A、The access layer is the initial point at which traffic enters the network. Traffic is marked （orremarked） at Layers 2 and 3 by the access switch as it enters the network， or is "trusted" that it isentering the network with the appropriate tag

B、No traffic marking occurs at the core layer. Layer 2/3 QoS tags are trusted from distributionlayer switches and used to prioritize and queue the traffic as it traverses the core

C、Traffic inbound from the access layer to the distribution layer can be trusted or reset dependingupon the ability of the access layer switches. Priority access into the core is provided based onLayer 3 QoS tags

D、IP precedence， DSCP， QoS group， IP address， and ingress interface are Layer 2 characteristics that are set by the access layer as it passes traffic to the distribution layer. Thedistribution layer， once it has made a switching decision to the core layer， strips these off

E、MAC address， Multiprotocol Label Switching （MPLS）； the ATM cell loss priority （CLP） bit， theFrame Relay discard eligible （DE） bit， and ingress interface are established by the voicesubmodule （distribution layer） as traffic passes to the core layer

F、The distribution layer inspects a frame to see if it has exceeded a predefined rate of trafficwithin a certain time frame， which is typically a fixed number internal to the switch. If a frame isdetermined to be in excess of the predefined rate limit， the CoS value can be marked up in a waythat results in the packet being dropped