Layer 2 QoS CoS on 8021QP Ethernet Frame
The IEEE defined the 802.1Q frame for the purpose of implementing trunks between LAN devices. The 4-byte 802.1Q header field that is inserted after the source MAC address on the Ethernet header has a VLAN ID field for trunking purposes. A three-bit user priority field (PRI) is available also and is called CoS (802.1p). CoS is used for QoS purposes; it can have one of eight possible values, as shown in Table 3-2.
|
CoS (bits) |
CoS (in Decimal) |
IETF RFC791 |
Application |
|
000 |
0 |
Routine |
Best-Effort Data |
|
001 |
1 |
Priority |
Medium Priority Data |
|
010 |
2 |
Immediate |
High Priority Data |
|
011 |
3 |
Flash |
Call Signaling |
|
100 |
4 |
Flash-Override |
Video Conferencing |
|
101 |
5 |
Critical |
Voice Bearer |
|
110 |
6 |
(inter-network control) |
|
|
111 |
7 |
Network |
Reserved (network control) |
Figure 3-1 shows the 4-byte 802.1Q field that is inserted into the Ethernet header after the source MAC address. In a network with IP Telephony deployed, workstations connect to the IP phone Ethernet jack (marked PC), and the IP phone connects to the access layer switch (marked Switch).
The IP phone sends 802.1Q/P frames to the workgroup switch. The frames leaving the IP phone toward the workgroup (access) switch have the voice VLAN number in the VLAN ID field, and their priority (CoS) field is usually set to 5 (decimal), which is equal to 101 binary, interpreted as critical or voice bearer.
|
Ethernet 802.1Q/P Frame |
|||||||
|
Preamble |
SFD |
DA |
SA |
802.1Q/P |
Type |
Data |
FCS |
|
TPID 0x8100 16 bits |
PRI a bits |
CFI 1 bit |
VLAN ID 12 bits |
Continue reading here: Layer 2 12 QoS Mpls Exp Field
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