will "[no] mls qos" cause network disruption?

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Apr 23rd, 2009
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Would like to confim the following:

(1) will enabling qos using "mls qos" cause any network disruption?

(2) will disabling qos using "no mls qos" cause any network disruption? (in case we need to fallback)

My client is concerned about enabling/disabling qos may cause network disruption to existing traffic flow due to the re-allocation of buffer at the interface.

My thoughts is that if the link is not congested, no packets will be in the queue, hence enabling/disabling qos should not impact existing traffic flows.

Any comments/thoughts are welcome.


Eng Wee

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Giuseppe Larosa Fri, 04/24/2009 - 11:21
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Hello Eng,

the immediate effect of enabling mls qos is that of seeing all packets QoS settings (DSCP byte) set to zero because by default all ports are not trusted.

see for example


This can be a problem for applications that check the DSCP settings.

This is the aspect to be aware of.

Hope to help


Edison Ortiz Fri, 04/24/2009 - 12:28
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You haven't described the type of switches you are planning to QoS on but if those switches are 6500s, you should be aware that each module type employs a different type of QoS queuing.

If you are etherchanneling and the link members belong to different module types, you may experience a disruption in the etherchannel and an error will come up such as "QoS card-type mismatch".




glen.grant Sat, 04/25/2009 - 04:52
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Yeah 6500 can be a pain as we found out the hard way . We tried to remove some mls qos commands off a single interface and its behavior was to pull it off all the ports in that particular asic which was like 12 ports , broke stuff on those other ports .


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