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Chapter 26 Configuring QoS Unde r sta ndi n g QoS
Figure 26-1 QoS Classification Layers in Frames and Packets
All switches and routers that access the Internet rely on the class information to provide the same
forwar din g treatm ent to p ac ke ts with t he sam e class in f orma tion and di fferent treatme nt to p ackets with
different class information. The class information in the packet can be assigned by end hosts or by
switches or routers along the way, based on a configured policy, detailed examination of the packet, or
both. Detailed examination of the packet is expected to happen closer to the edge of the network so that
the core switche s a nd ro ut ers a re not overloaded .
Switches and routers along the path can use the class information to limit the amount of resources
allocated per traffic class. The behavior of an individual device when handling traffic in the DiffServ
architecture is called per-hop behavior . If all de vices along a path provide a consistent per-hop behavior ,
you can construct an end-to-end QoS solution.
Implem ent ing QoS i n you r ne twork c an be a sim ple or c om plex ta sk and d epend s o n th e QoS f eatur es
offered by your inte rne twork ing devices, t he t raffic types and pa ttern s in you r netwo rk, a nd the
granularity of control that you need over incoming and outgoing traffic.

Basic QoS Model

Figure 26-2 shows the basic QoS model. Actions at the ingress interface include classifying traffic,
policing, a nd ma rk ing:
Note If you have the SI instal led on your swi tch, only th e queueing a nd sched uling feat ures are available.
•Classif ying dist ing uis hes o ne kind o f tr affic from ano t her. For m ore in forma ti on, se e the
“Classification” section on pa ge 26-4.
•Policing deter mines whether a packet is in or out of prof ile accordi ng to the config ured poli cer, and
the policer lim its the bandwidth consumed by a flow of traffic. The result of this determ ination is
passed to the marker. For mor e information, see the “Policing and Ma rking” section on page 26-6.
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Encapsulated Packet
Layer 2
header IP header Data
Layer 2 802.1Q/P Frame
Preamble Start frame
delimiter DA
Len
SA Tag PT Data FCS
Layer 3 IPv4 Packet
Version
length ToS
(1 byte) ID Offset TTL Proto FCS IP-SA IP-DA Data
3 bits used for CoS (user priority)
DSCP