CANOGA PERKINS CanogaOS Configuration Guide - Manual (Page 268)

CanogaOS Configuration Guide
Proprietary & Confidential Canoga Perkins Metro Ethernet Switches Page 268 of 350
Queueing maps packets to a queue. Each egress port can accommodate up 8 queues, prioritized
as 0 lowest and 7 highest.
The packet internal priority can be mapped to one of the 8 queues obtained from the filtering
mechanism result.
After the packets are mapped to a queue, they are scheduled.
Tail Drop
Tail drop is the default congestion-avoidance technique on the interface. With tail drop, packets
are queued until the thresholds are exceeded. The packets with color red are assigned to the first
threshold for drop precedence 0, yellow are assigned to the second threshold for drop precedence
1, and green are assigned to the third threshold for drop precedence 2. You can modify the three
tail-drop threshold to every egress queue by using the queue threshold interface configuration
command. Each threshold value is packet number, which ranges from 0 to 64.
WRED
Weighted Random Early Detection (WRED) differs from other congestion-avoidance techniques
because it attempts to anticipate and avoid congestion, rather than controlling congestion when it
occurs.
WRED reduces the chances of tail drop by selectively dropping packets when the output
interface begins to show signs of congestion. By dropping some packets early rather than waiting
until the queue is full, WRED avoids dropping large numbers of packets at once. Thus, WRED
allows the transmission line to be fully used at all times. WRED also drops more packets from
large users than small. Therefore, sources that generate the most traffic are more likely to be
slowed down versus sources that generate little traffic.
You can enable WRED and configure the two thresholds for a drop-precedence assigned to every
egress queues. The WRED’s color drop precedence map is the same as tail-drop’s. Each
min-threshold represents where WRED starts to randomly drop packets. After min-threshold is
exceeded, WRED randomly begins to drop packets assigned to this threshold. As the queue
max-threshold is approached, WRED continues to drop packets randomly with the rate of
drop-probability. When the max-threshold is reached, WRED drops all packets assigned to the
threshold. By default, WRED is disabled.
Scheduling
Scheduling forwards conditions packets using combination of WDRR and SP. Every queue
belongs to a class. The class rang from 0 to 3, and 3 is the highest priority. Several queues can be
in a same class, or non queue in some class. Packets are scheduled by SP between classes and
WDRR between queues in a class.
• Strict Priority-Based (SP), in which any high-priority packets are first transmitted.
Lower-priority packets are transmitted only when the higher-priority queues are empty. A
problem may occur when too many lower-priority packets are not transmitted.