Catalyst switch QoS is primarily based on the Layer 2 markings that are contained within a frame, i.e. the Class of Service (CoS) value. However, it can also be based on the Layer 3 markings contained within a packet, i.e. the IP Precedence or DSCP values. IP Precedence and DSCP were described in the previous section. The CoS value is contained in the VLAN field for 802.1Q and ISL-encapsulated frames.
IEEE 802.1Q inserts a 4 byte field into the original frame. The first 2 bytes are the Tag Protocol Identifier field, which is used to . . .
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