Cisco Catalyst 2000 Configuration Handbook page 245

224 Cisco LAN Switching Configuration Handbook
Low (1)
■
Medium (2)
■
■ High (3)
Traffic in the AF classes can be dropped, with the most likelihood of dropping in the
Low category and the least in the High category. In other words, service level AF class 4
with drop precedence 3 is delivered before AF class 4 with drop precedence 1, which is
delivered before AF class 3 with drop precedence 3, and so on.
Class 5 is also called the Expedited Forwarding (EF) class, offering premium service and
the least likelihood of packet drops. The Default class selector (DSCP 000 000) offers
only best-effort forwarding.
Class 6, Internetwork Control, and Class 7, Network Control, are both set aside for net-
work control traffic. This includes the Spanning Tree Protocol and routing protocols, traf-
fic that is not user-generated but usually considered high-priority.
Table 13-1 shows how the IP precedence names and bits have been mapped to DSCP val-
ues. DSCP is broken down by per-hop behavior (PHB), class selector, and drop prece-
dence. Many times, DSCP values are referred to by the codepoint name (AF23, for exam-
ple), which are also listed in the table. The DSCP bits are shown along with their decimal
equivalent. In many DSCP-related commands, you need to enter a decimal DSCP value,
even though it is difficult to relate the decimal numbers with the corresponding DSCP
service levels and PHBs. Use this table as a convenient cross-reference.
Table 13-1 Mapping of IP Precedence and DSCP Fields
IP Precedence (3 Bits)
Name
Routine
Priority
DSCP (6 Bits)
Per-Hop
Value Bits Behavior
0 000 Default
1 001 AF
Class Drop
Selector Precedence
— —
1 1: Low
2: Medium
3: High
Codepoint DSCP Bits
Name (Decimal)
Default 000 000 (0)
AF11 001 010 (10)
AF12 001 100 (12)
AF13 001 110 (14)