Different qos fields in IP packets, mpls packets, and vlan packets.

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When implementing the QoS, we will do that with the help of the diffrent qos field in different packets.


In IP packets, we use the ToS field to adjust the QoS, the first three bits of the ToS represents the IP precedence, the value range is from 0 to 7, the value is bigger, the priority is higher. Now, RFC has redifined the ToS field, the new is call DSCP, the first three bits is called CSCP(class selector code point), and it is compatible with the previous IP preference.


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In the case of the VLAN packets, the packet priority is defined by the PRI(PCP) field in 802.1q tag. We should notice that the vlan tag is defined by the 802.1q, while the PCP is not, PCP is defined in 802.1p.


Like the IP precedence, the 3 bits PRI defines 8 priority levels, from 0 to 7, the vlaue is bigger, the priority is higher.


111556ptovvzvy0t67s0ts.png


For MPLS packets, the priority is defined by the EXP field. the EXP field also contains 3 bits, defines the priority from 0 to 7, with the value is bigger, the priority is higher.


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