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

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66 C
HAPTER
6: U
SING
T
RAFFIC
P
RIORITIZATION
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
uses 64 values that map to user-defined service levels, allowing you to
establish more control over network traffic.
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 and therefore priority is preserved
across the Internet.
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
IEEE 802.1p
Priority Level IEEE 802.1D
Traffic Type