WRED (Weighted Random Early Detection)

[lagapidis (Lazarus Agapidis)]

Hello Network23

@network23

Wred randomly drops packets when there is congestion but I thought it also does it as a proactive measure to avoid congestion.

WRED is used as a proactive measure to avoid queue exhaustion, not congestion. When a packet arrives at an interface, if the interface is not yet at capacity, the packet will be served immediately. It will not enter the queue. A packet will enter the queue only if there is already congestion on the link.

Now documentation does use the term “congestion avoidance” but in actual fact, it’s “tail drop avoidance” that we are performing, for the purpose of ensuring that TCP connections will not go into slow start, thus slowing traffic down in general.

@network23

For example, let’s say we have a 1G link on a router interface and there is a class map for bulk data traffic. The link is not congested but there is bulk data traffic exceeding the minimum threshold. In this scenario would WRED begin to randomly drop traffic in order to avoid possible congestion?

If you use a class map to limit traffic of a certain type on an interface, such as bulk traffic as you mention, then you are creating in essence, a situation where congestion would take place. It may not be congestion because you reach the physical limit of the interface, but the configured limitation you set yourself. If that takes place, then yes you can then configure WRED to function simultaneously. But again, congestion must occur before a queue begins to fill up.

I hope this has been helpful!

Laz