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Configuration Note 7. 802.1p Priority/Layer 2 QoS
Version 6.8 25 Mediant MSBR
7 802.1p Priority/Layer 2 QoS
Layer 2 QoS is lightweight, easily implemented and independent to Layer 3. Due to its
independence, it can also be applied to non-IP networks where any QoS provisioning is
impossible or very difficult.
Layer 2 Ethernet switches rely on the 802.1p standard to provide QoS. The IEEE 802.1p
standard is a method for assigning priority for network packet transmission. This priority
works with the MAC header at the data link layer (Layer 2 in the OSI reference model).
It uses tagged frames inserted in Ethernet frames after the source address field. One of the
tag fields, the Tag Control Information, is used by 802.1p to differentiate between the
classes of service (CoS).
The 802.1p sets a 3-bit CoS value in the MAC header (when 802.1Q VLAN tagging is
present) to indicate prioritization. This 3-bit value provides priority levels ranging from 0 to
7, with level 7 representing the highest priority. This allows packets to cluster and form
different traffic classes, so that when network congestion occurs, those packets that are
assigned higher priority receive preference, while lower priority packets are queued.
When the priority command is used in MSBR interface, all incoming packets will be marked
with the VLAN priority bit.
Note: CoS only operates on 802.1Q VLAN Ethernet at the data link layer (layer 2), while
other QoS mechanisms (such as DSCP) operate at the IP network layer (layer 3-ToS).
7.1 Commands
The table below describes the 802.1p priority commands.
Table 7-1: 802.1p Priority Commands
Command Description
MSBR# configure data
Enter the data configuration menu.
(config-data)# interface
<physical_interface>
[slot/port.vlanID]
Accesses a specific interface configuration
mode.
• <physical_interface> - selects the type of
interface.
• [slot/port.vlanID] – slot and port number is
taken from the device panel.
Note : The interface configuration mode
changes after the command is entered.
(conf-if-GE SlotNum/PortNum)#
Priority [Number: 0-7]
Configures the priority interface traffic.
•
0 – Low Priority
•
7 – High Priority
7.2 Examples
This example shows how to change priority for a specific physical interface.
MSBR(config-data)# interface fiber 0/3
MSBR(conf-if-GE 0/0)# priority 7