For troubleshooting purposes, you might want to analyze packets flowing over a network. To capture packets (that is, store copies of packets on a local hard drive) for analysis, you could attach a network sniffer to a hub. Because a hub sends bits received on one port out all other ports, the attached network sniffer sees all traffic entering the hub.
Several standalone network sniffers are available. However, a low-cost way to perform packet capture and analysis is to use software such as wireshark, as shown in Figure 11_21.
A challenge arises, however, if you connect a network sniffer (for example, a laptop running the Wireshark software) to a switch port rather than to a hub port. Because a switch, by design, forwards frames out ports containing the frames' destination addresses, a network sniffer attached to one port would not see traffic destined for a device connected to a different port.
For example, in Figure 11-22, traffic enters a switch on port 1 and, based on the destination MAC address, exits via port 2. However, a network sniffer is connected to port 3 and is unable to see (and therefore capture) the traffic flowing between port 1 and 2.
Fortunately, some switches support a port mirroring feature, which makes a copy of traffic seen on one port and sends that duplicated traffic out another port (to which a network sniffer could be attached). As shown in Figure 11-23, this switch is configured to mirror traffic on port 2 to port 3, allowing a network sniffer to capture the packets that need to be analyzed. Depending on the switch, locally captured traffic could be forwarded to a remote destination for centralized analysis of that traffic.
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
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