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| 2.4.2.1 Fifo queuing discipline |
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| Fifo is the default queuing discipline used by most of the routers around the world. In fact, having its benefits and limitation, but being very simple to implement, has spread as the grass. |
| Its own name (First In - First Out) explains its simple behavior. Packets arriving from different flows are treated equally by placing them into the queue, respecting, strictly, their arriving order. Already in the queue they are dispatched in the same order they entered, this means, first packet that comes in is the first packet that goes out. |
| We drew a Fifo queue somewhere above. Let's improve a little the picture stealing a new one from Juniper Networks [1]. Have a look to figure #4. As you see different packet flows, represented by different colors, are placed in the queue at its tail and after waiting a little for their turn, are dispatched from the head. |
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| As was explained before the queue acts as a dam for temporary burst of packets
avoiding unnecessary dropping by storing them for a while, hoping that congestion alleviate and they can be dispatched. When congestion is heavy and the queue overflows new arriving packets have to be dropped not having the router any other choice for them. |
| Fifo is the queuing discipline most used as default for routers. Because of this you don't need to configure anything to have it just implemented and ready to be used. On Cisco routers, when no other queuing strategies are configured, all interfaces, except serial interfaces at E1 (2.048 Mbps) and below, use Fifo queuing discipline by default. |
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