[c-nsp] Ethernet interface QoS

[Bruce Pinsky]

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Steven Saner wrote:
> 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
> 


You need to shape the overall traffic to the bandwidth capacity you have
purchased on the link.  So, in your case, you have a 100Mbps ethernet
presentation that needs to be shaped to 5Mbps.  Then, within that 5Mbps,
you need to shape/police your various traffic classes.  Basically,
Hierarchical Queuing.

http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/qos_frhqf_support.html

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bep

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