A bridge port can be configured, for
example, to discard all of the above customers??™ traffic exceeding 50 Mbps, to set the DEI
bit for all frames between 15 Mbps and 50 Mbps, and to clear the DEI bit for frames
under 15 Mbps. Thus, frames are either ???green??? (DEI = 0), ???yellow??? (DEI = 1), or ???red???
(discarded). In a typical implementation, each queue (selected by the priority, not by
the DEI bit) has two thresholds for discarding frames that don??™t fit when the queue is
full??”the real size of the queue is used for green frames, and a smaller size is used for
yellow frames. So yellow frames can only be queued for transmission if the buffer is,
say, less than half full, while green frames can use the whole queue.
With the addition of the DEI, the 15/50 Mbps example works. The traffic from each
customer in excess of 15 Mbps is marked yellow. That traffic can compete for queue
space only to a point; if the queue starts to fill up, only green traffic can pass. This way,
even if alternate frames of a stream are marked yellow, all of the frames not dropped
go to the same queue and are delivered in order.
VID Translation Two different provider networks may need to be connected together so
that both can cooperate in serving some number of customers??™ needs that neither can
satisfy alone.
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