Cisco Catalyst 2000 User Manual

222 Cisco LAN Switching Configuration Handbook
0123
Tag Protocol ID (0x8100)
User
VLAN ID (12 bits)
Figure 13-2 802.1Q Trunk Encapsulation Format
Classification
and
Trust
Choose Queue
Ingress Port
or
VLAN
Egress
Port
Policers
Marking Scheduling
Congestion
Avoidance
Compute
Internal
DSCP
CoS
DSCP
IP Prec
Port Default CoS
Markdown
Map
Drop
Choose
Drop Thresholds
CoS
DSCP
IP Prec
Transmit
Frame
Catalyst Switch
Operations on QoS Values
...
Queue 1
Queue 2
Priority
Queue
%
Figure 13-1 Catalyst Switch QoS Operations and Internal DSCP
■ 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.
■ All Catalyst QoS operations are based around the concept of an internal DSCP
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.
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.

Cisco Catalyst 2000 Specifications

General

Brand	Cisco
Model	Catalyst 2000
Category	Switch
Language	English