Configuration Guide (Supporting R05.6.00) Manual
120 Multi-Service IronWare QoS and Traffic Management Configuration Guide
53-1003037-02
Configuring packet drop priority using WRED
4
Applying the WRED algorithm to device traffic
Packets are assigned to an Ingress queue type based on their individual destination port and one
of the 8 (0 - 7) internal priorities. Each of these priorities is assigned a queue type from 0 - 7
according to the internal priority it belongs to as shown in Table 29.
The WRED algorithm is applied to traffic on these individual queues based upon parameters
configured for its assigned queue type. When traffic arrives at a queue, it is passed or dropped as
determined by the WRED algorithm. Packets in an individual queue are further differentiated by
one of four drop precedence values which are determined by the value of bits 3:2 of the TOS or
DSCP bits in the IPv4 or IPv6 packet header as shown in Figure 5.
FIGURE 5 TOS or DSCP bits in packet header
The user configurable values applied per queue type and per drop precedence value are:
• Maximum Drop Probability
• Minimum and Maximum Average Queue Size
• Maximum Packet Size
Configuring packet drop priority using WRED
For a description of WRED, refer to “Weighted Random Early Discard (WRED)” on page 118. This
section describes how to configure the parameters described in that section to enable the use of
WRED on a Brocade device. In addition, there is a default configuration that can be enabled that
sets the parameters to the values shown in Table 31. If you use the default configuration, you do
not need to set the parameters individually.
To configure WRED, you must configure the following parameters:
• “Enabling WRED”
• “Setting the averaging-weight (Wq) parameter” (optional)
• “Configuring the maximum instantaneous queue size” (optional)
• “Configuring the drop precedence parameters” (optional)
• “Restoring default WRED parameters”
pkt-size (avg-q-size - min-avg-q size)
Pdrop = ----------------- * Pmax * -----------------------------------------
pkt-size-max (max-avg-q-size - min-avg-q size)
TABLE 29 Internal priority to queue type mapping
Internal priority 0 1 2 345 67
Queue type 01 23 4 5 6 7
0
7
6
5
4
3
2
1
DSCP
CU
DSCP = Differentiated Services Codepoint
CU = currently unused