LAN Switch QoS

[ikraehenmann]

Hello

When implementing QoS on a switch (e.g. 3550, etc), it uses WRR for output scheduling. Let's say, we run QoS on a 3550 gigabit interface. In WRR, the interface gets 4096 buffers to queue frames/packets (can be verified by #show mls qos interface <int> statistics). However, if I look at the interface (#show interfaces <int>), I see the output queue size set at 40 (the default).

I wonder if this number has any relevance or if this is just the queue size used when QoS is not turned on. What effect can I expect if I increase the output queue size on the interface ((config-if)hold-queue <n> out). Any comments?

Thanks, Igi.

[vkapoor5]

I hope the "hold-queue x" command applicable only when any type of manual queueing configuration is not done. So, in your case, with WRR configuration, 4096 buffers got allocated for that one. From one Cisco document, eventhough it's not talking about WRR, I thought of relevance...

"If priority output queueing is being used, the length of the four output queues is set using the priority-list global configuration command. The hold-queue command cannot be used to set an output hold queue length in this situation."

Reference documents...

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a7e9e.html#wp1142192

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_tech_note09186a00801558cb.shtml

[ikraehenmann]

Thanks for your input. I totally agree with you (although still not confirmed) that the queue size set with hold-queue is for FIFO queueing only.

Priority queueing is in fact an option on the switches, however, it is not configured through the priority lists as on the router.

Igi.