Need help with QoS

[ciscostudent1]

Hi, my teacher gave me the task to create a MPLS VPN network. I have successfully done everything he asked for, except for the QoS configuration. Since I'm a beginner I'm not familiar with QoS whatsoever. I have been trying to figure out how to do it for the past couple of days but I can't seem to figure it out so I'd be very glad if someone here could help me out. I'm supposed to configure QoS on both PE and CE routers. The QoS needs to have 3 classes - Real Time, Default and Data. I need to make it so that the CE router switches the DSCP values to EXP values for the PE router, and the other way around for PE - it needs to switch from EXP values to DSCP for the CE router. The topology is as shown below. Any instructions will be appreciated. Thanks in advance!

[Dieselboy]

Hello OP.

Qos comes in a few parts and qos can be completely pointless and/or useless unless it's addressing specific issues. When implementing qos it is done in a few parts, too. Firstly you need to classify the traffic that is to configure what are your classes and what / how the packets are being matched to put them into classes. Then this is wrapped into a policy and the policy applied to an interface direction. You mention conversion so you'll need a qos-map. So for a single interface you might need to create two policies (one in and one out) depending on what you want to do. And there are default qos maps when you enable qos which you can modify to your needs.

Generally qos specifies and gives prioritisation to packets because there is not enough bandwidth somewhere. Maybe the local edge router has gigabit interfaces but the restriction is further down the line. So maybe you want to release packets not at gigabit wire speed but perhaps buffer them and release them slowly at a certain rate so that they dont congest the path but also during the buffer period it gives the router time to slip in higher priority packets. It's important to note that all interfaces have allocated system memory where this packet buffer takes place but bear in mind that on the input direction, the packets have already traversed the link and probably gone through the congested part of the network.

If you have unlimited bandwidth it's likely you dont need any qos at all because qos only defines what packets get priority when packets would be dropped due to congestion. However, this only assumes that there is no way possible that sudden bursts of traffic can reach the maximum link speed even if the average throughput is low.

In terms of classification, sometimes it's done based on subnet address. For example voice traffic might be to and from specific voice subnets on each side. So classification is a bit simple there if you expect only voice traffic to be between those subnets. But in some cases it might be important to classify based on other packet criteria. Only once you classify traffic can you then later specify what to do with it, but you can also match any and all traffic into a class, too.

"end to end qos" is a term I read a lot while studying voice.


Remember it should not be about a right or wrong answer here but your journey and how you got there. Check out this link to Cisco MQC which should get you going. Qos seems difficult at first but it's really quite simple for the most part.

https://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfmdcli.html

[deanwebb]

Most QOS policies I've seen match on protocol type, but destination can also be factored in - bulk internet-bound traffic gets lowest priority in many places because cat videos are somehow not as important as actual work for some reason...

[Dieselboy]

because cat videos are somehow not as important as actual work

Blasphemy