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New Member

QOS basics

I am kind of new to QoS, purposefully avoiding it as long as possible. I have read the design guides and have a few questions.

1. Is one way QoS a good thing? Or should it be bidirectional?

  example: I am marking traffic between two hosts as AF21, the traffic from host B to host A gets marked properly, but the return traffic Host A to Host B is marked as default.

2. Should I have to mark up any routing traffic; or should my Cisco routers and switches automatically mark it as Class 6 or 7 for me?

3. Marking source, should I mark at my switches, or should marking traffic at all of my routers (MPLS network) be sufficient?

4. Not marking traffic at all on my core switch, could this cause an issue; currently everything is getting set to Default as it goes across the switch.




1. it depends in what

1. it depends in what direction is congestion

2. it depends how your mpls provider passes the data through its network

New Member

I really don't see congestion

I really don't see congestion on the links. Not what I would suspect to be congestion. But shouldn't the return traffic be just as important as the traffic towards the servers, or more important since it is the data we are really concerned about?


Our two Data centers are the same way, they are connected by a Metro Ethernet connection and traffic is prioritized towards the one data center, but not the other one. Again shouldn't the priority be bi directional?


Super Bronze

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#1 QoS can be a very good thing!  "Or .. bidirectional?"  Unsure why you preface an "or", but bidirectional, ideally, should be used.

#2 Depends on the device and protocol.  Often Cisco devices will mark some.  If not, you can mark.  (Also, you might need to provide [configure] "special" treatment even if the device auto marks.)

#3 The ideal is to mark (and verify) as early is possible.  But it's not required to use QoS.  (You can do QoS without markings.)

#4 Cores often don't mark, at that point within the network, they often "trust".  Cannot say what impact ". . . everything is getting set to Default as it goes across the switch."


BTW, QoS markings are just a "shortcut" when looking at frames/packets for QoS processing.  QoS markings aren't required to provide QoS treatments and QoS markings (alone) don't often guarantee QoS treatment.

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