I have a question regarding Fractional Fast-E and QoS. If you have a router subscribed to say a 20Mb MPLS circuit, it is advisable (even necesary) to use a FE interface for connectivity versus GE interface when you are running VoIP or any delay-sensitive apps. It seems that QoS is not enough to combat the discrepancy between the higher clock-rate that GE sends traffic on the wire in spite of a QoS/Shaping policy applied to the WAN interface. I have run into this situation in the past where VoIP quality was very poor until I switched the WAN interface to FE. My rule of thumb was if you subscribe to 100Mb or less, use FE ports.
If someone can point me to a URL/resource on CCO that explains/confirms this I would be very grateful for your help.
Thanks in advance,
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More likely, you're QoS isn't configured to account for the subtle "gotchas" that aren't normally discussed in most QoS text books.
In your other posting, I mention the importance of Tc, when shaping, and the fact (I believe) many shapers don't account for L2 overhead. Unless these are configured correctly, shaping behavior may not behave as you expect.
Shaping doesn't truly mimic an interface of the same bandwidth, even with optimal settings, because it works with average transmission rates over time. For example, a gig interface shaped at 100 Mbps will not be exactly like a FE interface at 100 Mbps. However, it can generally come close enough that most traffic will meet its service requirements, including VoIP.
BTW, even when doing QoS on physical interfaces, there are "gotchas". For instance, interfaces maintain a physical transmission queue (tx-ring) which is FIFO. Only when this queue overflows, is your QoS policy applied. So, to reduce the impact of a bunch of large bulk traffic packets in this queue when a VoIP packets arrives, you may need to configure a smaller tx-ring limit.