3Com 3C17203 - SuperStack 3 Switch 4400, Implementation Manual

Page: 65 / 153

How Traffic Prioritization Works 65
Traffic Marking After traffic has been identified through classification, it must be Marked
to ensure that other devices such as Layer 2 switches or routers on the
network know how to prioritize the application, device or user that
generated it. The Switch uses two of the industry-standard methods of
marking network traffic:
■
IEEE 802.1D — a layer 2 marking scheme.
■
Differentiated Services (DiffServ) — a layer 3 marking scheme.
IEEE 802.1D Traffic Marking
The IEEE Std 802.1D, 1998 Edition marking scheme is an enhancement to
the IEEE Std 802.1D to enable Quality of Service in 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 each
frame with an IEEE 802.1p priority level between 0 and 7, which
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
Layer 4
■
UDP / TCP Source and
Destination ports for IP
applications
Many applications use certain TCP or UDP
sockets to communicate. By examining the
socket number in the IP packet, the intelligent
network can determine what type of application
generated the packet. This is also known as Layer
4 switching.
OSI Layer and Protocols Summary of Protocols
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)