Figure 93 IP Header and ToS Field

IP Packet

3-bit

Precedence

Field

These three bits allow for eight different CoS types (0-7), listed in Table 9-2.

Table 9-2. ToS (IP Precedence)

Service Type

Purpose

Routine

Set routine precedence (0)

Priority

Set priority precedence (1)

Immediate

Set immediate precedence (2)

Flash

Set Flash precedence (3)

Flash-override

Set Flash override precedence (4)

Critical

Set critical precedence (5)

Internet

Set internetwork control precedence (6)

Network

Set network control precedence (7)

IP Precedence 6 and 7 are reserved for network information (routing updates, hello packets, and so on). This leaves 6 remaining precedence settings for normal IP traffic flows.

IP Precedence enables a router to group traffic flows based on the eight precedence settings and to queue traffic based upon that information as well as on source address, destination address, and port numbers.

You can consider IP Precedence an in-band QoS mechanism. Extra signaling is not involved, nor does additional packet header overhead exist. Given these benefits, IP Precedence is the QoS mechanism that large-scale networks use most often.

With Cisco IOS, you can set the IP Precedence bits of your IP streams in several ways. With Cisco's VoIP design, you can set the IP Precedence bits based upon the destination phone number (the called number). Setting the precedence in this manner is easy and allows for different types of CoS, depending upon which destination you are calling.

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