Home > CCNP, ONT > ONT – Congestion Avoidance

ONT – Congestion Avoidance

In this post I want to go over some of the congestion avoidance techniques that ONT cover.

Tail Drop is a mechanism routers use by default to drop packets. It uses TCP Synchronization, computers become synched with the bandwidth they can use on the network and use Unbiased Dropping when the router is 100% FULL. After the router reaches a peak of 100% there is a huge drop in traffic which result in wasted bandwidth (at the after peak moment) when all the connections are cut down.
This is the most basic mechanism which is simple but waste the most bandwidth.

RED, Random Early Detection is a way to prevent unbiased dropping. Randomly drop packets from TCP flow to decrease synchronization and dropping becomes more aggressive as queues fill. The router attempt to prevent data loss by reducing cutting some traffic before the peak.
Cisco does not support RED

WRED, Weighted RED is an improved version that can use multiple RED profiles.
This mechanism require constant monitoring to determine the thresholds being used.
MPD, Mark Probability Denominator is the size of the threshold that lay between the minimum and maximum threshold lines.
Min = start dropping.
Max = start tail drop.
MPD is in the middle with a default set on 10.

WRED ECN Enhancement, ECN stand for Explicit Congestion Notification which adds pro-activeness to WRED. It uses the last 2 bits of ToS byte:


The end point begin the take part in the process. When congestion is experienced the router change the value to 11 and the receiving computer will cut the speed until the 11 alert is gone. Optional marking are:
00->not ECN capable
01->endpoints are ECN capable
10->endpoints are ECN capable
11->congestion experienced

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