H3C S9500 Series Operating Manual page 5

Operation Manual – QoS
H3C S9500 Series Routing Switches
field. The first six bits denote DSCP (differentiated services codepoint) priority, in the
range of 0 to 63, and the latter two bits are reserved. EXP priority, a number ranging
from 0 to 7 which is obtained by a mapping of the first three bits (bit 0 to 2) of DSCP
priority, lies in MPLS (multiprotocol label switching) header.
2) 802.1p priority
802.1p priority is stored in the header of Layer 2 packets and is suitable for the case
where only Layer 2 QoS guarantee, not L3 header analysis, is required.
Figure 1-2 Ethernet frame with 802.1Q tag header
In the above figure, each host supporting 802.1Q protocol adds a 4-byte 802.1Q tag
header after the source address in Ethernet header.
The 802.1Q tag header contains a 2-byte tag protocol identifier (TPID), with the default
value 0x8100, and a 2-byte tag control information (TCI). The TPID is newly defined by
IEEE to indicate that a packet is 802.1Q tagged. The TCI field consists of the User
Priority, CFI, and VLAN ID fields.
The User Priority field in TCI stands for 802.1p priority, which consists of three bits.
There are eight priority levels, numbered as 0 to 7, for determining to send which
packets first when switch congestion takes place. Since their applications are defined in
detail in the 802.1p Recommendation, they are named as 802.1p priority levels.
VII. Queue scheduling
Queue scheduling is used to resolve problems of resource contention by many packets.
These algorithms are often used in queue scheduling: strict priority (SP) algorithm and
weighted round Robin (WRR) algorithm.
1) SP algorithm
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Chapter 1 QoS Configuration