Well, none of the above answers talk about QOS,

L2-QOS:
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It is clear that L2 is at MAC layer. In case of MAC level 
forwarding such as Bridge connction and a quality of 
service need to be assured, The following methods can be 
used:

1) MAC filtering (avoiding unnecessary traffic coming from 
known MAC sources)
2) VLAN & COS: VLAN will ensure that the traffic is 
classified based on various parameters like MAC address, 
incoming port, etc... COS: Class Of Service is a filed in 
VLAN header. This will be used to prioritize traffic. Later 
a QOS scheduler can use the COS filed to qualify the 
traffic in to different QOS queues.

L3 - QOS
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L3 QOS is required for IP level classification. 
Classification can be achieved by Diffserv implementations 
that takes many IP parameters in to consideration in 
prioritizing traffic. This priority can be set in TOS field 
of IP header. This TOS will later be used by Scheduling 
process to achieve QOS.