Configuring flow export

Introduction

Correct configuration of Flow Exporting Devices (FED) is key to Flow Based Network Analysis. In most cases, errors made during configuration will result in missing or duplicate data. It may be difficult to spot that because of this the flow data is compromized. Device vendors are not making it easy and most of them use different methods to enable network flow exports to the collector (in our case TVF or TVA).

Furthermore, some collectors require a special setting on FED in order to undersand its flow records correctly.

This article is trying to decribe the idea behind network flow reporting and we hope it will help you to determine the correct procedure on how to confgure your FEDs.

For a complete overview of NetFlow or IPFIX, we recommend the reading of RFC 3917.

 

Solution

Definitions

FED

A Flow Enabled Device is any L3 and in some cases L2 device sending flow records to a collector.

Flow

A flow is defined as a stream of packets between a given source and a given destination. For example, in client-server computing a TCP sessions consists of two flows: one flow from client to server and the second flow from server to client.

Flow record

A flow record contains information about a specific flow that was metered at an observation point.  A flow record contains measured properties of the flow (e.g., the total number of bytes of all packets of the flow, flow duration and the physical or virtual interface where the packets entered and exited the FED) and usually characteristic properties of the flow (e.g., source/destination IP address, protocol, port, ToS/DiffServ marking, ...).

Sensor

A sensor is a meter located at an observation point. It will "observe" packets going by the observation point and "build" the flow record decribing the flow.  A sensor will meter packets in one direction only (ingress/in or egress/out) but some vendors use sensors that are bidirectional. Historically, a sensor was ingress/in only, so if left unspecified by a vendor, you may assume the sensor being unidirectional and ingress/in only.

 

Global configuration

The way to enable flow monitoring and flow exports will vary from vendor to vendor.  Basically you will have to configure these parameters:

Destination

IP address of the collector

Port

UDP port the collector is listening

Active flow timeout

Interval to send flow record updates for a long flow. This should match the smallest granularity of the database on the collector.  In case of TruView, this is one minute.

Inactive flow timeout

Interval of inactivity (no packets) that marks a flow as inactive. The recommended value is 15 seconds.

Flow record format

With the introduction of CISCO's NetFlow 10 and the standardization as IPFIX, the PDU of a flow record is no longer uniformely defined. The fields of the PDU should reflect the collector's capabilities. In case of TruView this is