QOS/LLQ on ethernet subinterfaces

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Apr 28th, 2010
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Hypathetical scenario: I have a trunk port on a router going to a switch with subinterface fastethernet 0/0.1 for data, and 0/0.2 for voice. I want to perform LLQ or prioritize voice traffic on the 0/0.2 subif. Even though both data and voice have their own logical interface and therefore a FIFO queue for each, they eventually have to compete for the same physical transmit queue. How do I prioritize voice. Cisco documentation states that because logical interfaces do not support CBWFQ/LLQ, a higherarchical policy map needs to be created with the child map doing the CBWFQ/LLQ and the parent map doing shaping to create backpressure as a mechanism for CBWFQ to be engaged. However the documentation that I have read states to place the higherarchical policy map on the subinterface itself. This only works if I am trying to prioritize different flows that exist on the same subif but not if I want to prioritize flows on one subif over flows on the other subif. Do I place the higherarchical map on the interface itself (fastethernet 0/0)? If so, what shaping rate should be used and is there any documentation to this exact scenario. Maybe another question should be is there even a need to do this given the speed of fastethernet?

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Edison Ortiz Wed, 04/28/2010 - 15:28
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Your requirements can be accomplished with HQF

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

Please be aware of the IOS requirements.



Maybe another question should be is there even a need to do this given the speed of fastethernet?


If that link has potential for congestion due the aggregated traffic and you need to make sure VoIP does get priority, you need QoS.


Regards


Edison

djh278778 Tue, 05/04/2010 - 11:27
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I was hoping to find a specific document regarding Ethernet/Fastethernet/Gigethernet subif's and best practices along with configuration examples for this situation. Thanks though for the link, it did help and it is a great resource. I'll have to conclude based on the documentation that is out there that these logical interfaces are treated similar to that of Frame Relay subif's (as mentioned in the link) and serial interface subif's (as mentioned in Cisco Press and on the web), meaning that the parent map (that is doing the shaping) would be placed on the physical interface thereby creating shaping queue's that could then be prioritized as configured in the child/nestled map. The "Bc" value would have to be configured to force the "Tc" to 10 millseconds for voice to avoid too much delay.


Thanks.

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