Tuesday, August 30, 2011

The cable view

The last post was about logging and how it helps you to find out what is going on in times of trouble. But that is not the only tool that the snom m9 offers. There is another tool built in, it is called the “PCAP trace”.

PCAP means “packet capture” (see http://en.wikipedia.org/wiki/Pcap) and it is a format that several tools understand for the display of what happened on the cable (whatever that is). For example a popular tool is “Wireshark”, which you can easily run on the operating system of your choice. You can see all protocols, IPv4 or IPv6, LLDP, ARP, SIP, RTP, HTTP, and whatever might be in your network.

Usually, when you want to see what is going on in the network level, you need to use either an Ethernet hub (if you can still find one) or set up a port monitoring on the Ethernet switch. Then you can record the packets on the Ethernet interface of a PC. This method a “true” picture of what is going on; however it is a lot of work if you quickly want to see what is going on. And you need physical access to the device, and in cases when you are using things like 802.1X things might get really tricky.

The snom 300 series already had the PCAP trace possibility for years and it proved to be very useful. Instead of connecting an external device, the phone just records the traffic from the device and stores it internally. Then you can download it from the web interface. There is a limit on how much you can record, but usually a few hundred KB are not a problem and this usually helps a lot to find out what is going on.

The snom m9 does essentially the same thing; however it adds the possibility to start capturing right after the boot. This makes it possible to see for example the DHCP traffic or 802.1X in the beginning. In the case of the desktop phones, you really needed to connect physically to the device to see this traffic. On the m9, you can just set the switch and then the software will automatically turn the recording on in the beginning.

Because of the dual IPv4/IPv6 stack on the m9, it is now possible to “debug” DHCP. For example, if you have problems with DHCPv4, you can set the flag to start recording during bootup, then reboot, and log into the device using the IPv6 local link address and download the PCAP trace. For development, I can tell you guys, this was a killer feature. It was never so easy to see what is going on during the boot phase!

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