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3) In the third phase each internal priority is mapped in on of 4 or 8 Egress Queue Classes.
The setting is per ingress port but the different mappings is limited to three (one default one-to-one and
two user configurable). "EC" in Figure 68.
4) Based on the ingress port settings the frame (Green, Yellow or Red) will be switched to the egress port
based on the VLAN assignments. At the egress port it will go to the queue (or bypass the use if the class is
Strict Priority) as defined in the previous phase. For each class there is a Shaper Rate, Weighting Factor and
Queuing method as well as threshold values for the Queue length for dropping first Yellow and then Green
frames. The egress port also has a Shaper Rate that cannot be exceeded (in the modem port it depends on
the channel spacing and modulation and E1 channels in use, in other ports it can be set manually). "EQS" in
Figure 68). The scheduling mechanism is Deficit Weighted Round Robin (DWRR) and the drop mode is
either WTD (Weighted Tail Drop) or WRED (Weighted Random Early Discard).
QoS planning depends on the operator requirements and the settings should be modified for each case.
SAMPLE QOS POLICY
In this example "802.1Q User Priority" is used, with four priority classes: Class 0 (BE, Best Effort), Class 1
(BE+, Best Effort+), Class 2 (BC, Business Critical), Class 3 (RT, Real Time) and Class 4 (NC, Network Critical);
classes 5 and 6 are not used. The highest class (7) is given Strict Priority, SP and the other 5 classes use
Weighted Random Early Discard, WRED. The class weights are from low to high 1:5:29:89:3:1:1.
In this example the "Port Based QoS Mode" with incoming frame CoS(C-Tag) classification. Figure 69.
There is no Ingress Policer defined, in other words all ports may use as much bandwidth they want for any
priority (it is assumed that policing is handled by external equipment).
Setting Default Port Priority = 0 means that any untagged frames will be given internal priority = 0, provided
that VLAN settings allow any untagged frames.
Ingress 802.1q
frame p-bit
0 (000) or missing
tag
1 (001)
2 (010)
3 (011)
4 (100)
5 (101)
6 (110)
7 (111)
Table 16. CoS-classification into egress queues. There will be nothing in the egress port queue class 5 or 6.
Class
Assigned
internal
priority, IC
BE
0
BE+
1
BC
2
BC
3
RT
4
RT
5
NC
6
"7"
7
Egress Queuing Class
mapping, EC
0
1
2
2
3
3
4
7
79
Weight, WRED
1
5
29
89
3
(SP)
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