MDS105
Data Sheet
WRED is a method of handling traffic congestion in the absence of flow control mechanisms
1
. When flow control is
enabled, all devices that are connected to a switch node that is exercising flow control are effectively unable to
transmit, including nodes that are not directly responsible for the congestion problem. This inability to transmit
during flow control periods would wreak havoc with voice packets, or other high priority packet flows, and therefore
flow control is not recommended for networks that mix voice and data traffic.
WRED allows traffic to continue flowing into ports on a switch, and randomly drops packets with different
probabilities based upon each packet’s priority markings. As the switch congestion increases, the probability of
dropping an incoming packet increases, and as congestion decreases, the probability of dropping an incoming
packet decreases. Not surprisingly, packets designated high-drop are sacrificed with higher odds during congestion
than packets designated low-drop.
The following table summarizes the WRED operation of the MDS105. It lists the buffer thresholds at which each
drop probability takes effect.
WRED Threshold
Condition for High
Priority Queue
Level 0
Level 1
Level 2
Condition for Low
Priority Queue
Drop Percentage
Drop Percentage for
High-Drop Packet
50%
75%
100%
Drop Percentage for
Low-Drop Packet
0%
25%
50%
Total buffer space available in device is
≤LPBT
24 buffers occupied
72 buffers occupied
84 buffers occupied
Table 1 - WRED Operation of the MDS105
The WRED packet drop capabilities of the MDS105 are enabled by performing the following four steps:
1. Select the TOS/DS or VLAN Tag field as the decision-maker for dropping packets. The selection is made using
bit 7 of the Flooding Control Register (FCR).
-
-
FCR[7] = 0: Use VLAN Priority Tag field to resolve the drop level, if this field exists.
FCR[7] = 1: Use TOS/DS field for IP packet drop level resolution.
2. Select which TOS/DS Tag subfield to use for dropping packets provided that the TOS/DS field was selected in
step 1. The selection is made using bit 7 of the FCB Buffer Low Threshold Register (FCBST).
-
-
FCBST[7] = 0: Use DTR subfield to resolve the drop precedence.
FCBST[7] = 1: Use IP precedence subfield to resolve the drop precedence.
3. Create the mapping from the values in the TOS DS or VLAN Tag field to the packet flags representing high or
low drop precedence. The mapping is created using the VLAN Discard Map (AVDM) and TOS Discard Map
(TOSDM) registers.
4. Make sure that the desired ports are flow control disabled, using the ECR1Px registers.
Note that to apply the WRED QoS function of the MDS105, flow control must be disabled.
1. Flow control, of course, provides the advantage of not dropping packets. However, its primary disadvantage is that a flow controlled port may
experience head-of-line blocking. This means that if even 1 packet is destined to a congested output port, then all other packets originating
from the same source may, in the worst case, be delayed – even if these other packets have uncongested destinations. On the other hand,
WRED may cause some packet loss, but with no such head-of-line blocking problem. Which method of handling traffic congestion should be
chosen will depend on the application.
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