ProLiant BL p-Class GbE2 Interconnect Switch Application Guide

Quality of Service 83
Differentiated Services concepts
To differentiate between traffic flows, packets can be classified by their DSCP value. The Differentiated Services
(DS) field in the IP header is an octet, and the first six bits, called the DS Code Point (DSCP), can provide QoS
functions. Each packet carries its own QoS state in the DSCP. There are 64 possible DSCP values (0-63).
Figure 12 Layer 3 IPv4 packet
QoS levels
The following table shows the default service levels provided by the GbE2, listed from highest to lowest
importance:
Table 16 Default QoS service levels
Service Level Default PHB 802.1p Priority
Critical CS7 7
Network Control CS6 6
Premium EF, CS5 5
Platinum AF41, AF42, AF43, CS4 4
Gold AF31, AF32, AF33, CS3 3
Silver AF21, AF22, AF23, CS2 2
Bronze AF11, AF12, AF13, CS1 1
Using 802.1p priorities to provide QoS
GbE2 software provides Quality of Service functions based on the priority bits in a packet’s VLAN header. (The
priority bits are defined by the 802.1p standard within the IEEE 802.1q VLAN header.) The 802.1p bits, if
present in the packet, specify the priority that should be given to packets during forwarding. Packets with a
numerically higher (non-zero) priority are given forwarding preference over packets with lower priority.
The IEEE 802.1p standard uses eight levels of priority (0-7). Priority 7 is assigned to highest priority network
traffic, such as OSPF or RIP routing table updates, priorities 5-6 are assigned to delay-sensitive applications such
as voice and video, and lower priorities are assigned to standard applications. A value of 0 (zero) indicates a
“best effort” traffic prioritization, and this is the default when traffic priority has not been configured on your
network. The GbE2 can filter packets based on the 802.1p values, and it can assign or overwrite the 802.1p
value in the packet.
Figure 13 Layer 2 802.1q/802.1p VLAN tagged packet