Specifications

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assigned to the IPv4 Packet. DSCP (also referred to as DiffServ) is defined in RFC 2474 (Definition of the
Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers).
The Catalyst 6500 can manipulate (modify) the ToS priority bits and this can be achieved using both the PFC and/or
the MSFC. When a data frame comes into the switch, it will be assigned a DSCP value, which is derived from a
predefined default or from existing priority settings. This DSCP value is used internally in the switch to assign levels
of service (QoS policies) defined by the administrator. The DSCP can already exist in a frame and be used, or the
DSCP can be derived from the existing CoS, IP Precedence or DSCP in the frame (should the port be trusted). A
map is used internally in the switch to derive the DSCP (explained later). With 8 possible CoS/IP Precedence values
and 64 possible DSCP values, the default map will map CoS/IPPrec 0 to DSCP 0, CoS/IPPrec 1 to DSCP 8,
CoS/IPPrec 2 to DSCP 16, and so on. These default mappings can be overridden by the administrator. When the
frame is scheduled to an outbound port, the CoS can be re-written and the DSCP value is used to derive the new
CoS. A new feature of the PFC3 allows the DSCP to be left alone while at the same time rewriting CoS. This is a
new configurable option recently released for the PFC3 only.
4.1.2. Class of Service (CoS)
CoS refers to three bits in either an ISL header or an 802.1Q header that is used to indicate the priority of the
Ethernet frame as it passes through a switch network.
Figure 15. Inter Switch Link (ISL) Frame Format
In the ISL header, a VLAN tag (seen in red above) includes 3 bits, which are used to assign a priority value to this
frame.
For the purposes of this document, I will only be referring to the use of the 802.1Q header. The Class of Service bits
in the 802.1Q header are officially referred to as the 802.1p bits. Not surprisingly there are three CoS bits which
match the number of bits used for IP Precedence. In many networks, a packet may traverse both layer 2 and layer 3
domains, so to maintain QoS, the ToS can be mapped to CoS and vice versa.
Shown below is an Ethernet frame tagged with an 802.1Q field, which consists of a 2-byte Ether-type and a 2-byte
Tag. Within the 2-byte tag are the user priority bits (known as 802.1p).
Figure 16. Class of Service User Priority (802.1p) in an Ethernet frame