Deciding where to place probes in your network
Knowing where you want visibility has an impact on the number and type of ports needed on your probe. It must be decided prior to purchasing so that the proper number of TAPs and SFPs are included in the package that is shipped to you.
To guarantee that every packet passing between every device on the network, errors and all, is available to your analyzer is practically impossible on a network with multiple switches. It would require placing a TAP on every link to each switch. Fortunately, you need only place probes where the traffic is significant enough to warrant the expense, and a lot of traffic is not that critical.
Ultimately, where to deploy probes depends on the design of your particular network and where you require visibility. A probe only shows your analyzer the data that is visible to that probe. The visibility of Ethernet Probe, for example, is limited to what a particular switch's SPAN/mirror port can deliver. A specialized hardware probe connected through a TAP sees only the traffic traversing that link. If 100% coverage is important to you, install TAPs on all the high-speed critical links in or near the core of your network, and probes plugged into the SPAN/mirror ports of switches on the edge.
For example, placing TAPs on the full-duplex links that connect servers or server farms to core switches will give you complete visibility into all traffic between servers and their clients. Connecting additional half-duplex probe appliances to SPAN/mirror ports at the edge of the network will let you focus in on any segment or station on the network for detailed problem resolution.
Failure to deploy the right probes in the right place can result in “blind spots” on your network, and an incomplete picture can lead to inefficient troubleshooting and expensive mistakes.
Some of the main things a probe can be used for include:
monitor server, link, and application performance
tweak or troubleshoot trunk performance
troubleshoot workstation connections
Figure 92 shows your options and what you gain or lose by placing probes at certain locations. Click to download a PDF of the diagram.
 
Figure 92: Probe placement options