Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix M M–5 MN-CDM-425
M.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 M-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 M-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 M-1. IEEE 802.1q VLAN priority
Table M-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.