Vlan - Priority/Max (Vlan-Based Layer 2) Qos Mode; Table L-1. 802.1Q To Layer 2 Qos Priority Conversion - Comtech EF Data CDM-570A Installation And Operation Manual

L.1.4 VLAN – Priority/Max (VLAN-based Layer 2) QoS Mode
When selecting VLAN-based Layer 2 QoS, the priority field is extracted from the 802.1q VLAN
tag as shown in Figure L-1. The VLAN tag priority field is 3 bits, so the 802.1q protocol supports
eight priorities, from 0 to 7. However, because Layer 2 QoS has only four priorities as mentioned
previously, as shown in Table L-1 these eight VLAN priorities are automatically mapped to four
internal priories.
Similar to the Port-based scheduler, VLAN-based QoS also observes strict priority-based
scheduling. Should overdriving of VLAN priority traffic occur, the packet will be dropped and drop
stats will be incremented against that port.
Dest MAC
6 bytes
802.1q VLAN Priority Field
Even though it is not necessary to configure multiple ports with VLANs, the VLAN QoS will not
look in the port information when classifying the packets into queue.
In case VLAN Q-in-Q (double VLAN tag), the L2 VLAN QoS considers first VLAN
tag priority field only for traffic prioritization.
Advanced Quality of Service (QoS) Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
SRC MAC
6 bytes
Type(0x8100)
2 bytes
Figure L-1. IEEE 802.1q VLAN Priority

Table L-1. 802.1q to Layer 2 QoS Priority Conversion

7 (b111)
6 (b110)
5 (b101)
4 (b100)
3 (b011)
2 (b010)
1 (b001)
0 (b000)
VLAN Tag Data
Priority CFI VLANid
3bits 1 bit 12 bits
Layer 2 QoS Priority
L–11
Revision 5
FCS
4 bytes
Priority 1
Priority 2
Priority 3
Priority 4
Priority 5
Priority 6
Priority 7
Priority 8
MN-CDM570A