Are CoS and QoS ever interchangeable

cosqos

The terms CoS & QoS are often used interchangably, particularly in the SP space when providing "Class of Service profiles" etc as part of Product sets. I'm looking to re-affirm my understanding that CoS is just one way in which traffic can be marked at layer 2, whilst QoS determines what you actually do to that traffic at layer 3?

Can anyone provide more real world implementation examples of distinguishing between the two terms or alternatively advise me of any overlap?

Best Answer

QoS is an umbrella term which covers the use of features such as policing, shaping, traffic classification, and advanced queuing mechanisms.

CoS is a form of QoS limited to layer-2 Ethernet and it uses 3-bits (8 values) of the 802.1Q tag to differentiate your traffic. Therefore no trunking, no CoS.

DSCP is the most commonly acted upon value at layer-3 and it is found in 6-bits (64 values) of the IP header. Typically only 14 of the values are used and are referred to using the notations of best effort, expedited forwarding (EF), and assured forwarding (AF). AF values are in the form AFxy, where x is 1-4 and refers to the precedence and y is 1-3 and refers to the drop probability.

Telco vendors (such as AT&T specifically) use the term CoS to define the bandwidth allocation classes that they support in their MPLS offerings. Basically they turned CoS into a marketing term. Typically a Telco uses DSCP to know which of their CoS classes your traffic is supposed to be a part of.

Another important point for configuring these features is that in a Cisco switch or router you need to use a command such as 'mls qos trust dscp' or 'mls qos trust cos' or else the device will reset the QoS marking to zero. There are some exceptions based on model, configuration, and IOS version but this has been true for everything within the last few years in my experience. This configuration is required because the device needs to know which marking you want it to act upon since it cannot use both. I like using DSCP. I haven't found a solid reason to use CoS over it but I'm sure someone has one.

Related Solutions

QoS on HP Procurve 2520 and 3800 switches

If my memory serves, the older Procurve switches used to accept and forward any CoS/DSCP markings they received. You couldn't modify them unless you were on a 5400xl or better switch.

It looks like the 2520 and the 3800 allow you to modify whether or not you're using CoS or DiffServ TOS for classification. The top level command you want is qos type-of-service. The rest of the commands should be contained under that. If they aren't available, you may not be able to modify the QoS settings on that specific platform.

Juniper Destination Based CoS Configuration

Packet classification occurs when the packet enters the router. You're trying to apply it on the forwarding table. That's not supported.

The Juniper documentation shows the packet flow in the router:

Packet flow in MX Routers

So try to apply the policy on the input interface like so:

set interfaces ge-x/x/x.x family inet policer input MARK-POLICY-1

Another way to update a packet in the forwarding-table is to use the classification-override feature:

Overriding the Input Classification

set class-of-service forwarding-policy class POLICY-1 classification-override forwarding-class network-control
set policy-options policy-statement MARK-POLICY-1 term term-1 to as-path BGP-PATH-TO-MARK-COS
set policy-options policy-statement MARK-POLICY-1 term term-1 then class POLICY-1
set policy-options policy-statement MARK-POLICY-1 term term-2 then accept
set routing-options forwarding-table export MARK-POLICY-1

Best Answer

Related Solutions

QoS on HP Procurve 2520 and 3800 switches

Juniper Destination Based CoS Configuration

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