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Deploying probes in your network
Chapter 5: Deploying Probes in Your Network 209
display. Distributed analysis is the only practical way to make different parts
of a switched or wireless network visible and therefore manageable. From a
single analyzer you can monitor and view traffic from anywhere on the network
where a probe has been deployed, from any type of media or topology (Ethernet,
wireless, and so on).
Monitoring half-duplex and full-duplex Ethernet links
If your IT department is typical, you have a limited budget. Therefore, before you
spend any money on analyzers, TAPs, and probes, you should assess what kinds
of traffic you need to see and what kinds of traffic you want to see for effective
network management. This allows you to deploy the correct technology needed
to meet your particular goals.
On wired networks with multiple switches, most of the stations are plugged
into half-duplex ports, even if the backbone or server connections are Gigabit
Ethernet or greater. Being able to see the traffic local to each switch at the edge
can give you insight unavailable from tapping the core connections. For example,
client-to-client communications are invisible from the backbone or server
connections. It can also be useful to isolate a segment when troubleshooting
client-to-core connection problems. The best way to achieve this kind of visibility
is to configure SPAN/mirror sessions on each switch, and then direct the SPAN/
mirror output to half-duplex probes.
A SPAN/mirror port duplicates the traffic on a switch port or a group of ports,
and sends the copied data to an analyzer. Using a SPAN/mirror port and half-
duplex probes are inexpensive and convenient, but cannot give you all the
visibility you need to manage and troubleshoot a network that also includes
gigabit, WAN, and wireless infrastructure. For networks that include these other
topologies, other solutions are needed.
Because full-duplex Ethernet lies at the core of most corporate networks,
ensuring completely transparent analyzer access to full-duplex Ethernet
traffic is critical. A SPAN/mirror port access is fine for the half-duplex Ethernet
connections to stations at the edge, but may be unable to keep up with the
higher-traffic full duplex links to the core.
There are three common ways for a probe or analyzer to gain access to full-
duplex streams of data flowing on Ethernet cables:
♦
Connect the probe to a SPAN/mirror port. A SPAN/mirror port can provide
a copy of all designated traffic on the switch in real time, assuming
bandwidth utilization is below 50% of full capacity.
♦
Deploy a port aggregator (sometimes called an “Aggregator TAP”) on
critical full-duplex links.
♦
Deploy a TAP (Test Access Port) on critical full-duplex links to capture
traffic. For some types of traffic such as full-duplex gigabit links, TAPs are
the only way to guarantee complete analysis, especially when traffic levels
are high.
Connecting a probe to a switch SPAN/mirror port or aggregator can provide
adequate visibility into most of the traffic local to the switch, assuming that
bandwidth utilization is low. However, if the aggregate switch traffic ever
exceeds 50% bandwidth saturation, SPAN/mirror ports and aggregators simply
cannot transmit the data fast enough to keep up; dropped packets (and perhaps
sluggish switch performance) will result. This is because SPAN/mirror ports and