[c-nsp] Ethernet interface QoS

[Steven Saner]

I have a QoS question that I'm hoping someone can help me understand.

We use last mile technology that is ethernet based. So at one end of a
link we might have a router with a FastEthernet interface connected to
some device that acts as an ethernet bridge connected to some last mile
technology (ethernet-over-copper, xDSL, etc).

I need to perform some QoS to give priority to VOIP traffic. I
understand basically how to set up policy-maps on a Cisco router to
classify the traffic that I am interested in and then give priority to
that traffic. I also understand that such policy maps affect queuing
during times of congestion. My question is, does the interface know when
the network is congested?

The router interface may be 100 Mbps. It is then connected to a last
mile technology device and the bandwidth capacity through this last mile
technology is, say, 5 Mbps. So if the router tries to send data at a
rate that is greater than 5 Mbps, we have congestion in the network.
But, that is no where near the capacity of the router's physical
interface. Does the router know at that point that congestion is
happening, and do the policy maps go in to effect? If not, is there some
way to tell the 100 Mbps ethernet interface that it only has 5 Mbps to
work with? Or, is the only way to get QoS to work in this case, to do it
on the last mile technology gear where the bottleneck actually occurs?

I fear I'm missing some fundamental truth here. Thanks for any help.

Steve

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Steven Saner <steve at saner.net> KD0IJP
Andover, Kansas USA