CANOGA PERKINS CanogaOS Configuration Guide - Manual (Page 266)

CanogaOS Configuration Guide
Proprietary & Confidential Canoga Perkins Metro Ethernet Switches Page 266 of 350
Class of Service (CoS) is a 3-bit value used to classify the priority of Layer-2 frames upon entry
into a network.
QoS classifies frames by assigning priority-indexed CoS values to them, and gives preference to
higher-priority traffic.
Layer-2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS
value in the 3 most significant bits, called the User Priority bits. On interfaces configured as
Layer-2 802.1Q trunks, all traffic is in 802.1Q frames, except for traffic in the native VLAN.
Other frame types cannot carry Layer-2 CoS values.
CoS values range from 0 to 7, 7 being the highest priority.
DSCP Value
Differentiated Services Code Point (DSCP) is a 6-bit value used to classify the priority of
Layer-3 packets upon entry into a network.
DSCP values range from 0 to 63, 63 being the highest priority, 0 being best-effort traffic.
IP- Precedence Value
IP-Precedence is a 3-bit value used to classify the priority of Layer-3 packets upon entry into a
network.
IP-Precedence values range from 0 to 7, 7 being the highest priority.
EXP Value
EXP value is a 3-bit value used to classify the priority of MPLS packets upon entry into a
network.
MPLS EXP values range from 0 to 7, 7 being the highest priority.
Classification
Classification distinguishes one kind of traffic from another by examining the fields in the
packet. The process generates an internal priority for a packet, which identifies all future QoS
actions to be taken on the packet.
Each packet is classified upon entry into the network. At the ingress, the packet is inspected, and
the priority is determined based on ACLs or the configuration. The Layer-2 CoS value is then
mapped to a priority value.
The classification is carried in the IP packet header using 6 bits or 3 bits from the deprecated IP
TOS field to carry the classification information. Classification can also occur in the Layer-2
frame.
Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is
globally disabled, thus, no classification occurs.