How WRED Weights Packets
WRED gives preference to packets with certain IPP or DSCP values. To do so, WRED uses
different traffic profiles for packets with different IPP and DSCP values. A WRED traffic profile
consists of a setting for three key WRED variables: the minimum threshold, the maximum
threshold, and the MPD. Figure 13-6 shows just such a case, with two WRED traffic profiles (for
IPP 0 and IPP 3).
As Figure 13-6 illustrates, IPP 3’s minimum threshold was higher than for IPP 0. As a result, IPP 0
traffic will be discarded earlier than IPP 3 packets. Also, the MPD is higher for IPP 3, resulting in
a lower discard percentage (based on the formula discard percentage = 1/MPD).
Figure 13-6 Example WRED Profiles for Precedences 0 and 3
Table 13-8 lists the IOS default WRED profile settings for various DSCP values. You may recall
from Chapter 12 that Assured Forwarding DSCPs whose names end in 1 (for example, AF21)
should get better WRED treatment than those settings that end in 2 (for example, AF32). The IOS
Average
Queue Depth
20
Precedence
0 Minimum
Threshold
40
Maximum
Threshold
Discard
Percentage
100%
10%
5%
Precedence 0 Drop
Percentage (MPD = 10)
Precedence 3 Drop
Percentage (MPD = 20)
30
Precedence
3 Minimum
Threshold
defaults listed in Table 13-8 achieve that goal by setting lower minimum thresholds for the
appropriate AF DSCPs.
Table 13-8 Cisco IOS Software Default WRED Profiles for DSCP-Based WRED
DSCP Minimum Threshold Maximum Threshold MPD 1/MPD
AFx1 33 40 10 10%
AFx2 28 40 10 10%
AFx3 24 40 10 10%
EF 37 40 10 10%