Necessity for configuring WRED on a Cisco Router

Necessity for configuring WRED on a Cisco Router

Although the queue provides VoIP QoS congestion management, it still needs some congestion avoidance mechanisms, such as WRED. In particular, WRED can prevent the output queue from reaching the limit value, which will lead to the loss of all input packets. This article comes from Informit and discusses the necessity of configuring WRED on a Cisco router.

Recall how the window of Transmission Control Protocol (TCP) works in your early network technology learning. The sender sends an information segment. If the sender receives a successful response from the recipient, it sends two more information segments (that is, two "window sizes "). If the two message segments are successfully sent, the sender sends the four Information segments and continues, the window size increases exponentially.

However, if one information segment is lost, the TCP stream enters TCP slow start (TCP slow start), and the window size is reduced to 1. TCP streams increase the window size exponentially until half of the window size when congestion occurs. Then, the window size of the TCP stream increases linearly. TCP Slow Start is related to QoS, because when the output queue of an interface is full, all newly arrived packets are discarded (that is, "tail dropped "), all TCP streams start slowly at the same time.

Note: The process of multiple TCP streams entering TCP Slow Start is called global synchronization or TCP synchronization. When TCP synchronization occurs, the connection bandwidth cannot be fully utilized, resulting in a waste of bandwidth.

RED basic random early detection (RED) aims to discard packets randomly when the output queue of an interface is about to be saturated to prevent TCP synchronization. The speed at which RED discards data packets depends on the queue depth at that time. The following three parameters affect when a new packet is discarded:

· Minimum threshold
· Maximum threshold
· Tag probability denominator (MPD)

The minimum threshold specifies the number of data packets in the queue before the queue prepares to discard data packets. The possibility of dropping data packets increases as the queue depth reaches the maximum threshold. When the queue depth exceeds the maximum threshold, all other packets attempting to enter the queue are discarded.

The possibility of dropping data packets is 1/(MPD) when the queue depth is equal to the maximum threshold ). For example, if the tag probability denominator is set to 10, when the queue depth reaches the maximum threshold, the probability of being dropped is 1/10 (that is, the probability of being dropped is 10 ). The minimum threshold, the maximum threshold, And MPD constitute RED. It is very useful for RED ports that may cause congestion on the vro. For example, RED may be required for a WAN port.

CB-WRED

Cisco does not support RED, but does support better weighted random advance detection (WRED). Unlike RED, WRED can resolve each priority label. For example, the minimum threshold of a packet whose IP priority is 0 may be 20, but the minimum threshold of a packet whose IP priority is 1 may be 25. In this example, data packets with an IP priority of 0 are discarded first.

Although WRED can be set from the port configuration mode or virtual circuit structure, the MQC-based WRED settings are described here. To enable WRED and set the tags (IP priority or DSCP) That WRED needs to follow, enter the following policy graph configuration mode commands.

Router (config-pmap-c) # random-detect [dscp-based | prec-based]

If neither dscp-based nor prec-based is specified, WRED is prec-based by default. After WRED is configured, IOS allocates the default minimum threshold, maximum threshold, And MPD. You can use the following command to change the default parameter:

Router (config-pmap-c) # random-detect precedence precedence_value
Minimum-threshold maximum-threshold mark-probability-denominator
(For prec-based WRED)

Router (config-pmap-c) # random-detect dscp dscp_value
Minimum-thresholdmaximum-threshold mark-probability-denominator
(Used for dscp-based WRED)

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