7600 Sup32 egress rate-limit?

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Apr 6th, 2007
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I have the requirement to rate-limit egress traffic from a particular gig port on a Sup32 in a 7600 router. All 4 egress queues are in use; I need to limit total output throughput on the port - not just one particular queue.


I know the Sup32 doesn't support shaping, but is basic egress rate-limiting possible? (without going to SIP-400 & SPA's which provide more granularity and expense than I require)

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gmarogi Thu, 04/12/2007 - 06:09
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Policing TCP is always going to be unpredictable because of the nature of TCP flow-control. You may find the aggregate throughput is closer to the policed rate when there are more TCP flows. You could also try altering the burst rate, (from the QoS config guide):


"When configuring the burst rate, you need to take into account the fact that some protocols (like TCP) implement a flow-control mechanism that reacts to packet loss. For example, TCP reduces windowing by half for each lost packet; consequently, when policed to a certain rate, the effective link utilization will be lower than the configured rate. You can increase the burst in order to achieve better utilization. A good start for such traffic would be to double the burst size, monitor performance, and then adjust further if needed."


This URL should help you:

http://www.cisco.com/en/US/products/hw/routers/ps368/products_configuration_guide_chapter09186a0080160ec7.html#wp1465326

b.henshaw Thu, 04/12/2007 - 14:56
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Thanks for the response. I don't particularly want to police though, as this will be indiscriminate with regard to the various QoS classes. (or am I wrong with this assumption?)


I'm looking for something similar to the wrr-queue limit command on some of the fixed config switches which limits the rate at which the scheduler services the queues and outputs packets onto the wire, thus still allowing QoS to work.


I don't want to move to OSM or SIP (which will require Sup720 if I want connectivity at gig rate, and quadruple my price per chassis) and downlinking to another switch which can do shaping or rate-limiting is a bit messy - but maybe I don't have a choice. (happy for anyone to argue with me on this!)

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