User Manual Managed Switches
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IEEE 802.1D Traffic Marking
The IEEE Std 802.1D, 1998 Edition marking scheme, which is an enhancement to IEEE Std 802.1D,
enables Quality of Service on the LAN. Traffic service levels are defined in the IEEE 802.1Q 4-byte
tag, which is used to carry VLAN identification as well as IEEE 802.1p priority information. The 4-byte
tag immediately follows the destination MAC address and Source MAC address.
The IEEE Std 802.1D, 1998 Edition priority marking scheme assigns an IEEE 802.1p priority level
between 0 and 7 to each frame. The priority marking scheme determines the level of service that this
type of traffic should receive. Refer to the table below for an example of how different traffic types can
be mapped to the eight IEEE 802.1p priority levels.
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 of latency and jitter
6
Voice (interactive voice); less than 10 milliseconds of latency and jitter
7
Network Control Reserved traffic
Even though the IEEE 802.1D standard is the most widely used prioritization scheme in the LAN
environment, it still has some restrictions:
•
It requires an additional 4-byte tag in the frame, which is normally optional for 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 must implement IEEE 802.1Q VLAN tagging.
•
It is only supported on a LAN and not across routed WAN links, since 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
that allows you to choose how your network prioritizes different types of traffic. DSCP uses 64 values
that map to user-defined service levels, allowing you to establish more control over network traffic.
The advantages of DiffServ over IEEE 802.1D are:
•
You can configure how you want your switch to treat selected applications and types of traffic by
assigning various grades of network service to them.
•
No extra tags are required in the packet.
•
DSCP uses the IP header of a packet to preserve priority across the Internet
•
DSCP is backward compatible with IPV4 ToS, which allows operation with existing devices that
use a layer 3 ToS enabled prioritization scheme.
