VLAN challenge answer

The VLAN challenge is over and the results are in! I received 112 entries total, of which 60 had the correct answer: VLAN 2405. Congratulations to the submitters of the three randomly-selected correct answers! I'll be in contact with these individuals shortly to arrange shipment of their books.

• Josh Atterbury
• Istvan
• Dan Kirkland

Congratulations are due for everyone who came up with the correct answer, especially those who worked to arrive at it. Let's look at how the answer could be found...

Solution

The answer actually appears in the packet capture 29 times; once per STP BPDU. What's special about STP? Take a close look at the bridge priority:

Notice the bridge priority is advertised as 35173. However, referencing our configuration file we can see that no STP priority has been manually configured, so the switch should have the default priority of 32768. The difference in these numbers is an effect of IEEE 802.1t (enabled with the line spanning-tree extend system-id), which sets the last 12 bits of the 16-bit bridge priority to the VLAN identifier. So, we can deduce 35173 - 32768 = 2405.

Additionally, the UDLD packets offer a very obvious indication that the capture was taken from interface Fa0/6, though this doesn't contribute to the answer.

The majority of the incorrect answers I received suspected the capture was taken from VLAN 1, due to the inclusion of ISL-encapsulated DTP packets. This is an oddity of Cisco's DTP, and as a few readers pointed out, the 2960 doesn't even support ISL trunking. A few entries had the right idea about the bridge priority, but misinterpreted the bytes (taking the first two bytes of the MAC address as the priority), and unfortunately couldn't be counted.

So, if you got the answer, give yourself a pat on the back, and if not, don't sweat it. Now everyone knows!