Specifications

CHAPTER 8

Voice and Video in a Campus Network

n Allows service to be tailored to network needs

n Allows mission-critical applications to share the network with other data

People sometimes think that there is no need for QoS strategies in a LAN. However, switch ports can experience conges-

tion because of port speed mismatches, many people trying to access the switch backbone, and many people trying to

send traffic to the same switch port (such as a server port). Voice and video traffic contends with, and can be affected by,

data traffic within both the WAN and the LAN.

QoS Actions

Three QoS strategies are commonly implemented on interfaces in which traffic enters the switch:

n Classification: Distinguishing one type of traffic from another. After traffic is classified, other actions can be

performed on it. Some classification methods include access lists, ingress interface, and NBAR.

n Marking: At Layer 2, placing an 802.1p CoS value within the 802.1Q frame tag. At Layer 3, setting IP Precedence

or Differentiated Services Code Point (DSCP) values in the packet’s IP header.

n Policing: Determining whether a specific type of traffic is within preset bandwidth levels. If so, it is usually allowed

and might be marked. If not, the traffic is typically marked or dropped. CAR and class-based policing are examples

of policing techniques.

Other QoS techniques are typically used on outbound interfaces:

n Traffic shaping and conditioning: Attempts to send traffic out in a steady stream at a specified rate. Buffers traffic

that goes above that rate and sends it when there is less traffic on the line.

n Queuing: After traffic is classified and marked, one way it can be given special treatment is to be put into different

queues on the interface to be sent out at different rates and times. Some examples include priority queuing, weighted

fair queuing, and custom queuing. The default queuing method for a switch port is FIFO.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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