that when a flow isexported, the counters represent the totals seen for the full lifetime of the flow and not the additionalpackets and bytes seen since the last export was sent.

Flow Exporters

Flow exporters export the data in the flow monitor cache to a remote system, such as a server runningNetFlow collector, for analysis and storage. Flow exporters are created as separate entities in theconfiguration. Flow exporters are assigned to flow monitors to provide data export capability for the flowmonitors. You can create several flow exporters and assign them to one or more flow monitors to provideseveral export destinations. You can create one flow exporter and apply it to several flow monitors.

Flexible NetFlow Data Export Format Version 9

The basic output of Flexible NetFlow is a flow record. Several different formats for flow records haveevolved as NetFlow has matured. The most recent evolution of the NetFlow flow-record export formatis known as Version 9. The distinguishing feature of the NetFlow Version 9 export format is that it istemplate-based. Templates provide an extensible design to the record format, a feature that should allowfuture enhancements to NetFlow services without requiring concurrent changes to the basic flow-recordformat. Using templates provides several key benefits:

• Third-party business partners who produce applications that provide collector or display services forNetFlow do not have to recompile their applications each time a new NetFlow feature is added.Instead, they should be able to use an external data file that documents the known template formats.

• New features can be added to NetFlow quickly, without breaking current implementations.

• NetFlow is “future-proofed” against new or developing protocols because the Version 9 format canbe adapted to provide support for them.

The Version 9 export format consists of a packet header followed by one or more template flow or dataflow sets. A template flow set provides a description of the fields that will be present in future data flowsets. These data flow sets may occur later within the same export packet or in subsequent export packets.Template flow and data flow sets can be intermingled within a single export packet, as illustrated inFigure 5.

Figure 5 Version 9 Export Packet



NetFlow version 9 will periodically export the template data so the NetFlow collector will understandwhat data is to be sent and also export the data flow set for the template. The key advantage to FlexibleNetFlow is that the user configures a flow record, which is effectively converted to a version 9 templateand then forwarded to the collector.Figure 6 is a detailed example of the NetFlow Version 9 exportformat, including the header, template flow and data flow sets.

Note The NetFlow Version 5 export format is a fixed export format that would provide limited information forFlexible NetFlow data. This is why Flexible Netflow uses the Version 9 export format.

Figure 6 Detailed Example of the NetFlow Version 9 Export Format

For more information on the Version 9 export format, refer to the white paper entitledCisco IOS NetFlowVersion 9 Flow-Record Format white paper available at this url:http://www.cisco.com/en/US/tech/tk648/tk362/technologies_white_paper09186a00800a3db9.shtml.

Flow Samplers

Flow samplers are used to reduce the load that Flexible NetFlow places on the networking device tomonitor traffic by limiting the number of sampled packets that are analyzed. You configure a rate ofsampling that is 1 out of a range of 2 to 32768 packets. For example, a rate of 1 out of 2 results in analysisof 50 percent of the packets sampled. Flow samplers are applied to interfaces in conjunction with a flowmonitor to implement sampled Flexible NetFlow.

To enable flow sampling, you configure the record that you want to use for traffic analysis and assign itto a flow monitor. When you apply a flow monitor with a sampler to an interface, the sampled packetsare analyzed at the rate specified by the sampler and compared with the flow record associated with theflow monitor. If the analyzed packets meet the criteria specified by the flow record, they are added to theflow monitor cache.

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_white_paper09186a00800a3db9.shtml





Benefits of Flexible NetFlow ComponentsThe user-defined flow records and the component structure of Flexible NetFlow make it easy for you tocreate various configurations for traffic analysis and data export on a networking device with a minimumamount of configuration commands. Each flow monitor can have a unique combination of flow record,flow exporter, and cache type. If you change a parameter such as the destination IP address for a flowexporter, it is automatically changed for all the flow monitors that use the flow exporter. The same flowmonitor can be used in conjunction with different flow samplers to sample the same type of networktraffic at different rates on different interfaces.

Feature Comparison of Original NetFlow and Flexible NetFlowTable 1 provides a feature-by-feature comparison of original NetFlow and Flexible NetFlow.

Table 1 Feature-by-Feature Comparison of Original NetFlow and Flexible NetFlow

FeatureOriginalNetFlow

FlexibleNetFlow Comments

NetFlow DataCapture

Supported Supported Data-capture is available with the predefined1

and user-defined records in Flexible NetFlow.

NetFlow Data Export Supported Supported Flow exporters export data from the FlexibleNetFlow flow monitor caches to remotesystems

NetFlow for IPv6 Supported Not supported —

MPLS-AwareNetFlow

Supported Not supported —

MPLS EgressNetFlow


NetFlow BGP NextHop Support

Supported Supported Available in the predefined and user definedkeys in Flexible NetFlow records.

Random PacketSampled NetFlow

Supported Supported Available with Flexible NetFlow sampling.

NetFlow v9 ExportFormat

Supported Supported Available with Flexible NetFlow exporters.

NetFlow SubinterfaceSupport

Supported Supported Flexible NetFlow monitors can be assigned tosubinterfaces.

NetFlow MultipleExport Destinations

Supported Supported Available with Flexible NetFlow exporters.

NetFlow ToS-BasedRouter Aggregation

Supported Supported Available in the predefined and user-definedrecords in Flexible NetFlow records.

NetFlow MinimumPrefix Mask forRouter-BasedAggregation

Supported Supported Available in the predefined and user-definedrecords.

NetFlow Input Filters Supported Not supported —

NetFlow MIB Supported Not supported —

Cisco IOS Flexible NetFlow Overview Where to Go Next


Where to Go NextTo implement a basic Flexible NetFlow configuration that emulates original NetFlow traffic analysis anddata export, refer to the “Getting Started with Configuring Cisco IOS Flexible NetFlow”. To implementother Flexible NetFlow configurations, refer to the Flexible NetFlow configuration modules forCisco IOS, Release 12.4T. For access to other information for Flexible NetFlow refer to the“AdditionalReferences” section on page 12.

Additional ReferencesThe following sections provide references related to Flexible NetFlow.

Related Documents

NetFlow MIB andTop Talkers


NetFlow MulticastSupport

Supported Not supported Flexible NetFlow collects statistics formulticast flows. However, specific additionalfields such as replication counts for bytes(OBYTES) and packets (OPACKETS) are notsupported by Flexible Netflow in Cisco IOSRelease 12.4(9)T.

NetFlow Layer 2 andSecurity MonitoringExports


Egress NetFlowAccounting

Supported Supported Flexible NetFlow monitors can be used tomonitor egress traffic on interfaces andsubinterfaces.

NetFlow ReliableExport with SCTP


NetFlow DynamicTop Talkers CLI


1. Flexible NetFlow has several predefined keys that emulate data analysis with original NetFlow.

Table 1 Feature-by-Feature Comparison of Original NetFlow and Flexible NetFlow (continued)

FeatureOriginalNetFlow

FlexibleNetFlow Comments

Related Topic Document Title

Flexible NetFlow Feature Roadmap “Cisco IOS Flexible NetFlow Features Roadmap”

Emulating original NetFlow with Flexible NetFlow “Getting Started with Configuring Cisco IOS Flexible NetFlow”

Customizing Flexible NetFlow for your network “Customizing Cisco IOS Flexible NetFlow Flow Records and FlowMonitors for Your Network”

http://lbj/push_targets1/ucdit/cc/td/doc/product/software/ios124/124tcg/tfnf_c/chap05/index.htm

Cisco IOS Flexible NetFlow Overview Additional References


RFCs

Technical Assistance

Copyright © 2006 Cisco Systems, Inc. All rights reserved.This module first published June 19th, 2006. Last updated June 19th, 2006.

Configuring data export for Flexible NetFlow “Configuring Data Export for Cisco IOS Flexible NetFlow withFlow Exporters”

Configuring flow sampling to reduce the overhead ofmonitoring traffic with Flexible NetFlow

“Using Cisco IOS Flexible NetFlow Flow Sampling to Reduce theCPU Overhead of Analyzing Traffic”

Configuring Flexible NetFlow using predefinedrecords

“Configuring Cisco IOS Flexible NetFlow with Predefined Records”

RFC Title

RFC3954 Cisco Systems NetFlow Services Export Version 9

Description Link

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Cisco IOS Flexible NetFlow Overview Additional References
