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CHAPTER 3
Configuring J-Flow Statistics
Overview
Interface Sampling
Copyright © 2010, Juniper Networks, Inc.
This chapter describes how to configure J-Flow statistics on your ERX router; it contains
the following sections:
Overview on page 91
Platform Considerations on page 94
Before You Configure J-Flow Statistics on page 94
Configuring Flow-Based Statistics Collection on page 94
Monitoring J-Flow Statistics on page 101
The JunosE J-Flow feature provides a method by which you can collect IP traffic flow
statistics on your routing devices. J-Flow does not require any special protocol for
connection setup. It also does not require any external changes to networked traffic,
packets, or any other devices in the network. In other words, J-Flow is transparent to the
existing network, including end stations and application software and network devices
such as LAN switches.
The JunosE implementation of J-Flow allows you to export data to the UDP port of a
remote workstation for data collection and further processing. In addition, the ability to
enable J-Flow on an individual virtual router, interface, or subinterface allows you to
collect network statistics for specific locations within your network.
For any given IP interface, enabling J-Flow causes packets from the input stream to be
sampled at a globally configured rate. For each packet sampled, the main flow cache is
examined to see if there is an existing entry. If no entry exists, J-Flow creates a new entry
and records attributes of the flow. If the packet matches an existing entry, J-Flow updates
the existing flow.
In general, the system samples packets that it can forward. In other words, the system
does not sample packets that it discards. As sampling occurs, the system records flow
characteristics as they would appear for a packet that the virtual router transmits. This
means, for example, that if a packet uses the address of an output interface or next-hop
value altered by a policy setting, the system records the altered value in the flow record.
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