how does FIFO work

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Dec 19th, 2008
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Hi all

When we talk about FIFO in switches, is it true that it would service the large packets first? so if we have 2 streams of traffic going through the same port, would the stream with the large frame size get through first?


thanks


Carl

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Jon Marshall Fri, 12/19/2008 - 02:38
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Carl


FIFO = First in First out. It's pretty much as simple as that. No packets are prioritised over any other packets. So it would service the large packets first if they arrived first. If they arrived after some smaller packets then they would be serviced after the smaller packets.


Jon

ropethic Fri, 12/19/2008 - 03:11
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here is a very good link explaining all types of congestion management

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfconmg.html


FIFO Queueing

In its simplest form, FIFO queueing-also known as first-come, first-served (FCFS) queueing-involves buffering and forwarding of packets in the order of arrival.


FIFO embodies no concept of priority or classes of traffic and consequently makes no decision about packet priority. There is only one queue, and all packets are treated equally. Packets are sent out an interface in the order in which they arrive.


When FIFO is used, ill-behaved sources can consume all the bandwidth, bursty sources can cause delays in time-sensitive or important traffic, and important traffic can be dropped because less important traffic fills the queue.


When no other queueing strategies are configured, all interfaces except serial interfaces at E1 (2.048 Mbps) and below use FIFO by default. (Serial interfaces at E1 and below use WFQ by default.)


FIFO, which is the fastest method of queueing, is effective for large links that have little delay and minimal congestion. If your link has very little congestion, FIFO queueing may be the only queueing you need to use


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