Specifications

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4.1 Priority Mechanisms in IP and Ethernet

For any QoS services to be applied to data, there must be a way to “tag” (or prioritize) an IP Packet or an Ethernet

frame. The Type of Service (ToS) in the IPv4 header and the Class of Service (CoS) fields in the Ethernet header are

used to achieve this. These are described in more detail below.

4.1.1. Type of Service (ToS)

Type of Service (ToS) is a one-byte field that exists in an IPv4 header. The ToS field consists of 8 bits of which the

first 3 bits were used to indicate the priority of the IP Packet. These first 3 bits are referred to as the IP Precedence

bits. These bits can be set from 0 to 7, (0 being the lowest priority and 7 being the highest priority). Support has been

around for setting IP Precedence in IOS for many years. Support for resetting IP Precedence can be done by the

MSFC or by the PFC (independent of the MSFC). A Trust setting of “untrusted” (discussed later in this paper) can

also wipe out any IP Precedence settings on an incoming frame (please refer to the section on “Trust” later in this

paper).

The values that can be set for IP Precedence are:

Table 13. IP Precedence Priority Bit Settings

IP Precedence bits IP Precedence Value

000 Routine

001 Priority

010 Intermediate

011 Flash

100 Flash Override

101 Critical

110 Internetwork Control

111 Network Control

The diagram below is a representation of the IP Precedence bits in the ToS header. The three Most Significant Bits

(MSB) are interpreted as the IP Precedence bits.

Figure 14. ToS Byte Settings

More recently, the use of the ToS field has been expanded to encompass the Six Most Significant Bits, referred to as

DSCP. DSCP (Differentiated Services Code Point) results in 64 priority values (2 to the power of 6) that can be