Average Queue Size For Wred - Cisco ASR 9000 Series Configuration Manual

Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers
WRED makes early detection of congestion possible and provides for multiple classes of traffic. It also
protects against global synchronization. For these reasons, WRED is useful on any output interface in
which you expect congestion to occur.
However, WRED is usually used in the core routers of a network, rather than at the edge of the network.
Edge routers assign IP precedences to packets as they enter the network. WRED uses these precedences
to determine how to treat different types of traffic.
WRED provides separate thresholds and weights for different IP precedences, allowing you to provide
different qualities of service in regard to packet dropping for different traffic types. Standard traffic may
be dropped more frequently than premium traffic during periods of congestion.
WRED treats non-IP traffic as precedence 0, the lowest precedence. Therefore, non-IP traffic, in general,
is more likely to be dropped than IP traffic.
WRED is useful only when the bulk of the traffic is TCP/IP traffic. With TCP, dropped packets indicate
congestion, so the packet source reduces its transmission rate. With other protocols, packet sources may
not respond or may resend dropped packets at the same rate. Thus, dropping packets does not decrease
congestion.
Figure 1
Figure 1
Incoming packets

Average Queue Size for WRED

The router automatically determines the parameters to use in the WRED calculations. The average queue
size is based on the previous average and current size of the queue. The formula is:
average = (old_average * (1-2
where x is the exponential weight factor.
For high values of x, the previous average becomes more important. A large factor smooths out the peaks
and lows in queue length. The average queue size is unlikely to change very quickly, avoiding a drastic
change in size. The WRED process is slow to start dropping packets, but it may 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.
OL-17239-01
Information About Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers
D r a f t — C i s c o C o n f i d e n t i a l
illustrates how WRED works.
Weighted Random Early Detection
Discard test
Classify
Discard test based on:
Buffer queue
depth
IP Precedence
RSVP session
-x
)) + (current_queue_size * 2
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
Transmit
queue
FIFO scheduling
Queueing
buffer
resources
-x
)
Outgoing
packets
QC-65