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QMultiFlex-400™ Installation and Operating Handbook
In this example, the Expedited Forwarding (EF) class is given the highest priority (0) and is
guaranteed 10% of the available bandwidth. Two Assured Forwarding classes (3 and 4) are
guaranteed 10% of the available bandwidth for each drop probability (1, 2 and 3). Each AF
class is given the appropriate level of priority corresponding to its precedence and drop
probability. All classes are allowed to burst up to 100% of the available bandwidth in this
example. All other traffic is given lower priority. No explicit bandwidth reservations are made
for any other DSCP class in this particular case.
The default class (Def) corresponds to the 'best effort' DSCP class 000000.
8.7.8.4.4 IEEE 802.1p Priority Tag
Classification may be done on the 3-bit Priority Code Point field of an IEEE 802.1q VLAN tag
(also referred to as an IEEE 802.1p Priority Tag). This is part of a 32-bit field inserted into an
Ethernet frame between the MAC address and length field.
The priority tag has eight possible values, each of which maps directly to an equivalent
internal class within the modem (for which a BIR, CIR, etc. can be set). Each packet passed
to the modem must have this field set to the appropriate value in order for the modem to
recognize the different data streams.
The example shown in Figure 8-31 shows traffic shaping based on priority tagging.
Figure 8-31 Traffic Shaping using IEEE 802.1p Priority Tags
In this example, the IEEE 802.1p priority classes are shown in the left-hand column. Each is
assigned varying amounts of guaranteed bandwidth and each is allowed to burst up to the
maximum bandwidth. IEEE 802.1p priorities (with 7 being the highest) are inverted in relation
to those in the modem's queuing mechanism (where 0 is highest). The right-hand column
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