MDS212 Datasheet, PDF (48/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
Note that, at the remote device, the frame is written into the transmit FIFO of the remote destination port. To reduce
the latency, the frame is not stored in the FDB of the remote device again.
12.2.3 Multicast Data Frame
In this scenario, we assume that the multicast frame involves both local and remote ports. The received multicast
frame is written to the local FDB by the Frame Engine. After resolving the destinations, the Switch Engine provides
local destination port addresses and remote port addresses to the Frame Engine. If the address resolution cannot
be completed by the Switch Engine, the HISC and/or the CPU are queried.
The Frame Engine pushes the jobs to the corresponding transmission queues (per job per local port). When a local
port is ready for this multicast frame, the Frame Engine moves the frame to the corresponding TXFIFO. There is a
counter to track of the number of copies to be sent. The number is provided by the Search Engine and the frame
engine keeps track of this counter. When a frame is sent, the counter is decreased by one. The FDB will be
released when the counter becomes zero.
When the destination ports include remote ports, the frame is transferred over the XPipe to the remote Frame
Engine, which writes a single copy of it into the remote FDB. That is, we use double store-and- forward for remote
multicast. After receiving the whole frame, the remote Frame Engine utilizes the control information in the internal
header, which indicates the associated destination ports in the remote device to push the jobs into the
corresponding transmission queues. When a port is ready for this multicast frame, the Frame Engine moves the
frame to the corresponding TXFIFO. Similarly, the Frame Engine also keeps track of the number of copies of a
frame to be sent and releases the frame when the counter is reduced to be zero.
12.3 Flow For CPU Control Frame
In a managed system, the CPU may transmit or receive CPU control frames, e.g., Protocols, SNMP frames to/from
a MAC port via a CPU unicast frame. On the other hand, the CPU may receive a multicast frame from a MAC port.
Moreover, the CPU can transmit a multicast frame to multiple ports. The following four scenarios illustrate the four
possible forwarding flows.
12.3.1 CPU Transmitting Unicast CPU Frame
The CPU initiates Unicast control messages, by first writing the frame into the FDB, and then sending a message to
the HISC. The HISC forwards a switch response to the Frame Engine, which transmits the frame to the destination
MAC port. After receiving switch response, Frame Engine performs the same unicast forwarding as for unicast data
frame. Refer to previous subsection for unicast data frame mechanism.
12.3.2 CPU Transmitting Multicast CPU Frame
When the CPU sends a multicast control message to the ports, the CPU first writes the frame to the local FDB. The
CPU then sends a message to the HISC, which provides a switch response message to the local Frame Engine.
After receiving the switch response, the Frame Engine performs the same multicast forwarding as for the multicast
data frame. Refer to previous subsection for multicast data frame mechanism.
12.3.3 CPU Receiving Unicast Frame
The receiving CPU frame is moved to the FDB and the Frame Engine forwards a switch request, including the
frame header, to the Search Engine. After the Search Engine decodes the header and determines to forward it to
the HISC to process, HISC informs the CPU via a mail, which indicates the handle of the FDB. The CPU then
obtains the frame through the MDS212. After reading the frame from the FDB, the CPU will inform the HISC to
release the FDB. Finally, the HISC passes the release command to the Frame Engine to release the FDB
accommodated CPU frame.
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