- Bronze, 100 points or more
Hi every body!
My book says"The voice packets must be carried over a unique voice vlan known as the voice vlan id or vvid or over the regular data vlan known as native vlan or the port vlan id,pvid. the qos information from the voice packets also must be carried."
1) Is voice vlan or vlan 0 is created by default on all switches?
2)based on following from the above excerpt:
"or over the regular data vlan known as native vlan or the port vlan id,pvid. the qos information from the voice packets also must be carried."
1) Can we use other vlan say vlan 3 for voice packets? or as the book say voice vlan or native vlan only?
If we use native vlan for voice packets then cos information would be lost(assuming 802.1q). When switch receives the frame without cos value, it will assume cos=0 or switch change cos=0 value to some other value in accordance with configured policy on the switch. This will cause an overhead as switch has to do extra work compared to if voice packets were carried over regular vlan say vlan 0 , because switch's port would be configured to trust the cos values in frame from ip phone and thereby do not have to check against configured policy. Is my understanding correct?
thanks a lot!
If the switch port allows configuration the command switchport voice vlan dotlp, we know that it supports 802.1p standard.
I learn something new every day :-)
So for my benefit to summarise
1) Use non-native vlan for voice. This will have 802.1q tag and so can carry Cos value.
2) The dot1p option - this is the one i wasn't aware of. A vlan tag is added but the vlan id is 0, kind of like a placeholder, just so the vlan tag can carry CoS markings.
3) No tag at all and therefore no CoS.
Just to add to what has already been said. The native vlan by definition is untagged but you can change this by telling the switch to tag ALL vlans including the native vlan which would then allow the native vlan to carry CoS markings.
It's not that native vlan frames are not tagged.
PC1 - Vlan 20 connected to switch 1
PC2 - Vlan 20 connected to switch 2
On both switch 1 and 2 , native vlan on the trunk port connecting the switches are set to vlan 20.
If PC1 generates a frame to PC2 or vice versa, there is still dot1.q header associated with frame specifying the VLAN ID . A simple capture of frames on the trunk port on either switch 1 or switch 2 will reveal this.
i.e Even frames in native vlan are tagged with native vlan ID. It's just that frames appearing untagged on a trunk port are put into the native vlan configured.
Getting back to IP phones, IP phones do generate [voice] frames with 802.1q header, thus being able to set priority bits.
**correction to my previous post, which said native vlan means VID is not set. Even when frames are destined for the native vlan, the VID is set to the native vlan configured. When they say native vlan frames are not tagged, they are referring to untagged frames appearing at a for instance trunk port and are tagged with native VLAN configured on the trunk port.
starting from the following may help
SW-RM-SXR000-A-C-1(config-if)#switchport voice vlan ?
<1-4094> Vlan for voice traffic
dot1p Priority tagged on PVID
none Don't tell telephone about voice vlan
untagged Untagged on PVID
These are the possible options for voice vlan.
The best is to use a dedicated voice vlan with its own vlan-id an 802.1Q header with a non-zero vlan-id and 802.1p bits set to 5.
Then comes the dot1p option:
frames stay within the access vlan of the port as PC frames but an 802.1Q header is added the vlan-id is set to 0 to allow the switch to accept the frame, the 802.1p field has 5 value.
none : the switch doesn't tell anything about the voice vlan on CDP/LLDP messages out the port.
Note: for security reasons frames with arbitrary vlan-id in 802.1Q tags are discarded in modern switches (to avoid single vlan hopping attacks)
untagged: the switch says voicevlan = access vlan and instructs the phone to not use any form of tagging.
Clearly only the first two options can provide end-to-end QoS with the first to be preferred for different reasons: for example to avoid to have PCs and phones to compete for IP addresses on the same DHCP scope, for better security and so on
Hope to help
True native vlan means, VID (VLAN ID) in the tag field is not set. But user priority bits in the tag field is set, where COS is marked. Hence why I said two parts in the tag field VID and priority bits. And the IP phone is doing this.