• You should monitor your network and take actions with respect to situations like device and line failures.
• You should analyze line utilizations, errors on the line and be sure about network performance.
• You should be aware of who talks with whom? How much bandwidth is needed for every single application?
• And sometimes, you need to see exact data flow over the network.

If you have all these information ready, then people will think twice before they point finger at you. How can you achieve this?
We need a layered approach to understand network monitoring. I am not talking about network layers, but network monitoring layers. We have to involve deeply to monitoring layers before decide about network monitoring software needs. A simple summary could be like below.

• Preconditions of network monitoring.
• Up/Down monitoring
• Performance Monitoring / SNMP monitoring
• Who talks with whom? / Netflow monitoring
• Data capture / Data sniffing

Preconditions of Network Monitoring
Network documentation is essential to monitor a network. Trying to set up network monitoring tools before going through the documentation is complete waste of time. You will see everything green on the screen, but this maybe due to one of the redundant lines that are down. You will sit staring without knowing what is happening. Always remember, documentation comes first and everything follows.
Suggested monitoring tools: Powerpoint/Visio, NetViz

Up/Down monitoring
You have a map in which you can see some red and green lights glowing. Green means up and red means down. It is simple yet powerful. You will immediately come to know that there is some problem if the red light glows.
This is based on ping. Almost every IP devices support echo/echo reply. So, you can monitor all IP devices in your network by using ping. You go one step further by monitoring one application at a time present on a device instead of whole device. All network applications utilize TCP/UDP ports. You can monitor the applications by trying to access with telnet to its TCP/UDP ports. The port being open suggests that the application is running

Suggested monitoring tools: WhatsupGold, nmap

Performance monitoring / SNMP monitoring
The lines are up, the devices are up, but life is not perfect. People complain about performance of data lines. Are they saturated? Do we have package losses on the lines? Are routers running out of memory? We need SNMP to monitor heart beat of the network.

Suggested monitoring tools: MRTG, Solarwinds Orion, PRTG

Who talks with whom? / Netflow monitoring
You realized that the line is full. Someone / some applications make increase traffic load enormously. Who are they? Is it necessary traffic? In Cisco devices, by using “ip accounting” command we can get an idea of current traffic sources and destinations. Nevertheless, to analyze and to optimize the traffic we need flow monitoring. We need to know source and destination IP addresses and TCP/UDP ports and number of packages/bytes.
Everyone blames the network speed until you publish the network usage report that clearly shows only 15% of the traffic is ERP traffic and rest comes Internet access.
You should know that flow monitoring tools requires more server resources, since they collect enormous amount of data.

Suggested monitoring tools: Fluke Netflow monitor, Paasler

Data capture / RMON – Sniffer tools
Sometimes you need to observe the exact data flow on the line and not just information about it. Just have a look at this sample scenario. After you find out that the web service causes inappropriately high network traffic, the owner of the application just can say “No, we are not pushing this much of data to network. We just respond Yes or No in this web service and it is just 100 bytes”. Therefore, you should sniff the data flow on the line. Maybe, you will find that web service responds yes or no (100 bytes) and with the definition of web service (6 kilobytes).

Suggested monitoring tools: Wireshark

You can have a look at Network Monitoring Tools in Stanford University web site for a great list of network monitoring tools. You can find another tidy list at Network Traffic Monitoring in Alan Kennington’s topology.org.

All routers including the oldest (e.g. Cisco 2500 series) and smallest (e.g. Cisco 800 series) support NetFlow. Some functions does not exist in older IOS versions.
Catalyst 6500 series switches support NetFlow. Catalyst 4500 series switches support NetFlow with Supervisor IV/V + WS-F4531 Catalyst 4500 NetFlow Services Card.

Standalone or stackable switches do not support NetFlow. This means Catalyst 4948, Catalyst 3750 or Catalyst 3560 series switches do not support NetFlow. You can see the necessary commands on config mode, but they are not effective. It is not about IOS version or feature set. You need a modular switch for NetFlow.

Unfortunately, the answer of “What Cisco switches support netflow?” is only the modular switches.