11-21
Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 11 Managing Packet Queue Congestion
Interfaces Supporting Layer 3 Packet Drop Policies
Average Queue Size and the Exponential Weight Constant
The router calculates the average queue size based on the previous average and the current size of the
queue, using the following formula:
Average = (old-average * (1 – 2 –
n
)) + (current-queue-size * 2 –
n
)
where
n
is the exponential weight constant
You can configure the exponential weight constant; however, we recommend that you use the default
value of 9. Valid values are from 1 to 16.
•
A high exponential weight constant smooths out the peaks and lows in queue length. The average
queue size is unlikely to change very quickly, avoiding drastic swings in size. WRED might be slow
to start dropping packets and can continue dropping packets for a time after the actual queue size
has fallen below the minimum threshold. The slow-moving average accommodates temporary bursts
in traffic.
•
If the exponential weight constant is too high, WRED does not react to congestion and packets are
transmitted or dropped as if WRED were not in effect.
•
If the exponential weight constant is too low, the average queue size might fluctuate with changes
in the traffic levels. As a result, WRED responds quickly to long queues, overreacts to temporary
traffic bursts, and drops traffic unnecessarily. After the queue falls below the minimum threshold,
WRED stops dropping packets.
Interfaces Supporting Layer 3 Packet Drop Policies
The following describes interface support for tail drop, random early detection (RED), and weighted
random early detection (WRED) drop policies using the
queue-limit
and
random-detect
commands:
Interfaces Supporting the queue-limit and random-detect Commands (Outbound Only)
•
Physical
•
Multilink PPP and Multilink Frame Relay
•
ATM shaped (peak cell rate is specified) unspecified bit rate (UBR) PVCs and point-to-point
subinterfaces *
•
ATM constant bit rate (CBR) PVCs and point-to-point subinterfaces *
•
ATM variable bit rate (VBR) PVCs and point-to-point subinterfaces *
•
Label-controlled ATM (LC-ATM) subinterfaces **
•
Frame Relay PVCs, point-to-point subinterfaces, and map classes **
•
Ethernet VLANs **
* The PRE3 does not support the
queue-limit
and
random-detect
commands on ATM subinterfaces
because the PRE3 only supports MQC policy maps on ATM PVCs.
** Requires a specific type of hierarchical policy. For more information, see the
Chapter 13, “Defining
QoS for Multiple Policy Levels.”
Note
The router supports the
queue-limit
and
random-detect
commands on outbound interfaces only.
