WRED and IP Precedence
WRED is the Cisco implementation of RED that combines the capabilities of
the RED algorithm with IP precedence to provide lower drop rates for higher
priority, or higher precedence, packets.The router attributing different minimum
and maximum threshold levels to each precedence level accomplishes this. By
default, the minimum threshold in packets for IP precedence level 0 is one half
the maximum.The values for the remaining precedences fall between half the
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Figure 8.3 Weighted Random Early Detection
Incoming Packets
Classify
Discard
Test
Buffers
Bit
Bucket
Outgoing Packets
Advanced QoS for AVVID Environments • Chapter 8 249
maximum threshold and the maximum threshold at evenly spaced intervals.Table
8.4 shows the default values for both WRED and Distributed WRED
(DWRED), which is available on the Versatile Interface Processors (VIP)-based
Route/Switch Processor (RSP) platform. See the discussion later in this chapter
on “Running in Distributed Mode.”
NOTE
Although WRED gives lower drop probabilities to higher IP precedence
values, it can be configured to change the weighting of each precedence
level, or even to ignore precedence altogether, and thus function as
normal RED. By using the random-detect precedence command, you
can set the minimum and maximum threshold levels to something other
than the default values shown in Table 8.3. By making all the thresholds
the same, you essentially make WRED function as normal RED.