From: Ramasubramanian Sethuraman (snrmanian@hotmail.com)
Date: Fri Feb 27 2004 - 01:52:32 GMT-3
I have a question regarding 3550 Qos. I read that based on the internal COS
value of the packet, one of the 4 o/p queues is selected. But the COS
value itself is based on the DSCP value of the packet after policiing, which
implies there is a one-to-one correspondence between DSCP and
the selected queue (each queue has unique set of DSCP's)
The point i don't understand is there is also a option(ingress interface)
to map dscp to either of the 2 thresholds for each o/p queue using the
command "wrr-queue dscp-map 1|2 <dscp set>". This tells that
same DSCP can exist on multiple queues which contradicts my earlier
understanding that each queue has a unique set of DSCP's.
Pls clarify if i'm missing something.
This is the sample config from CCO
Switch(config)# interface gigabitethernet0/1 <-- egress
Switch(config-if)# wrr-queue random-detect max-threshold 1 50 100
Switch(config-if)# wrr-queue random-detect max-threshold 2 70 100
Switch(config-if)# wrr-queue random-detect max-threshold 3 50 100
Switch(config-if)# wrr-queue random-detect max-threshold 4 70 100
Switch(config-if)# exit
Switch(config)# interface gigabitethernet0/2 <-- ingress
Switch(config-if)# mls qos trust dscp
Switch(config-if)# wrr-queue dscp-map 1 0 8 16 24 32 40 48 56
Switch(config-if)# wrr-queue dscp-map 2 10 20 30 40 50 60
explanation
---------------
As a result of this configuration, when the queues 1 and 3 are filled above
50 percent, packets with DSCPs 0, 8, 16, 24, 32, 40, 48, and 56 are randomly
dropped. The same packets are randomly dropped when queues 2 and 4 are
filled above 70 percent. When the second threshold (100 percent) is
exceeded, all queues randomly drop packets with DSCPs 10, 20, 30, 40, 50,
and 60.
thanks,
subbu
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