How Traffic Prioritization Works 51
determines the level of service that type of traffic should receive. Refer to
Table 6
for an example of how different traffic types can be mapped to
the eight IEEE 802.1p priority levels.
Table 6 IEEE recommendation for mapping 802.1p priority levels to 802.1D
traffic types
The traffic marking and prioritization supported by the Switch using layer
2 information is compatible with the relevant sections of the IEEE Std
802.1D, 1998 Edition (incorporating IEEE 802.1p).
The IEEE 802.1D standard is the most widely used prioritization scheme in
the LAN environment, but it does however have some restrictions:
■ It requires an additional 4-byte tag in the frame, which is normally
optional in Ethernet networks. Without this tag, the scheme cannot
work.
■ The tag is part of the IEEE 802.1Q header, so to implement QoS at
layer 2, the entire network has to implement IEEE 802.1Q VLAN
tagging.
■ It is only supported on a LAN and not across routed WAN links,
because the IEEE 802.1Q tags are removed when the packets pass
through a router.
Differentiated Services (DiffServ) Traffic Marking
DiffServ is a Layer 3 marking scheme that uses the DiffServ Code Point
(DSCP) field in the IP header to store the packet priority information.
DSCP is an advanced intelligent method of traffic marking because you
can choose how your network prioritizes different types of traffic. DSCP
IEEE 802.1p
Priority Level
IEEE 802.1D
Traffic Type
0 Best Effort (default)
1 Background
2 Standard (spare)
3 Excellent Effort (business critical)
4 Controlled Load (streaming multimedia)
5 Video (interactive media), less than 100 milliseconds latency and
jitter
6 Voice (interactive voice), less than 10 milliseconds latency and jitter
7 Network Control Reserved traffic
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