Configuring 802.1P Class Of Service; How Class Of Service Works; Port Priority; Port Scheduling - Cisco WS-C2955T-12 Software Manual

Chapter 8 Configuring VLANs

Configuring 802.1p Class of Service

The Catalyst 2900 XL and Catalyst 3500 XL switches provide quality of service (QoS)-based IEEE
802.1p class of service (CoS) values. QoS uses classification and scheduling to send network traffic from
the switch in a predictable manner. QoS classifies frames by assigning priority-indexed CoS values to
them and gives preference to higher-priority traffic such as telephone calls.

How Class of Service Works

Before you set up 802.1p CoS on a Catalyst 2900 XL or Catalyst 3500 XL switch that operates with the
Catalyst 6000 family of switches, refer to the Catalyst 6000 documentation. There are differences in the
802.1p implementation, and they should be understood to ensure compatibility.

Port Priority

Frames received from users in the administratively-defined VLANs are classified or tagged for
transmission to other devices. Based on rules you define, a unique identifier (the tag) is inserted in each
frame header before it is forwarded. The tag is examined and understood by each device before any
broadcasts or transmissions to other switches, routers, or end stations. When the frame reaches the last
switch or router, the tag is removed before the frame is resent to the target end station. VLANs that are
assigned on trunk or access ports without identification or a tag are called native or untagged frames.
For ISL or IEEE 802.1Q frames with tag information, the priority value from the header frame is used.
For native frames, the default priority of the input port is used.

Port Scheduling

Each port on the switch has a single receive queue buffer (the ingress port) for incoming traffic. When
an untagged frame arrives, it is assigned the value of the port as its port default priority. You assign this
value by using the CLI or CMS software. A tagged frame continues to use its assigned CoS value when
it passes through the ingress port.
CoS configures each transmit port (the egress port) with a normal-priority transmit queue and a
high-priority transmit queue, depending on the frame tag or the port information. Frames in the
normal-priority queue are forwarded only after frames in the high-priority queue are forwarded.
Table 8-12
Table 8-12 Transmit Queue Information
1
Transmit Queue Category
Catalyst 2900 XL switches,
Catalyst 2900 XL Ethernet modules
(802.1p user priority)
Catalyst 3500 XL switches, Gigabit
Ethernet modules (802.1p user priority)
1. Catalyst 2900 XL switches with 4 MB of DRAM and the WS-X2914-XL and the WS-X2922-XL modules only have one transmit queue and do not support
QoS.
78-6511-08
shows the two categories of switch transmit queues.
Transmit Queues
Frames with a priority value of 0 through 3 are sent to a normal-priority queue.
Frames with a priority value of 4 through 7 are sent to a high-priority queue.
Frames with a priority value of 0 through 3 are sent to a normal-priority queue.
Frames with a priority value of 4 through 7 are sent to a high-priority queue.
Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
Configuring 802.1p Class of Service
8-31