ZyXEL Communications VMG1312-B Series, User Manual

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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 t hree precedence bit s in t he ToS oct et so t hat non- DiffServ
com pliant , ToS- enabled net work device will not conflict wit h t he DSCP m apping.
The DSCP value det erm ines t he forwarding behavior, t he PHB ( Per- Hop Behavior) , t hat each packet
get s across t he DiffServ net work. Based on t he m arking rule, different kinds of t raffic can be
m arked for different kinds of forwarding. Resources can t hen be allocat ed according t o t he DSCP
values and t he configured policies.
IP Precedence
Sim ilar t o I EEE 802.1p priorit izat ion at layer- 2, you can use I P precedence t o priorit ize packet s in a
layer- 3 net work. I P precedence uses t hree bit s of t he eight- bit ToS ( Type of Service) field in t he I P
header. There are eight classes of services ( ranging from zero t o seven) in I P precedence. Zero is
t he lowest priorit y level and seven is t he highest .
Automatic Priority Queue Assignment
I f you enable QoS on t he Device, t he Device can aut om at ically base on t he I EEE 802.1p priorit y
level, I P precedence and/ or packet lengt h t o assign priorit y t o t raffic which does not m at ch a class.
The following t able shows you t he int ernal layer- 2 and layer- 3 QoS m apping on t he Device. On t he
Device, t raffic assigned t o higher priorit y queues get s t hrough fast er while t raffic in lower index
queues is dropped if t he net work 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
1 2
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