Layer 2 Qos Classification And Marking - Cisco Catalyst 2000 Configuration Handbook

222 Cisco LAN Switching Configuration Handbook
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All Catalyst QoS operations are based around the concept of an internal DSCP
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value. This value is determined by an ingress port's trust state and is carried through-
out the QoS process with each frame. Upon egress, the internal DSCP can be used to
mark other QoS values within the frame. The bottom portion of Figure 13-1 shows
the internal DSCP operations.
Ingress Port
or
VLAN
CoS
DSCP
IP Prec
Port Default CoS
Figure 13-1 Catalyst Switch QoS Operations and Internal DSCP

Layer 2 QoS Classification and Marking

At Layer 2, individual frames have no mechanism for indicating the priority or impor-
tance of their contents. Therefore, the delivery of Layer 2 frames must be on a "best
effort" basis.
When virtual LANs (VLAN) are trunked over a single link, however, the trunk provides a
means to carry priority information along with each frame. Layer 2 CoS is transported as
follows:
■ IEEE 802.1Q trunk: Frames are tagged with a 12-bit VLAN ID. The CoS is contained
in the three 802.1p priority bits in the User field. Frames in the native VLAN are not
tagged at all; they are given the default CoS or priority for the switch port. Figure 13-
2 shows the format of the 802.1Q encapsulation tag.
Figure 13-2 802.1Q Trunk Encapsulation Format
Marking: Assigns a value to either the Layer 3 Differentiated Services Code
Point (DSCP), the Layer 2 class of service (CoS), or both for each frame.
Scheduling: Assigns traffic to a specific switch port queue for either ingress or
egress traffic.
Congestion Avoidance: Reserves bandwidth in the switch port queues. Traffic
that exceeds a threshold can be dropped or reduced in priority, making space
for other traffic in the queues.
Classification
and Policers
Trust
Compute
Internal
DSCP
0
Tag Protocol ID (0x8100)
Catalyst Switch
Marking
CoS
Markdown
Map
DSCP
Drop
IP Prec
Operations on QoS Values
1 2
User
Congestion
Scheduling
Avoidance
Choose
Choose Queue
Drop Thresholds
Queue 1
Queue 2
Queue
...
Priority
%
3
VLAN ID (12 bits)
Egress
Port
Transmit
Frame