Layer 2 Marking 8021p CoS

Pream.

SFD

DA

SA

Type

TAG 4 bytes

PT

Data

3 bits used for CoS (802.1p user priority)

PRI CFI VLAN ID

Ethernet Frame

3 bits used for CoS (802.1p user priority)

PRI CFI VLAN ID

802.10/p Header

CoS Typical Application

802.1p User Priority field is also called class of service (CoS).

Different types of traffic are assigned different CoS values.

CoS 6 and 7 are reserved for network use.

802.10/p Header

CoS Typical Application

m

Dl

5 i

Voice Bearer

4

Videoconferencing |

3

Call Signaling |

2

High-Priority Data |

1

Medium-Priority Data

0

Best-Effort Data

QoS Layer 2 classification occurs by examining information in the Ethernet or 802.1Q header, such as destination MAC address or VLAN ID. QoS Layer 2 marking occurs in the Priority field of the 802.1Q header. LAN Layer 2 headers have no means of carrying a QoS value, so 802.1Q encapsulation is required if Layer 2 QoS marking is to occur. The Priority field is 3 bits long and is also known as the 802.1p User Priority or class of service (CoS) value.

This 3-bit field supports CoS values ranging from 1 to 7, 1 being associated with delay-tolerant traffic such as TCP/IP. Voice traffic, which by nature is not delay tolerant, receives higher default CoS values, such as 3 for Call Signaling. A CoS value of 5 is given to voice bearer traffic, which is the phone conversation itself, in which voice quality is impaired if packets are dropped or delayed.

As a result of Layer 2 classification and marking, these QoS operations can occur:

■ Input queue scheduling: When a frame enters a port, it can be assigned to one of a number of port-based queues before being scheduled for switching to an egress port. Typically, multiple queues are used where traffic requires different service levels.

■ Policing: Policing is the process of inspecting a frame to see if it has exceeded a predefined rate of traffic within a certain time frame that is typically a fixed number internal to the switch. If a frame is determined to be in excess of the predefined rate limit, it can either be dropped, or the CoS value can be marked down.

■ Output queue scheduling: The switch will place the frame into an appropriate outbound (egress) queue for switching. The switch will perform buffer management on this queue by ensuring that the buffer does not overflow.

Building Cisco Multilayer Switched Networks (BCMSN) v3.0

© 2006 Cisco Systems, Inc.

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