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Congestion avoidance techniques monitor traffic flow in an effort to anticipate and avoid congestion at common network bottlenecks. Avoidance techniques are implemented before congestion occurs as compared with congestion management techniques that control congestion after it has occurred.
Congestion avoidance is achieved through packet dropping. The router supports these QoS congestion avoidance techniques:
Tail drop is a congestion avoidance technique that drops packets when an output queue is full until congestion is eliminated. Tail drop treats all traffic flow equally and does not differentiate between classes of service. It manages the packets that are unclassified, placed into a first-in, first-out (FIFO) queue, and forwarded at a rate determined by the available underlying link bandwidth.
Packets satisfying the match criteria for a class accumulate in the queue reserved for the class until they are serviced. The queue-limit command is used to define the maximum threshold for a class. When the maximum threshold is reached, the enqueued packets to the class queue result in tail drop (packet drop).
When configuring the queue-limit command, you must configure one of the following commands: priority , shape average , bandwidth or bandwidth remaining , except for the default class.
You have to accomplish the following to complete the tail drop configuration:
Creating (or modifying) a policy map that can be attached to one or more interfaces to specify a service policy
Associating the traffic class with the traffic policy
Specifying the maximum limit the queue can hold for a class policy configured in a policy map.
Specifying priority to a class of traffic belonging to a policy map.
(Optional) Specifying the bandwidth allocated for a class belonging to a policy map or specifying how to allocate leftover bandwidth to various classes.
Attaching a policy map to an output interface to be used as the service policy for that interface.
Router# configure
Router(config)# policy-map test-qlimit-1
Router(config-pmap)# class qos-1
Router(config-pmap-c)# queue-limit 100 us
Router(config-pmap-c)# priority level 7
Router(config-pmap-c)# exit
Router(config-pmap)# exit
Router(config)# interface HundredGigE 0/6/0/18
Router(config-if)# service-policy output test-qlimit-1
Router(config-if)# commit
policy-map test-qlimit-1
class qos-1
queue-limit 100 us
priority level 7
!
class class-default
!
end-policy-map
!
Router# show qos int hundredGigE 0/6/0/18 output
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- output policy
NPU Id: 3
Total number of classes: 2
Interface Bandwidth: 100000000 kbps
VOQ Base: 11176
VOQ Stats Handle: 0x88550ea0
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class (HP7) = qos-1
Egressq Queue ID = 11177 (HP7 queue)
TailDrop Threshold = 1253376 bytes / 100 us (100 us)
WRED not configured for this class
Level1 Class = class-default
Egressq Queue ID = 11176 (Default LP queue)
Queue Max. BW. = 101803495 kbps (default)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
TailDrop Threshold = 1253376 bytes / 10 ms (default)
WRED not configured for this class
The Random Early Detection (RED) congestion avoidance technique takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it decreases its transmission rate until all packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause TCP to slow transmission of packets. TCP not only pauses, but it also restarts quickly and adapts its transmission rate to the rate that the network can support.
RED distributes losses in time and maintains normally low queue depth while absorbing traffic bursts. When enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select during configuration.
The random-detect command with the default keyword must be used to enable random early detection (RED).
If you configure the random-detect default command on any class including class-default, you must configure one of the following commands: shape average , bandwidth , and bandwidth remaining .
You have to accomplish the following to complete the random early detection configuration:
Creating (or modifying) a policy map that can be attached to one or more interfaces to specify a service policy
Associating the traffic class with the traffic policy
Enabling RED with default minimum and maximum thresholds.
(Optional) Specifying the bandwidth allocated for a class belonging to a policy map or specifying how to allocate leftover bandwidth to various classes.
(Optional) Shaping traffic to the specified bit rate or a percentage of the available bandwidth.
Attaching a policy map to an output interface to be used as the service policy for that interface.
Router# configure
Router(config)# policy-map test-wred-2
Router(config-pmap)# class qos-1
Router(config-pmap-c)# random-detect default
Router(config-pmap-c)# shape average percent 10
Router(config-pmap-c)# end-policy-map
Router(config)# commit
Router(config)# interface HundredGigE 0/6/0/18
Router(config-if)# service-policy output test-wred-2
Router(config-if)# commit
policy-map test-wred-2
class qos-1
random-detect default
shape average percent 10
!
class class-default
!
end-policy-map
!
interface HundredGigE 0/6/0/18
service-policy output test-wred-2
!
Router# show qos int hundredGigE 0/6/0/18 output
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- output policy
NPU Id: 3
Total number of classes: 2
Interface Bandwidth: 100000000 kbps
VOQ Base: 11176
VOQ Stats Handle: 0x88550ea0
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class = qos-1
Egressq Queue ID = 11177 (LP queue)
Queue Max. BW. = 10082461 kbps (10 %)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 10000000 kbps
TailDrop Threshold = 12517376 bytes / 10 ms (default)
Default RED profile
WRED Min. Threshold = 12517376 bytes (10 ms)
WRED Max. Threshold = 12517376 bytes (10 ms)
Level1 Class = class-default
Egressq Queue ID = 11176 (Default LP queue)
Queue Max. BW. = 101803495 kbps (default)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 50000000 kbps
TailDrop Threshold = 62652416 bytes / 10 ms (default)
WRED not configured for this class
The Weighted Random Early Detection (WRED) drops packets selectively based on any specified criteria, like discard-class. WRED uses this matching criteria to determine how to treat different types of traffic.
You can configure WRED using the random-detect command and different discard-class values. The value can be range or a list of values that are valid for that field. You can also use minimum and maximum queue thresholds to determine the dropping point. Ensure that the WRED maximum threshold value is close to the queue limit. When the maximum threshold value is reached, packets start to get dropped.
When a packet arrives, the following actions occur:
The average queue size is calculated.
If the average queue size is less than the minimum queue threshold, the arriving packet is queued.
If the average queue size is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic.
If the average queue size is greater than the maximum threshold, the packet is dropped.
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 -x)) + (current_queue_size * 2 -x)
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.
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For low values of x, the average queue size closely tracks the current queue size. The resulting average may fluctuate with changes in the traffic levels. In this case, the WRED process responds quickly to long queues. Once the queue falls below the minimum threshold, the process stops dropping packets.
This configuration task is similar to that used for RED except that the random-detect command is not configured in RED.
You cannot use the random-detect command in a class configured with the priority command, because WRED cannot be configured in a class that has been set for priority queueing (PQ).
When configuring the random-detect command, you must configure one of the following commands: shape average , bandwidth , and bandwidth remaining .
Creating (or modifying) a policy map that can be attached to one or more interfaces to specify a service policy
Associating the traffic class with the traffic policy
Enabling WRED by specifying the match criteria (discard-class).
(Optional) Specifying the bandwidth allocated for a class belonging to a policy map or specifying how to allocate leftover bandwidth to various classes.
(Optional) Shaping traffic to the specified bit rate or a percentage of the available bandwidth.
(Optional) Changing queue limit to fine-tune the amount of buffers available for each queue.
Attaching a policy map to an output interface to be used as the service policy for that interface.
Router# configure
Router(config)# policy-map test-wred-1
Router(config-pmap)# class qos-1
Router(config-pmap-c)# random-detect default
Router(config-pmap-c)# random-detect 10 ms 500 ms
Router(config-pmap-c)# shape average percent 10
Router(config-pmap-c)# commit
Router(config)# interface HundredGigE 0/6/0/18
Router(config-if)# service-policy output policy1
Router(config-if)# commit
policy-map test-wred-1
class qos-1
random-detect default
random-detect 10 ms 500 ms
shape average percent 10
!
class class-default
!
end-policy-map
!
interface HundredGigE 0/6/0/18
service-policy output test-wred-1
!
Router# show qos int hundredGigE 0/6/0/18 output
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- output policy
NPU Id: 3
Total number of classes: 2
Interface Bandwidth: 100000000 kbps
VOQ Base: 11176
VOQ Stats Handle: 0x88550ea0
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class = qos-1
Egressq Queue ID = 11177 (LP queue)
Queue Max. BW. = 10082461 kbps (10 %)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 10000000 kbps
TailDrop Threshold = 1073741824 bytes / 858 ms (default)
Default RED profile
WRED Min. Threshold = 12517376 bytes (10 ms)
WRED Max. Threshold = 629145600 bytes (500 ms)
Level1 Class = class-default
Egressq Queue ID = 11176 (Default LP queue)
Queue Max. BW. = 101803495 kbps (default)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 50000000 kbps
TailDrop Threshold = 62652416 bytes / 10 ms (default)
WRED not configured for this class
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