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Campus QoS Design

What can somebody tell me about QoS after I passed the Cisco QoS (642-642) exam just one hour ago ?!?!?! ;-). A lot as I noticed from the session.

When designing QoS SLA’s are very important. What are the required latency, jitter and data loss for the different applications, which traffic is really mission-critical and which traffic isn’t?? Remember e-mail is NOT mission-critical.

Voice traffic shouldn’t have more 150ms one-way latency. A call, which traffics the network, will have different kind of latencies, like CODEC, Queuing, Serialization, Propagation and Network Latency.

When designing QoS, classification and marking is the first step taken as closely to the edge as possible. After classifying and marking packets with the correct CoS (Class of Service) and DSCP (Differentiated Services Code Point), you have to configure all uplinks between switches/routers to trust these CoS and DSCP markings.

Policing is one way for congestion management. ISP use policing on customer links. Policing, in an ISP perspective, just drops all traffic about a defined rate (exceeding traffic). With policing you have also the option of marking traffic with different CoS or DSCP values, when exceeding the defined maximum rate. In times of congestion this newly marked packets can be dropped, but when the network isn’t congested, these packets are just allowed through.

Shaping is comparable to policing, but is less aggressive in dropping packets. In cases where packets that exceed a defined maximum rate might be discarded, the sending device may choose just to slow down its sending rate, so that the packets aren’t discarded.

Defining the trust boundary between trusted and untrusted devices is also very important when design QoS implementations. Normally a PC isn’t a trusted devices, so all marking from a PC on packets shouldn’t be trusted. A IP phone normally is a trusted device, so CoS markings from an IP phone should be trusted. QoS can be port-based (default) or VLAN-based. To configure VLAN-based QoS, use the command mls qos vlan-based.

There are a lot more best-practices and considerations when designing and implementing QoS in a network, but this is to much to write down in this blog post. I find QoS very interesting, so if you have any questions about QoS, don’t hesitate to contact me.