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Figure 1. CoS Field
®
Cisco Systems
supports the Inter-Switch Link (ISL) VLAN tagging option on selected Cisco Catalyst 6500 line cards. ISL is a Cisco proprietary
VLAN tagging option that also supports an inbuilt 3-bit CoS field just like the IEEE 802.1Q option mentioned earlier.
Type of Service
Built into every IP packet is an IP header, and like in the Ethernet example earlier, the IP header also contains a field that defines a priority value
for this packet. This field is the type-of-service (ToS) field. Unlike the CoS field, the ToS field is an 8-bit field. There are two ways to set a priority
value in the ToS field. One method, called IP precedence, uses the 3 most significant bits of the ToS field to yield eight priority values.
Differentiated Services Code Point (DSCP) is a second method for assigning a priority to an IP packet. DSCP uses the 6 most significant bits of the
ToS field to yield 64 different priority values. Figure 2 shows where the ToS bits are found in the IP header.
Figure 2. Reading IP Precedence and DSCP from the ToS Byte
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