Classification and Marking on Cisco Switch

[lagapidis (Lazarus Agapidis)]

Hello Azm

@azmuddincisco

1. By default, a switch does not trust anything and whenever it receives any frame with marking, it erases the marking and put the best effort tag(00) on it before it sends the frame out to a different device. When a switch receives a frame with no tag on it, it does not do anything to that frame meaning tag will be eventually 00/best effort. Please validate that statement.

Yes, you are correct.

@azmuddincisco

2. In order to have a switch trust another device’s marking, the switch needs to be told to trust that. It is enabled by using auto qos trust command. This action can be device specific by using auto qos voip cisco-phone or auto qos video ip-camera command.

Yes. The auto qos trust command will configure the CoS-to-DSCP map while the auto qos voip and auto qos ip-camera (and others) will create detailed class maps and policy maps based on best practices. You can find out more in this excellent Cisco documentation.[quote=“azmuddincisco, post:17, topic:857”]

3. Is there any command to enable qos in a switch globally in MQC model that will analyze the traffic and create class-maps, policies etc. automatically to optimize traffic flow?
   [/quote]
   As far as I know, there is no “silver-bullet” QoS solution for Cisco devices where traffic is analysed and QoS is automatically and dynamically configured. You must specify something (like auto classify, auto qos voip cisco-phone or similar commands) to more specifically indicate to the switch how to implement QoS for specific ports.

@azmuddincisco

4. When Cisco Phones mark traffic, do they mark traffic in the frame level, packet level or in both?

A Cisco phone will mark traffic using both CoS (Layer2) and DSCP (Layer3).

@azmuddincisco

5. What does a router(no qos configuration) do when it receives a packet with marking before it send the packet out to a different device? Does it strip off the QoS marking or it send the packet out as is?

Because CoS information is found in Layer 2 and DSCP information is found in Layer 3, in the scenario that you describe, the CoS information will be stripped away along with the Layer 2 header. The Layer 3 header however will retain the DSCP information and thus will retain the related QoS markings.

@azmuddincisco

6. If QoS is enabled on an access switch to mark traffic, Do the trunk link going to the distribution switch have to have any qos configuration to forward the marked traffic to the upstream distribution switch or it will do automatically?

In order for the access switch to send QoS info to a trusted device on the other end of the trunk, you must apply QoS configuration on the trunk. For example, if you are configuring QoS for VoIP, you need to apply QoS throughout the path that VoIP travels.

@azmuddincisco

7. Does a cisco switch recognize third party vendors’ phones’ marking?

A Cisco switch will recognise QoS markings of 3rd party phones as long as they are using QoS standard markings. However, configurations such as auto qos voip cisco-phone will NOT work correctly because the QoS labels of incoming packets are trusted ONLY when the telephone’s presence is detected using CDP. In order to make non-cisco IP phones function, the ports must be configured manually as if connecting to a trunk port to a switch, and then the command auto qos voip trust must be used as if configuring the QoS on a trunk.

@azmuddincisco

8. mls qos is an old method to configure qos. The modern platform is MQC and it does not support mls qos. Needs validation.

This really depends on the IOS version being used. Some will support both for the purposes of interoperability with older devices, however I’m not sure from which version on the old method is still supported. You will have to check for each IOS version that you are using.

I hope this has been helpful!

Laz