Comtech EF Data CDM-625 - Manual (Page 438)

CDM-625 Advanced Satellite Modem Revision 15
Quality of Service (QoS) MN-CDM625
20–6
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
SRC MAC
6 bytes
Data
FCS
4 bytes
Type(0x8100)
2 bytes
Priority
3bits
CFI
1 bit
VLANid
12 bits
VLAN Tag
Figure 20-1. IEEE 802.1q VLAN priority
Table 20-2. 802.1q to Layer 2 QoS Priority Conversion
802.1q VLAN Priority Field Layer 2 QoS Priority
7 (b111)
Priority 4
6 (b110)
5 (b101)
Priority 3
4 (b100)
3 (b011)
Priority 2
2 (b010)
1 (b001)
Priority 1
0 (b000)
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.