Vlan-Based Layer 2 Qos; Figure 20-1. Ieee 802.1Q Vlan Priority - Comtech EF Data CDM-625 Installation And Operation Manual

CDM-625 Advanced Satellite Modem
Quality of Service (QoS)

20.2.4 VLAN-based Layer 2 QoS

When selecting VLAN-based Layer 2 QoS, the priority field is extracted from the 802.1q VLAN tag
as shown in Figure 20-1. 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 20-2 these eight VLAN priorities are automatically mapped to four
internal priories.
Dest MAC
6 bytes
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.
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.
In case VLAN Q-in-Q (double VLAN tag), the L2 VLAN QoS considers first VLAN tag
priority field only for traffic prioritization.
SRC MAC
6 bytes
Type(0x8100)
2 bytes

Figure 20-1. IEEE 802.1q VLAN priority

Table 20-2. 802.1q to Layer 2 QoS Priority Conversion
802.1q VLAN Priority Field
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
Layer 2 QoS Priority
Priority 4
Priority 3
Priority 2
Priority 1
20–6
Revision 13
MN-CDM625
FCS
4 bytes
12 bits