Product designation
3-32
PHOENIX CONTACT
6874_en_16
3.4.4
Prioritization
The switch supports two priority queues for adjusting the internal packet processing
sequence (traffic classes according to IEEE 802.1D). Data telegrams that are received are
assigned to these classes according to their priority, which is specified in the
VLAN/prioritization tag:
–
Data packets with values between "0" and "3" in the priority field are low (default)
priority.
–
Data packets with values between "4" and "7" in the priority field are transmitted via the
switch with high priority.
In addition, the switch enables port-based prioritization of data streams.
3.4.4.1
VLAN/prioritization tag
The MMS/MCS processes incoming data packets with regard to the prioritization
information contained in the Ethernet packet (VLAN/prioritization tag).
The tag enables the specification of a priority level from 0 to 7, which the switch assigns to
one of its two internal queues. By default upon delivery, the packets with priorities from 0 to
3 are treated as low-priority packets whereas packets with priorities from 4 to 7 are high-
priority Ethernet packets.
The assignment of priority levels for both internal priority levels of the MMS can be modified
via the "dot1dTrafficClassTable" of the P bridge MIB.
Processing rules
The switch controller in the MMS/MCS forwards received packets to one of the receive
queues according to the following decisions:
–
BPDU packets (Spanning Tree, LLDP) and IGMP packets are always assigned to the
high-priority queue.
–
Packets with unknown unicast addresses are always assigned to the low-priority
queue.
–
Packets are assigned to the high-priority queue if the priority from the VLAN/priority tag
is mapped to the "high" level (default priority 4 to 7).
–
The internal port priority "high" results in priority level 7 handling, i.e., the basic settings
for data packet assignment to the high-priority queue are made.
–
All residual data is assigned to the low-priority queue.
3.4.4.2
Port prioritization
In addition to the processing sequence according to the priority information from the tag, the
user can set the internal prioritization for every individual switch port at the MMS/MCS. In
this way, the processing of Ethernet data for a particular port can be prioritized.
The port prioritizing method is suitable for termination devices that do not support tagging
and thus cannot generate priorities.
"High" port prioritization sets the internal priority of the packets received at this port to
priority level 7. In the switch, these packets will be processed with priority information 7
within the tag (preferred handling and forwarding method only within the switch, however,
when forwarding to the receiver, the packets are in the original state). The "Low" port
prioritization means that the priority of packets received at this port is not influenced by the
switch. This implies that existing tags must be taken into consideration or that other priority
rules must be observed.
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