Information Technology Reference
In-Depth Information
The DS field takes the form shown in Figure 8-2. The DS field provides more granular lev-
els of packet classification by using 6 bits for packet marking. DS has 2
6
= 64 levels of clas-
sification, which is significantly higher than the 8 levels of the IP precedence bits. These
64 levels are called codepoints, and they have been defined to be backward compatible
with IP precedence values. RFC 2474 defines three sets of PHBs: Class Selector (CS), As-
sured Forwarding (AF), and Expedited Forwarding (EF). The CS PHB set is for DSCP val-
ues that are compatible with IP precedence bits. The AF PHB set is used for queuing and
congestion avoidance. The EF PHB set is used for premium service. The CS per-hop be-
haviors (PHB), in the form of xxx000, make it backwards compatible with IP precedence.
The network designer uses DSCP to give priority to IP packets using Cisco routers.
Routers should be configured to map these codepoints to PHBs with queuing or other
bandwidth-management techniques. Table 8-6 compares DSCP and IP precedence values
used to assign priority and apply policies to IP packets.
Ta b l e 8 - 6
DSCP and IP Precedence Values
IP Precedence
Limitation
DSCP
Service Type
Decimal
Binary
Class
Decimal
Codepoint
Routine
0
000
Best effort
0
000xxx
Priority
1
001
Assured Forwarding (AF)
Class 1
8
001xxx
Immediate
2
010
AF Class 2
16
010xxx
Flash
3
011
AF Class 3
24
011xxx
Flash override
4
100
AF Class 4
32
100xxx
Critical
5
101
Expedited Forwarding
(EF)
40
101xxx
Internetwork con-
trol
6
110
Control
48
110xxx
Network control
7
111
Control
56
111xxx
RFC 2597 defines recommended values for AF codepoints with low, medium, and high
packet-drop precedence. Table 8-7 shows the recommended AF codepoint values.
Ta b l e 8 -7
DSCP AF Packet-Drop Precedence Values
Precedence
AF Class 1
AF Class 2
AF Class 3
AF Class 4
Low drop precedence
001010
010010
011010
100010
Medium drop precedence
001100
010100
011100
100100
High drop precedence
001110
010110
011110
