Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 977)

Quality of Service (QoS) Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 38.5
The Differential Services Field
Figure 38-3 shows an IP header containing a Differentiated Services field. The format of
this redefined field is explained in RFC 2474; the main difference being that the old ToS
field has been replaced by a 6 byte Differentiated Services Code Point (DSCP) field, which
now provides for up to 64 defined values.
By applying this model only the QoS edge routers need to fully interrogate the incoming
data packets; the QoS core routers are then relieved of this processing task and need only
to inspect the DCSP before applying its appropriate forwarding, queueing, and shaping
rules.
Figure 38-3: The DSCP bits of the DS field in the IPv4 header
On the switch you can use the match inner-vlan command to select frames containing a
particular DSCP value, and associate them with a particular class-map and policy-map.
Because the model offers considerable flexibility, and the mapping of traffic types to
DCSPs is individual for each network, this locally applied definition is known as a
Differential Services Domain. The previous section introduced the concept of a Per Hop
(service quality) Behaviors or PHBs. RFC 2597 defines a specific PHB group called Assured
Forwarding (AF). The AF PHB group provides delivery of IP packets in four independently
forwarded AF classes. Within each AF class, an IP packet can be assigned one of three
different levels of drop precedence.
Table 38-2 shows a list of recommended AF code
points.
Table 38-2: Recommended DSCP Code Points
(Lowest
Priority)
Class 1
(001xxxx)
Class 2
(010xxxx) Class 3
(011xxxx)
(Highest
Priority)
Class 4
(100xxxx)
Low Drop Precedence 001010
AF11
Decimal 10 010010
AF21
Decimal 18 011010
AF31
Decimal 26 100010
AF41
Decimal 34
Medium Drop
Precedence 001100
AF12
Decimal 12 010100
AF22
Decimal 20 011100
AF32
Decimal 28 100100
AF42
Decimal 36
High Drop Precedence 001110
AF13
Decimal 14 010110
AF23
Decimal 22 011110
AF33
Decimal 30 100110
AF43
Decimal 38
DSCP value
ver IHL DiffServ total length identification flags
fragment
offset
TTL protocol
3 7 15 79 0 31 47 51 63 71 95
header
checksum
1 2 3 4 5 6 0 7
Precedence
Type of
Service
MBZ
Type of Service field
0 1
DSCP