in general u r right COS L2 and DSCP L3
marking at Layer 2 takes place in the 3-bit User Priority field called the Class of Service (CoS), which resides inside an Ethernet header. The CoS field only exists inside Ethernet frames when trunking (either 802.1Q or Inter-Switch Link [ISL]) is used. The field can be used to set eight different binary values, which the classification features of other QoS tools can use
Layer 2 switches perform QoS features based on the CoS field within the Ethernet headers without regard for the markings within the IP header
The two most popular marking fields for QoS are the IP Precedence and IP DSCP fields that were introduced in QoS tools frequently use these two fields in part because the IP packet header exists from endpoint to endpoint in a network
DSCPs use the same 3 bits as IP Precedence and combine these with the next 3 bits of the ToS byte to provide a 6-bit field for QoS marking. Thus, DSCP values range from 0 (000000) to 63 (111111). This range provides unprecedented richness in marking granularity.
DSCP values can be expressed in numeric form or by special keyword names, called per-hop behaviors (PHB). Three defined classes of DSCP PHBs exist: Best-Effort (BE or DSCP 0), Assured Forwarding (AFxy), and Expedited Forwarding (EF). In addition to these three defined PHBs, Class-Selector (CSx) codepoints have been defined to be backward compatible with IP Precedence (in other words, CS1 through CS7 are identical to IP Precedence values 1 through 7). The RFCs describing these PHBs are 2547, 2597, and 3246
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