Cisco Flexible Netflow configuration


Recently, a customer called me to configure Netflow on these routers because he just installed NetFlow Analyzer software from ManageEngine. This software is an “all in one” NetFlow collector, database, WebUI software, able to build pretty nice reports.

In my opinion, Netflow is one of the absolutely required software to have a good visibility when you operate a network.

NetFlow, Flexible NetFlow (FNF), IP Flow Information Export (IPFIX)

NetFlow is a Cisco technology available in Cisco IOS since 1996. It provides statistics on packets flowing through a router or a switch. NetFlow collect and export the data to enable network and security monitoring, network planning, traffic analysis, and IP accounting.

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

FNF is supported (at least) on the following Cisco platforms: ISR, ISR-G2, ISR4K, ASR1K, ASR9K, CRS-1, CSR1000v, Catalyst 3750X, 3560X, 3850, 4500 & 6500 SUP2T, Nexus 7K, Nexus 1000V.  It is available since IOS 12.4(9)T for some platforms, or IOS 15.0(1)M for others (from 12.2(50)SY on 6500/Sup2T). So since a very long time.

IP Flow Information Export (IPFIX) is the IETF standard version of NetFlow. There are changes in the terminology but this is the same principles like Cisco NetFlow version 9.

Flow Exporter, Record, Monitor and Activation.

FNF is divided into three distinct functions: the selection of data we want to analyse (flow record), the collection of the data (flow monitor), and the export of the data to a collector (flow export). To complete the configuration, a flow monitor can be applied to one or multiple interface(s).


Flow Record

Flow records are used to analyze traffic data for a specific purpose. A customized flow record must have at least one match criterion for use as the key field and typically at least one collect criterion for use as a non-key field. There are hundreds of possible permutations of customized flow records:

Router(config)# flow record my-record
Router(config-flow-record)# match   <-- Specify a Key Field
Router(config-flow-record)# collect <-- Specify a Non-Key Field

Router(config-flow-record)# match ?
 application    Application Fields
 datalink       Datalink(layer 2) fields
 flow           Flow identifying fields
 interface      Interface fields
 ipv4           IPv4 fields
 ipv6           IPv6 fields
 routing        routing attributes
 transport      Transport layer field

Router(config-flow-record)# collect ?
 application    Application Fields
 counter        Counter fields
 datalink       Datalink(layer 2) fields
 flow           Flow identifying fields
 interface      Interface fields
 ipv4           IPv4 fields
 ipv6           IPv6 fields
 routing        IPv4 routing attributes

Please refer to the Cisco IOS Flexible NetFlow Command Reference for the complete list of key and non-key fields.

Flow Export or Exporters

Flow exporters are created as separate components in a router’s configuration. Exporters are assigned to flow monitors to export the data from the flow monitor cache to a remote system such as a NetFlow collector or the Netflow Analyser of my customer. Flow monitors can support more than one exporter. Each exporter can be customized to meet the requirements of the flow monitor or monitors in which it is used and the NetFlow collector systems to which it is exporting data.

Flow Monitor

Each flow monitor has a separate cache assigned to it. Each flow monitor requires a flow record to define the contents and layout of its cache entries. These record formats can be one of the predefined formats or a user-defined format. In the flow monitor we also specify the exporter(s) we will use.

Activation per Interface

The start the show, we finally must activate the flow monitor on an Interface. This is the most simple configuration:

Router(config-if)# ip flow monitor my-monitor [input | output] [unicast | multicast]

Flexible NetFlow configuration example

Here is an example of configuration I used to export netflow datas to ManageEngine Netflow Analyser. This configuration was made on a 6500/SUP2T.

flow exporter EXPORTER
 description --- To NetFlow Analyser Software ---
 source Loopback0
 transport udp 9996
 template data timeout 60
flow record NETFLOW-RECORD
match ipv4 protocol
match ipv4 source address
match ipv4 destination address
match transport source-port
match transport destination-port
match interface input
collect routing source as
collect routing destination as
collect routing next-hop address ipv4
collect ipv4 source mask
collect ipv4 destination mask
collect transport tcp flags
collect counter bytes long
collect counter packets long
collect timestamp sys-uptime first
collect timestamp sys-uptime last
flow monitor NETFLOW-MONITOR
 exporter EXPORTER
 cache timeout active 60
 cache timeout inactive 15
int ten1/2/3
ip flow monitor NETFLOW-MONITOR input

Go further

This is one flexible NetFlow configuration example, but we can go much further.

Flexible NetFlow with NBAR Integration

We can, for example, add the power of NBAR2 into a FNF configuration:

router(config)# flow record app_record
router(config-flow-record)# match interface input
router(config-flow-record)# match ipv4 source address
router(config-flow-record)# match ipv4 destination address
router(config-flow-record)# match application name account-on-resolution
! match "application name" calls NBAR "account-on-resolution" (ASR1K).
router(config-flow-record)# collect counter packets
router(config-flow-record)# collect counter bytes

router(config)# flow exporter app_collector
router(config-flow-exporter)# destination <ip address>
router(config-flow-exporter)# option interface-table
router(config-flow-exporter)# option application-table
router(config-flow-exporter)# option application-attributes
router(config-flow-exporter)# option sub-application-attributes
! The last three lines export the full list of applications and the full list of attributes per app.

router(config)# flow monitor app_monitor
router(config-flow-monitor)# record app_record
router(config-flow-monitor)# exporter app_collector
router(config-flow-monitor)# cache timeout event transaction-end

Then, we can see the application on the netflow collector or directly into the cache of the router:

router# show flow mon app_mon cache

=============== =============== ===============        nbar edonkey        nbar ssh        nbar http        NBAR custom-app

In the output above: nbar (lowercase) = standard applications and NBAR (uppercase) = custom applications.

Top Talkers

This command is also very useful to see, for example, the top-10 IP addresses that are sending the most packets:

Router# show flow monitor <monitor> cache aggregate ipv4 source address sort highest counter bytes top 10

Or the top-5 destination addresses to which we are routing most traffic from the prefix

Router# show flow monitor <monitor> cache filter ipv4 destination address aggregate ipv4 destination address sort highest counter bytes top 5

Or the top-10 protocols:

Router#show flow monitor <monitor> cache aggregate ipv4 protocol sort highest counter bytes top 10

The applications are without limits!

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