Figure 24: Determining The Cos Of A Received Frame; Figure 25: Use Of Cos When Forwarding Frames - RuggedCom RuggedSwitch RS1600 User Manual

RuggedSwitch™ User Guide
Note that a frame's CoS will be high if any of the above factors set it high. The
frame's CoS will be low only if all of the above factors set it low.
Received frames are first examined to determine if they are IP frames. If the
frame is IP and the TOS DSCP to CoS feature is selected, the CoS is determined
from the DSCP field itself. The frame is then examined for 802.1P and 802.1Q
tags and the priority field is mapped to a CoS. If a tag is not present the default
CoS for the port is used. The source and destination MAC addresses are then
used to determine if a high CoS has been selected.
Received
Frame
Use TOS
DSCP ?
Y
Map TOS to CoS

Figure 24: Determining The CoS Of A Received Frame

After inspection, the frame is the forwarded to the egress port for transmission.
Forwarding Phase
The inspection phase results in the CoS of individual frames being determined.
When these frames are forward to the egress port they are collected into either a
normal or high priority queue.
CoS weighting selects the degree of preferential treatment that is attached high
CoS frames. The ratio of the number of high CoS to low CoS frames transmitted
can be programmed. If desired, the user can program that low CoS frames are
transmitted only after all high CoS frames have been serviced.
Normal CoS Queue
High CoS Queue

Figure 25: Use of CoS When Forwarding Frames

If the port is configured to transmit VLAN tags and the frame was received
without a tag, the priority field value is selected from the CoS value.
Frame Use default CoS
tagged ? For Port
Y
Priority >
Y
Use High CoS
Threshold ?
Use Normal CoS
CoS
Weighting
Selector
Y
Mac CoS
High ?
Port Frame
tagged ? tagged ?
High Cos?
To CoS Queues
Use High CoS
of other Ports
Leave CoS As Is
Srip Tag
Transmitted
Frame
Leave Tag As Is
Use High Tag
Use Normal Tag
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