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Quality of service

carl_townshend
Spotlight
Spotlight

Hi all, with COS, does this actually provide you with a mechanism to prioritize the traffic as it stands for class of service, not quiality of service, and this all happens at layer 2 ?

19 Replies 19

mohammedmahmoud
Level 11
Level 11

Hi,

Layer 2 Classification and Marking uses the CoS 3 User Priority bits which resides in an Ethernet Frame Header (inside the 802.1Q Tag Field or the ISL header) -> 3 bits -> 8 values - The same as IP Precedence - Layer 2 to layer 3 marking mapping is applicable on Cisco switches.

HTH,

Mohammed Mahmoud.

can we actually prioritize the traffic at layer 2 using cos then ?

Hi,

Yes, and also we can map the cos to the DSCP in order to be further used on the layer 3 devices in the path.

HTH,

Mohammed Mahmoud.

thanks for that

so, what is the way of identifying this traffic at layer 2, and also how is it normally done, do you basically identify the traffic then apply a cos value to it ? which gives it priority ?

Pavel Bykov
Level 5
Level 5

QoS is actually a whole system of traffic flow architecture.

CoS marks are part of QoS, and can be used for QoS operations like congestion management (prioritizing traffic) or congestion avoidance (dropping aggressive traffic) just like other markings.

CoS only IDENTIFIES traffic. QoS doesn't do anything to it until you tell it to.

I thought that you can prioritize using cos ? so what is the layer 2 version of qos, can anyone explain to me what services apply at layer 2 and layer 3 then, im confused here?

many thanks

Hi,

CoS is used for layer 2 marking (User Priority bits which resides in an Ethernet Frame Header - inside the 802.1Q Tag Field or the ISL header), while IPP/DSCP are used for layer 3 marking (inside the IP header).

Layer 2 devices can take actions based on CoS (although newer Cisco switches understand also layer3 QoS), and layer 3 devices can take actions based on IPP and DSCP.

HTH,

Mohammed Mahmoud.

thanks for that

So , can you tell me firstly, how the switch identifies the traffic, and second, how it applies the policy to it ?

Hi Carl,

During classification, the switch performs a lookup and assigns a QoS label to the packet, after a packet is classified and has a DSCP-based or CoS-based QoS label assigned to it now the network device can act upon these values.

The following document is perfect for your question:

http://www.cisco.com/en/US/products/ps6406/products_configuration_guide_chapter09186a008055fca1.html#wp1020443

HTH,

Mohammed Mahmoud.

Hi

So soon as the switch has classified the traffic, by assigning say priority 5, does the switch automatically then prioritize it ? what happens once it has been given the priority ?

Hi,

Classification and Marking just classifies the traffic and tags it, what ever QoS configuration you need further can be configured based on this classification and marking, but it must be explicitly configured, please refer to the examples in the document i've provided you above.

I hope that i've been informative, please do come back if you have further questions.

HTH,

Mohammed Mahmoud.

Hi there

So are you saying once you have actually tagged the traffic the switch or router wont actually do anything? what needs to be done after then ?

cheers

Carl

Hi Carl,

What you need to do then is to configure the required QoS tools (Policing, shaping, ...), while referencing to the earlier marking.

HTH,

Mohammed Mahmoud.

For router to do something with it, you need to use L3 marking. Switches have COS>DSCP maps, if you are using COS. This map converts COS (Layer 2) to DSCP (Layer3) marking, and enables device to do something with it.

But COS, DSCP, IPP are all just bits - they are marks. No device does anything to them, unless you tell it to. (Except default behaviour for routing traffic in routers, and queues in switches, that are hardware dependant).

So for basic QoS you need to classify and mark traffic (like mentioned CoS, DSCP etc), and create policy for every marking.

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