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What is Netflow?

What is Netflow? A Tutorial and Overview

Netflow, a protocol developed by Cisco, is used to collect and record all IP Traffic going to and from a Cisco router or switch that is Netflow enabled. It allows you to collect traffic and analyze it through a program (Usually called a Netflow Collector or Analyzer) which then organizes the flow records into a format that allows the IT administrator or Network engineer to further analyze the traffic (Source, destination, etc).

 

The protocol allows you to really drill down into your network traffic to see where the traffic source is coming from and to where it is destined too when troubleshooting slow LAN or WAN network connections. The protocol itself does not analyze the traffic, but as mentioned previous, when configured properly it sends traffic to a Collector or Analyzer, which is either a hardware device or more often than not, a software program.

 

Free Real-Time Netflow Analyzer download

 

Cisco originally developed the protocol for its products and soon after it has been the standard that many other manufacturers are implementing into their products as well, including Juniper (which has “JFlow”), 3Com/HP, Dell and Netgear (SFlow), Ericsson (RFlow), Huawei (NetStream) and Alcatel-Lucent (which uses CFlow).

Basics of the Protocol

Overview

Netflow is made up of a couple components:

NetFlow Cache (sometimes referred to as Data source or Flow Cache) – Stores the IP Flow information.
Netflow Export or Transport Mechanism – This sends data to the Collector to further data reporting and analyzing.

 

When a packet enters an interface that the router/switch hasn't seen before, it will decide whether or not to route the datagram, and if it forwards the datagram it will make an entry in the Flow Cache (in the router or switch) based on matching criteria in the packet.

 

Each packet within a Switch/Router that is forwarded is examined for a certain set of IP packet attributes that identify the packet and amongst others. ( You can think of this as a fingerprint of sorts.) An Ip Flow is made up of a Set of 5 attributes and can have up to 7 total.

 

The Attributes1 of each IP Packet are as follows:

  • IP source address
  • IP destination address
  • Source port
  • Destination port
  • Layer 3 protocol type
  • Class of Service
  • Router or switch interface

 

Packets the have the following attributes that are the SAME, are grouped into whats called a FLOW, which are then tallied: Source/Destination IP address, Source/Destination ports, Protocol interface and Class of Service.

Analyze Netflow Traffic using this Free Software

Determining a Flow is scalable because this data and information is organized into a Database of Netflow information called the Netflow Cache, or Flow Cache.

 

Netflow and Flow cache entry image
via Cisco.com

 

Flow Cache

The Flow Cache entry contains information about the Flow including the following:

  • Destination IP Addresses
  • Source IP Addresses
  • Destination Port Number
  • Source Port Number
  • Source interface,
  • Layer 3 Protocol Type,
  • ToS Byte – (means Type of Service Byte and takes into account the Precedence, Speed, Throughput and Reliability
  • Input Logical Interface (ifIndex) (The interface of the Router or Switch)

 

The packet is then routed out the destination interface. As the following packets that match an existing flow entry come into the router, the byte and packet counters keep increment through each additional data-gram until the connection between the host involved in the flow is torn down.

 

So packets that enter the router that don't have a matching flow entry are first determined to be routeable and if they're accepted, they're then forwarded after a flow cache entry is made. A Flow Cache can contain hundreds of thousands of entries, and in some cases, into the Millions of entries. When the flows expire, they're exported off to the Netflow Collector, which will constantly analyze and archive the flows for future reference. The Netflow Collector can then provide details on things like, the threats detected, the network topology, top interfaces and graph those trends. Netflow is used for finding bandwidth hogs, hunting down network threats, isolating application slowness issues and even for usage based billing by some ISP's.

 

Netflow version 9, which is now a IETF standard known as IP Information Export (IPFIX), is the new standard for transporting information from Switches and Routers to a Collector.  Many hardware vendors are now adopting IPFIX, which is the official standard for all flow technologies. Both Netflow and IPFIX can be performed in hardware or software, they can be used to export information in real-time, right down to the second, and they can be used for both flow and packet sampling, much like SFLOW.

Free Network Bandwidth Analyzer Software

History of Netflow

As mentioned previously, Netflow was conceived at Cisco Systems, and is now a major standard that is included in almost every business grade router and switch that Cisco and other manufacturers produced. Prior to Netflow, Network engineers and Administrators relied primarily on SNMP (Simple Network Management Protocol) to monitor network and internet traffic on their LAN and WAN connections.

 

SNMP is great for monitoring networked devices and capacity planning, but it lacks in more granular and in-depth traffic and bandwidth usage and utilization. Roland Dobbins, a network engineer in the Internet Services group at Cisco said, “We needed a more granular understanding of how Cisco bandwidth is used,”1. Soon after, Cisco introduced its Netflow Technology in 1996 which allowed for deeper insight, characterization and the ability to further analyze network traffic flows using a specialized Application-Specific Integrated Circuit (ASIC) coupled with features within the Cisco IOS Firmware and Catalyst OS Software.

 

By 2003, Cisco's Netflow Version 9 would be chosen to become a IETF (Internet Engineering Task Force which proposes Standards for the Internet, primarily TCP/IP) standard formally called IPFIX, or IP Flow Information Export.

 

For additional Reading on What Netflow is and How you can use it to monitor and analyze your Network, Check out these following Whitepapers for more in-depth reading.

Netflow Tips & Tricks

3 Easy Steps To Diagnosing Bandwidth Problems & Complaints

 

Network Monitoring made Easy!