Applied CBWFQ to ATM pvc, but QoS not working, see config

Unanswered Question


I have applied CBWFQ config to 7200 router ATM pvc, but when I send 2 test files of 200Mbit from the LAN port over the ATM pvc, one from class LBE (lower than best effort using CS1) and one from class default, QoS is not working.

Looking at the stats using 'policy-map int atm 1/0.188 vc 100/192' the packets have been transmitted in class CS1 ok, but it also appears that no packets/bytes have been matched.

Please see my cut down config, showing the 2 classes, policy map and show commands.

Has anybody got any ideas what I have done wrong?

Would appreciate some help. Thanks in advance.



I have this problem too.
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John Patrick Lopez Wed, 02/07/2007 - 19:41

I think if you need strict prioritization with your QoS, you should use Low Latency Queueing. With CBWFQ, the QoS will not take effect until the link is congested. It seems that you have a lot of bandwidth and the link is not yet congested that's why it's not in effect.

"To characterize a class, you assign it bandwidth, weight, and maximum packet limit. The bandwidth assigned to a class is the guaranteed bandwidth delivered to the class during congestion."



mheusinger Thu, 02/08/2007 - 06:22


looks like you did not overload the interface.

CBWFQ will only care about packets QUEUED, i.e. if no overload condition exits, then no packets are "matched". TCP will adjust to the available bandwidth ... thus the interface might not have been overloaded. Basically then you do not test your CBWFQ config.

On the other hand, 200 MB and 18 seconds give about 90 Mbps!?! So I am not sure about your measurements.

First imho you should check, whether a SINGLE download could potentially fill the pipe, i.e. achieve the full throughput of 25 Mbps.

Additionally UDP flooding the link would make sure there is an overload condition.

Regards, Martin

Hi Martin,

I'm keeping you busy at the moment, hope you are well.

The files I sent were 200Mbit not Bytes in size. Thus 2 x 200Mbit files sent together down 25Mbit line arriving in 18sec is reasonable.

I have already tried the same policy on a serial line and tested with suitable size files (via FTP), and it all tested as expected.The TCP windowing did not affect my results.

Thanks again



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