As was seen in the previous article <Understanding Packet User Priority>, the IP header incorporates a Differentiated Services Code Point (DSCP) field which provides the packet priority.
Another level of priority was introduced in the 802.1p standard which featured a 3 bit priority field in the Ethernet header. The 802.1p standard was merged into the 802.1Q standard and the 3 bit priority field became the 3 bit Priority Code Point (PCP) field in the 802.1Q tag. The 3 bits priority is also termed as Class of Service (CoS).
Since the PCP field is 3 bits in length – it provides 8 levels of packet priority. These 3 bits of packet priority can be directly co-related to the 3 bits of the DSCP field which also provide priority in the IP Header.
As an example – if the 802.1Q PCP value is 6 – then the corresponding DSCP values for the same will be 48-55 in decimal. Expanding the above statement with an example
- 802.1Q tag 6 (in decimal) – 110
- DSCP value – | 110 | 000 | — 48 and | 110 | 001 | — 49 and so on
The 802.1Q PCP field and the Ethernet frame format is shown below
Fig Courtesy: Wikipedia