Chapter 7: Class of Service
Class of Service Overview
Mapping Ports to Egress Queues
When the egress queues on a port in an Ethernet switch contains more packets than the port can handle in a timely manner, the port may be forced to delay the transmission of some packets. A port may be forced to delay transmission of packets while it handles other traffic and, in some situations, some packets destined to be forwarded from the port are discarded.
Minor delays are often of no consequence to a network or its performance. But there are applications referred to as delay- or
CoS allows you to manage the flow of traffic through a switch by setting the switch ports to give higher priority to some packets, such as delay- sensitive traffic, over other packets. This is referred to as prioritizing traffic.
CoS applies primarily to tagged packets. A tagged packet contains information that specifies the VLAN to which the packet belongs and can also contain a priority level. Network switches and other networking devices use the priority level to determine how important that packet is compared to other packets. High priority packets are handled before low priority packets.
CoS, as defined in the IEEE 802.1p standard, has eight levels of priority— 0 to 7, with 0 the lowest priority and 7 the highest. Each port has four egress queues, labeled Q0, Q1, Q2, and Q3. Q0 is the lowest priority queue and Q3 is the highest. A packet in a high priority egress queue is typically transmitted out a port sooner than a packet in a low priority queue.
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