After a bit of a break, I am back studying for a re-sit of the BCMSN exam. I am a little confused with Voice / Auxiliary VLANs where it comes to the dot1p option. The Cisco Exam Certification Guide states
"The default condition for every switch port is none, where a trunk is not used. All modes except
for none use the special-case 802.1Q trunk. The only difference between the dot1p and untagged
modes is the encapsulation of voice traffic. The dot1p mode puts the voice packets on VLAN 0,
which requires a VLAN ID (not the native VLAN) but doesn't require a unique voice VLAN to be
created. The untagged mode puts voice packets in the native VLAN, requiring neither a VLAN
ID nor a unique voice VLAN."
I thought the default VLAN was VLAN 1, where does VLAN 0 come from?
If someone can provide a description on how this works, or a pointer to some Cisco documentation that would be great.
The default vlan is 1. So no problem there.
When a port is configured to be a dot1q trunk, then multiple vlan's are supported but they all have a "tag" to identify which vlan the traffic belongs to. Except of course, the native vlan which we know, by default, is vlan 1. This has no 802.1q header extension in the ethernet frame and is said to be untagged.
You can change the native vlan to be something else- lets say we changed our switchport such that vlan 2 is the native vlan. Now vlan 2 traffic is untagged and vlan 1 traffic is tagged.
Now to your question- usually we want our voice traffic to be logically separate from our data traffic. Our data traffic might be in the native vlan (we know what the default is:-) and our voice traffic is usually in a separate vlan. But how about if we wanted our voice traffic to use the same (native) vlan as data?
Well, we need to make sure our voice traffic has priority over data for qos so it's not that simple. We have to consider the "other" part of the 802.1Q header- known as the dot1p field (aka COS). We issue the "switchport voice vlan dot1p" command inside the interface so the switch will mark voice traffic to COS 5 (data will be COS 0 therefore lower priority). But we still *have* to populate the other half of the 802.1Q header with something and can't simply send half the bits. Since we want this to use the native vlan we set the Vlan to 0- the switch will treat anything running on vlan 0 as being in the native vlan (untagged).
Hope that clarifies it.
Vik Malhi - CCIE #13890, CCSI #31584
Sr Technical Instructor - IPexpert, Inc.
A Cisco Learning Partner - We Accept Learning Credits!
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