Spanning trees and IP routing only complicate the matter of trying to find out where to put your probe. Switched networks, by design, try to limit the traffic only to the ports involved with the communication in order to improve performance. If some of the packets you are looking for don't reach the network interface of the machine running Wireshark, you've got a problem. Monitoring: It's All About Location 9398s1.qrk In this example, I am capturing traffic on the PBX's only interface, eth0. Ensure that the correct interface is being used otherwise, you won't see the traffic. Launch Wireshark as root, and select Capture→Options to bring up the Capture Options dialog, as shown in Figure 1. Wireshark is used here to look at the traffic between the phone and the PBX. The first example is of a phone that's not working - every time a number is dialed, the phone idles, and no ringing is heard in the earpiece. The rest of this article focuses on using Wireshark's tools to solve three common VoIP problems. Fortunately, Wireshark provides some excellent tools to help interpret the data. This requires an understanding of both the underlying protocols and some telephony concepts. To find the cause of VoIP problems, you must be able to follow the flow of calls from start to finish and ensure that the correct events are happening. With this data in your hands, you can isolate the cause of VoIP problems. Wireshark uses the information from the call setup to better understand the voice flow. For example, the procedure to set up a call involves a different protocol than the voice traffic itself. VoIP involves a complex set of protocols that Wireshark can decode and relate to each other. Its job is to listen to network traffic, display it in a format that makes sense and then help you find problems. Wireshark, formerly known as Ethereal, is a network protocol analyzer. Fortunately, Wireshark is an open-source package that can do everything these tools can and more. After all, voice quality suffers if packets are late, missing or out of order. Vendors would have you believe you need to spend thousands of dollars on protocol analyzer software to troubleshoot Voice over IP (VoIP) networks.
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