zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 24

• Manufacturer Part Number: zl50415
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

5.3
5.3.1
The search block performs source MAC address and destination MAC address (or destination IP address for IP
multicast) searching. As we indicated earlier, if a match is not found, then the next entry in the linked list must be
examined and so on until a match is found or the end of the list is reached.
In tag based VLAN mode, if the frame is unicast and the destination port is not a member of the correct VLAN, then
the frame is dropped; otherwise the frame is forwarded. If the frame is multicast, this same table is used to indicate
all the ports to which the frame will be forwarded. Moreover, if port trunking is enabled, this block selects the
destination port (among those in the trunk group).
In port based VLAN mode, a bitmap is used to determine whether the frame should be forwarded to the outgoing
port. The main difference in this mode is that the bitmap is not dynamic. Ports cannot enter and exit groups
because of real-time learning made by a CPU.
The MAC search block is also responsible for updating the source MAC address timestamp and the VLAN port
association timestamp, used for aging.
5.3.2
The learning module learns new MAC addresses and performs port change operations on the MCT database. The
goal of learning is to update this database as the networking environment changes over time.
When CPU reporting is enabled, learning and port change will be performed when the CPU request queue has
room, and a memory slot is available, and a “Learn MAC Address” message is sent to the CPU. When fast learning
mode is enabled, learning and port change will be performed when memory slot is available and a latter “Learn
MAC Address” message is sent to the CPU when CPU queue has room.
When CPU reporting is disabled, learning and port change will be performed based on memory slot availability only.
In tag based VLAN mode, if the source port is not a member of a classified VLAN, a “New VLAN Port” message is
sent to the CPU. The CPU can decide whether or not the source port can be added to the VLAN.
5.3.3
Aging time is controlled by register 400h and 401h.
The aging module scans and ages MCT entries based on a programmable “age out” time interval. As we indicated
earlier, the search module updates the source MAC address and VLAN port association timestamps for each frame
it processes. When an entry is ready to be aged, the entry is removed from the table, and a “Delete MAC Address”
message is sent to inform the CPU.
Supported MAC entry types are: dynamic, static, source filter, destination filter, IP multicast, source and destination
filter, and secure MAC address. Only dynamic entries can be aged; all others are static. The MAC entry type is
stored in the “status” field of the MCT data structure.
5.3.4
The table below provides a mapping from VLAN ID to VLAN index. It is maintained by system software and is
checked by the hardware search engine for every incoming frame. This table has 4 K entries and is stored in
external SRAM. It is organized as 512  8 entries (total of 4 K VLAN indexes) as shown. Each VLAN index is 8 bits.
Search, Learning, and Aging
MAC Search
Learning
Aging
VLAN Table
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ZL50418
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Data Sheet