Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
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DSCP is backward com pat ible wit h t he three precedence bits in the ToS oct et so t hat non-DiffServ
com pliant , ToS-enabled net work device will not conflict w ith t he DSCP m apping.
The DSCP value det erm ines t he forwarding behavior, t he PHB ( Per-Hop Behavior), that each packet
get s across the DiffSer v net work. Based on the m arking rule, different kinds of traffic can be
m arked for different kinds of forwarding. Resources can then be allocated according to the DSCP
values and t he configured policies.
IP Precedence
Sim ilar to I EEE 802.1p prioritization at layer-2, you can use I P precedence t o priorit ize packet s in a
layer- 3 net w ork. I P precedence uses three bits of the eight -bit ToS ( Type of Ser vice) field in t he I P
header. There are eight classes of services ( ranging from zero to seven) in I P precedence. Zero is
the low est priorit y level and seven is the highest .
Automatic Priority Queue Assignment
I f you enable QoS on t he Device, the Device can aut om at ically base on t he I EEE 802.1p priority
level, I P precedence and/ or packet lengt h t o assign priorit y t o traffic which does not m atch a class.
The following t able shows you t he int ernal layer- 2 and layer-3 QoS m apping on the Device. On t he
Device, traffic assigned t o higher priorit y queues gets t hrough faster w hile t raffic in lower index
queues is dropped if t he network is congest ed.
DSCP (6 bit s) Unused ( 2 bit s)
Table 50 I nt ernal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY
(ETHERNET
PRIORITY)
TOS (IP
PRECEDENCE)
DSCP
IP PACKET
LENGTH (BYTE)
0 1 0 000000
12
2 0 0 000000 > 1100
3 3 1 001110
001100
001010
001000
250~ 1100
4 4 2 010110
010100
010010
010000
5 5 3 011110
011100
011010
011000
< 250
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