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

Page: 67 / 153

How Traffic Prioritization Works 67
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DSCP is backward compatible with IPV4 TOS, which allows operation
with any existing devices with layer 3 TOS enabled prioritization
scheme in use.
Traffic Re-Marking Traffic entering the Switch may get downgraded or discarded depending
on the network policies and Service Level Agreements (SLA) pre-defined
by the network administrator. If for example a traffic packet enters the
Switch with a priority marking higher than the network SLA, the rules set
up by the network administrator can either be to Re-Mark the packet
with a different 802.1D priority or new DSCP value, or alternatively to
discard the traffic.
Traffic Prioritization Your Switch supports Basic and Advanced Quality of Service (QoS) traffic
prioritization. Basic traffic prioritization classifies traffic based on layer 2
of the OSI 7 layer model, and the Switch will prioritize the received traffic
according to the priority information defined in the received packet.
Advanced traffic prioritization can classify traffic at layers 2, 3 and 4 of
the OSI 7 layer model, and treat traffic according to the rules set up by
the network administrator.
Basic Traffic Prioritization
Incoming traffic is classified based upon the IEEE 802.1D frame and is
assigned to the appropriate priority queue based upon the IEEE 802.1p
service level value defined in that packet. Service level markings (values)
are defined in the IEEE 802.1Q 4-byte tag, and therefore traffic will only
contain 802.1p priority markings if the network is configured with VLANs
and VLAN tagging. All products in the Switch 4400 Series support basic
traffic prioritization. The traffic flow through the Switch is as follows:
1 A packet received by the Switch may or may not have an 802.1p tag
associated with it. If it does not, then it is given a default 802.1p tag
(which is usually 0). A packet may also be discarded by the Switch in
which case the packet would go no further. Alternatively, the packet may
be remarked with a new 802.1p value, which will result in all knowledge
of the old 802.1p tag being lost.
2 Because the 802.1p priority levels are fixed to the traffic queues (as
shown in
Figure 15 on page 68 ), the packet will be placed in the
appropriate priority queue, ready for transmission through the
appropriate egress port(s). When the packet reaches the head of its
queue and is about to be transmitted the device determines whether or