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I'm going over some practice exams, to get ready to take TSHOOT. and have a quick question.

The practice question was asking what the default native VLAN was on a Catalyst switch using IOS. Obviously the answer is VLAN1, but one of the other answers was VLAN 0. It went on to state that VLAN 0 is invalid and can never be used. Odd thing is, if you type "switchport voice vlan dot1p" to an interface connected to a cisco phone and a PC, it creates a trunk, and designates VLAN 0 for voice traffic, and sends the PC data as untagged packets to the native VLAN.

So, which one is correct? Is VLAN 0 never valid, or only in a specific instance, like dot1p?

Ignoring Cisco specific stuff, VLAN 0 is never valid because it is .1p rather than .1q. VLANs are .1q hence valid from 1-4094, 0 and 4095 are reserved values in 802.1Q

notfred wrote:

Ignoring Cisco specific stuff, VLAN 0 is never valid because it is .1p rather than .1q. VLANs are .1q hence valid from 1-4094, 0 and 4095 are reserved values in 802.1Q

AH! Ok, that makes sense. Much thanks!

I'm not really into networking with Cisco gear, but I think this might help: https://supportforums.cisco.com/discussion/11369936/what-vlan0

Techgoudy wrote:

I'm not really into networking with Cisco gear, but I think this might help: https://supportforums.cisco.com/discussion/11369936/what-vlan0

Good link! Thanks!

I've never heard those bits referred to as 'CoS bits' before, when someone says CoS, that to me means the IP layer QoS field, the COS, TOS, PREC, DSCP field depending on which set of standards you want to follow. The VLAN bits are always referred to as Priority bits or P-bits.

notfred wrote:

I've never heard those bits referred to as 'CoS bits' before, when someone says CoS, that to me means the IP layer QoS field, the COS, TOS, PREC, DSCP field depending on which set of standards you want to follow. The VLAN bits are always referred to as Priority bits or P-bits.

Here's how Wendell Odom explains it in the SWITCH courseware:

"IEEE 802.1Q—Each frame is tagged with a 12-bit VLAN ID and a User field. The
User field contains three 802.1p priority bits that indicate the frame CoS, a unitless
value ranging from 0 (lowest-priority delivery) to 7 (highest-priority delivery). Frames
from the native VLAN are not tagged (no VLAN ID or User field), so they receive a
default CoS that is configured on the receiving switch."

I think the dude in the linked thread conflated the two.

(I can't bag on the guy, I totally flaked that dot1p was different from dot1q...)