QoS CoS vs DSCP
CoS and DSCP are two important parameters that user used when implementing QoS.
Class of Service or CoS is a value that exists within the VLAN tag at Layer 2 and is used to prioritize frames that traverse a trunk. Particular frames of specific VLANs are given priorities within that VLAN 802.1Q tag.
A CoS value exists in a frame as long as there is a VLAN tag. Once the tag is removed, the CoS value is no longer associated with the frame.
CoS uses 3 bits and can thus have values between 0 and 7 where 0 is the lowest priority and 7 is the highest. The IEEE broadly defines these values according to the 802.1Q standard and uses the following terms:
|4||VI||Video, < 100 ms latency and jitter|
|5||VO||Voice, < 10 ms latency and jitter|
Differentiated Services Code Point or DSCP operates at Layer 3, and is part of the DS field in the IP header. The DS field contains 8 bits and can thus have values between 0 and 255. The values are not simply used in this range, but depending on how they are interpreted, allow you to deliver a more granular and customized prioritization of packets.
In addition, unlike CoS which exists only on a particular trunk link, DSCP is something that remains within the packet from end to end, so an IP packet can be prioritized accordingly throughout its journey.
Keep in mind that DSCP values can be changed by network devices along the way, so even if you set these values when you send out the packet, if it traverses a network that you do not administer, those values can be changed.