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Weighted Fair Queue vs FIFO at serial interface.

Posted on 2002-04-11
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Last Modified: 2012-08-13
I have a test setup. At two ends are Cisco2501(IOS 12.05) and Cisco2611(IOS 12.07). The serial link(64kbps) between the two routers has ftp and netmeeting(VoIP) traffic to simulate congestion conditions. According to my test I start the ftp and let it run for about 2 minutes. Then I start VoIP. I have noticed that with WFQ(Weighted Fair Queueing) at the serial interfaces , the VoIP call never gets established(I think the VoIP packets are simply dropped!!!!!!!!). However , with FIFO(First In First Out policy) at the same interface , the VoIP works fine. Could someone explain the observation?  
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Question by:Siddhartha_s
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scraig84 earned 150 total points
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To know for sure what is going on, one would need to view a trace of the traffic to see exactly what is getting across and what is not.  Remember that WFQ is just a simple auto-queuing method and has really never done much in my opinion.

However, I have done extensive testing with Class-Based WFQ (CBWFQ) and that works well.  With this you can manually define traffic classes and play with them in all sorts of different ways.

I will also say that if you are concerned with QoS, it is likely that finding an acceptable queuing method won't be enough.  When dealing with latency-sensitive protocols such as H.323 and ICA, problems come in when competing with heartier protocols such as FTP, HTTP, etc.  The average voice packet is fairly small, whereas the average file transfer packet is as large as possible.  Even when little priority is given to the transfer protocols, some packets get through and wind up interleaved into the data stream, causing jitter.  In order to deal with this, you need to chop up the larger protocols to come down to size with the sensitive protocol.  Cisco employs a method called LFI, or you can use simple IP fragmentation by reducing MTU size.  Take a look at the following for more info:

http://www.cisco.com/warp/public/732/Tech/link/

Hope that helps!
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