Ip Differentiated Services Code Point Marking; Dscp Per-Hop Behavior - Cisco 10000 Series Configuration Manual

Quality of service configuration guide

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IP Differentiated Services Code Point Marking

IP precedence marking might seem too restrictive and limiting because only eight classes are available

for marking. You might choose instead to use the IP differentiated services code point (DSCP) marking

model, which offers up to 64 different values (0 through 63).

The differentiated services (DiffServ) functionality of the Cisco IOS software is fully compliant with the

Internet Engineering Task Force (IETF) standards defined in the following request for comments (RFCs)

The router leverages the IETF definition of the IPv4 1-byte type of service (ToS) field in the IP packet

header by using the six most significant bits of this field (the DSCP bits) to classify traffic into any of

the 64 possible classes. After the router classifies packets, you can use the modular QoS CLI to

implement IETF-defined per-hop behaviors (PHBs), including assured forwarding (AF) and expedited

forwarding (EF).

The router also uses bits in the ToS field to prioritize packets using an IP precedence value. Because the

IP precedence value is actually part of the DSCP value, you cannot simultaneously set both the IP

precedence and DSCP values. If you attempt to, an error message displays.

You can enter DSCP values as numeric values or as special keyword names called per-hop behaviors

(PHBs). For example, DSCP EF is the same as DSCP 46 and DSCP AF31 is the same as DSCP 26.

The router supports the following classes of DSCP PHBs:

Cisco 10000 Series Router Quality of Service Configuration Guide

shows the DSCP bits in the ToS field.

DSCP Bits in the IP ToS Byte

7 6 5 4 3 2 1 0

Best effort (BE)—DSCP 0

Assured forwarding (AF)—AF classes 1 through 4

Expedited forwarding (EF)—DSCP 46

Class selector code points—CS1 through CS7

Marking Traffic

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