MDS212 Datasheet, PDF (33/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
the other MDS212. The HISC processor can also use this method to make simple edits to the MCT entries for port
changes (i.e. source MAC address is now connected to a different port on the MDS212).
6.3 Flooding and Packet Control
Packets, for which there are no matching destination MCT entries, are by default flooded to all output ports. This
can result in broadcast storms and cause the number of flooded packets to increase rapidly. The MDS212 provides
the user a means for setting a level of flooding, by providing a Flooding Control Register (FCR). The FCR allows the
user to define a time base (100us to 12.8ms) during which packet flooding at each output port will be counted.
Three separate flood control fields allow the user to specify flooding limits for:
• Unicast to Multicast (flooded) packets per source port
• Unicast to CPU packets per chip
• Multicast to CPU packets per chip
During the time base period, three separate counters at each port count the number of packets meeting the flood
control types. Once a counter exceeds the allowed quantity, the Search Engine will then discard the packet and any
other packets of that type that enter the port during the remainder of the time base period. When the time base
period is completed, the three flood counters at each port are reset, and the counting process starts over.
The flooding control register is global for setting the limits on all register ports, but the individual ports have separate
counters to keep track of the number of flooded.
6.4 Packet Filtering
Packet filtering occurs during the address search phase. For static source or destination MAC address filtering,
there is a corresponding bit in the MCT entry that tells the Search Engine that the source or destination packet is to
be filtered.
When a match is found to a destination MAC address search, the “Destination Filter” (D) field in the MCT is
checked to determine if the destination address is to be filtered. If “D” is asserted, the Search Engine discards the
packet by sending a message to the Frame Engine telling it to release the Frame Control Buffer (FCB) where the
packet has been stored in the frame buffer memory. The packet thereby deleted from memory.
When a match is found to a source MAC address search, the “Source Filter” (F) field in the MCT is checked to
determine if the source address is to be filtered. If “S” is asserted, then the Search Engine discards the packet by
sending a message to the Frame Engine telling it to release the FCB for the packet.
6.5 Address Aging
Entries in the MCT database are removed if they have not been used within a user selectable timeframe. This aging
process is handled by inspecting a single MCT entry during each clock period. If the entry is valid and subject to
aging, an aging flag in the MCT entry is cleared. If the aging flag is already set to zero during the inspection, an
aging message is sent to the HISC processor to delete and free up the aged MCT entry. Each time an MCT entry is
matched by way of a Search Engine, source search process, the aging flag is asserted to restart the aging process
for that entry.
Some entries may be static and not subject to aging. These MCT entries have a status field that identifies them as
being static, and will therefore always have their aging flag asserted. The network manager, using Zarlink software,
establishes static entries during a switch configuration session.
6.6 IP Multicast
The Search Engine supports the ability of the MDS212 to provide IP Multicast by identifying Internet Group
Multicast Protocol (IGMP) packets when parsing the packet header information provided by the Frame Engine.
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