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7-6
Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 7 Marking Traffic
IP Differentiated Services Code Point Marking
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)
documents:
•
RFC-2474
•
RFC-2475
•
RFC-2597
•
RFC-2598
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.
Figure 7-1
shows the DSCP bits in the ToS field.
Figure 7-1
DSCP Bits in the IP ToS Byte
DSCP Per-Hop Behavior
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:
•
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
VER
HL
TOS
LEN
ID
FLAG
OFFSET
TTL
PROTO
CS
SIP
7 6 5 4 3 2 1 0
DSCP
1-Byte TOS
DIP
87065