Chapter 10 Quality of Service (QoS)
SBG3500-N Series User’s Guide
199
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be
marked for different kinds of forwarding. Resources can then be allocated according to the DSCP
values and the configured policies.
IP Precedence
Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to prioritize packets in a
layer-3 network. IP precedence uses three bits of the eight-bit ToS (Type of Service) field in the IP
header. There are eight classes of services (ranging from zero to seven) in IP precedence. Zero is
the lowest priority level and seven is the highest.
Automatic Priority Queue Assignment
If you enable QoS on the SBG3500-N Series, the SBG3500-N Series can automatically base on the
IEEE 802.1p priority level, IP precedence and/or packet length to assign priority to traffic which
does not match a class.
The following table shows you the internal layer-2 and layer-3 QoS mapping on the SBG3500-N
Series. On the SBG3500-N Series, traffic assigned to higher priority queues gets through faster
while traffic in lower index queues is dropped if the network is congested.
DSCP (6 bits) Unused (2 bits)
Table 52 Internal 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
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