When the queue is full, packets that are trying to queue up for transmission literally have nowhere to be put! These packets are then subject to tail drop, which is a fancy way of saying "you're being dropped because we have no place to put you".

You know that TCP has a detection and recovery scheme when it comes to missing segments, so tail drop is no big deal, right? Quite the opposite, it's a huge deal.

The problem starts innocently enough, as the senders realize their TCP packets are being dropped. As we'd expect, the senders then throttle back on their transmission speed. After doing so, the senders will then gradually speed their transmission rates back up.

As multiple senders increase their transmission rates, the queue will fill up again, and the senders will again almost simultaneously slow their tranmission rates, followed by another near-simultaneous increase.

As a result of this global synchronization, the links are perpetually in one of two states - congested or underused. Basically, the network ends up being either hammered or not being used to its full potential, and those are both circumstances we want to avoid.

One way to avoid global synchronization is though the use of Random Early Detection (RED).

Look for more Cisco practice exams on my website as well as this one!

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