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two questions regarding QoS


(I'm only talking about SERVER to DISK traffic)

Let's say we want to prioritize SERVER-DISK1 traffic over SERVER-DISK2 traffic.

QoS is possible, we simply configure QoS on MDS1 and put SERVER-DISK1 traffic in higher priority than SERVER-DISK2 traffic. No config is necessary on MDS2 (simply enable QoS). With this configuration, QoS will prioritize traffic coming into MDS2, because source and destination ports are the same.


1. What about MDS1? Does marking happen before prioritization? Will this configuration prioritize traffic in the SERVER F_Port VOQs?

2. What if there was a port channel between MDS1 and MDS2, and src-dst load balancing was in effect? Would QoS work on MDS2, if SERVER-DISK1 traffic would always arrive on different port channel member than SERVER-DISK2 traffic? Are there VoQs on Port Channel interfaces?


New Member

Re: two questions regarding QoS

1. marking happens

before scheduler (based on Class/Policy maps), so that scheduler(dwrr) processes the VoQs based on marked priority.

Class maps classifies based on s/did or pwwn or interface.

Policy maps helps to mark based on service levels.

DWRR determines how many times more high queue

is processed during congestion than lower priority


Prioritization of traffic is done at line card level.

2. once the traffic of disk1 and disk2 is in

MDS 2, disk1 traffic from Server will be processed

at default high queue priority, if you have not defined the qos maps/dwrr in MDS 2.

Even if it arrives in different port channel

ports, eventually disk1 and disk2 will contest for

VoQs in the same line card/port as they are AL ports.

Re: two questions regarding QoS

1. Thank you! So the marking happens before scheduling. Therefore SERVER-to-DISK1 traffic would be preferred over SERVER-to-DISK2 traffic in MDS1 switch.

2. Are you 100% sure on this one? I"m only talking about SERVER to DISK traffic. It will arrive from MDS1 to MDS2 across the port channel, traffic will be already marked, but it might arrive on different members of the port channel. I was under impression that in order for QoS to work, competing traffic has to arrive on the same interface and going to the same destination interface. In this case destination interface is the same, the FL port. But! The source interfaces are different, the two different members of port channel. Each physical interface has its own VOQs, only traffic within a single port can be prioritized. Am I wrong?

New Member

Re: two questions regarding QoS

Hi Rodichev,

Regarding for your question two, there are two answers for this, yes and no.

If you are using the first generation line card, you are correct. You can not implement the QoS with port channel since it requires that the traffic for the same destination should come from the same input interface on the MDS2.

However, this restriction does not exist in the second generation line cards. They enforce the QoS on the engress ports and do not require that the traffic for the same destination should come from the same input interface on the switch. So, in your case, you can use port channel or more than one ISL between the MDs 1 and 2.

For detailed explanation, you may want to check the following URL :

I hope it helps,