02-08-2008 03:14 AM - edited 03-05-2019 09:02 PM
Hi all, quick question after reading up on a few topics, can you only add a priority tag to a frame that has vlan tagging in it?
02-08-2008 03:59 AM
Yes only the tagging has the priority COS field.
-amit singh
02-08-2008 06:35 AM
so does that mean if you want to prioritize frames, you need to make sure that the packet is tagged with a vlan ?
02-11-2008 03:49 AM
can anyone help on this ?
02-11-2008 03:53 AM
Yes ,tagging is needed..
LAN QoS uses ClassOfservice(COS) bits for qos classification and marking.These bits are send along with vlan tag information.
For dot1q,its the 3 bit priority field and for isl tag its 3 bit from user field..
arun
02-11-2008 05:08 AM
so for layer 2 cos, the frame must have a vlan membership that is not tagged? so am i right in saying we cannot prioritize the native vlan ?
02-11-2008 05:33 AM
It is possible to have a frame with a Null tag, i.e. it looks like it is on VLAN 0. It does this, just so that it can use the 3 bits for the CoS. The zero-VLAN means native, whatever that might be.
Cisco phones often use that technique. You can configure the port as an access port. The PC sends frames like it was on a normal access port. The telephone sends frames tagged with VLAN 0 and a CoS 5. These telephone frames are implicitly on the VLAN that you have configured on the port as the voice VLAN.
Kevin Dorrell
Luxembourg.
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