Layer 3 Qos Classification And Marking - Cisco Catalyst 2000 Configuration Handbook

■ Inter-Switch Link (ISL) trunk: Frames are tagged with a 15-bit VLAN ID. The CoS is
contained in the lower three bits of the User field. Although this is not standardized,
Catalyst switches copy the 802.1p CoS bits from a frame in an 802.1Q trunk into the
User field of frames in an ISL trunk. Figure 13-3 shows the format of the ISL tag.
0
DA (cont'd)
0x03
VLAN ID (15 Bits) + BPDU (1 Bit)
Reserved
Figure 13-3 ISL Trunk Encapsulation Format

Layer 3 QoS Classification and Marking

QoS is also built around the concept of Differentiated Service (DiffServ), where the QoS
specification is carried within each Layer 3 packet. IP packets have a type of service
(ToS) byte that is formatted according to the top row of Figure 13-4. Bits P2, P1, and P0
form the IP precedence value. Bits T3, T2, T1, and T0 form the ToS value.
For DiffServ, the same byte is called the Differentiated Services (DS) byte and is also
formatted according to the bottom row of Figure 13-4. Bits DS5 through DS0 form the
Differentiated Services Code Point (DSCP). The DSCP is arranged to be backward com-
patible with the IP precedence bits because the two quantities share the same byte in the
IP header.
ToS Byte:
DS Byte:
Figure 13-4 ToS and DSCP Byte Formats
Bits DS5, DS4, and DS3 form the DSCP class selector. Classes 1 through 4 are termed the
Assured Forwarding (AF) service levels. Higher class numbers indicate higher-priority
traffic. Each class or AF service level has three drop precedence categories:
1
DA (40 Bits; 0x01-00-0c-00-00)
Type User
SA (cont'd)
Length
High Bits of SA (0x00-00-0c)
... 8 to 196600 Bytes ...
CRC
P2 P1 P0 T3
DS5 DS4 DS3 DS2
(Class Selector)
Chapter 13: Quality of Service 223
2
SA (48 Bits)
0xAA
Index
Encapsulated Frame ...
T2 T1 T0
DS1 DS0 ECN1 ECN0
(Drop Precedence)
3
0xAA
Zero