MDS212 Datasheet, PDF (32/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
The search process begins when the Frame Engine transfers the first 64 bytes of a packet header to the Search
Engine. These bytes are parsed to extract the information needed to perform the search for the MCT entries that
match the source and destination MAC address, generate the search hash keys, and lookup VLAN membership
and other packet status information.
6.1 Layer 2 Search Process
When the MDS212 is in either a “forwarding” state (able to forward packets) or a “learning” state (able to learn new
addresses), the Search Engine is capable of performing address searches. The search process begins when
packet header information is transferred to the Search Engine from the Frame Engine.
The Search Engine first checks to determine if the MDS212 is configured to support Virtual Local Area Networks
(VLAN). If VLANs are enabled, the Search Engine will search for both the destination MAC address, to get
destination resolution information, and the source MAC address, to get the port’s VLAN membership and verify the
validity of the port’s VLAN membership. If VLANs are disabled, the Search Engine will search for the destination
and source MAC addresses but will not do a VLAN table check.
6.1.1 VLAN Unaware
When VLANs are not enabled or configured, the Search Engine will search the internal switch database memory for
an MCT that matches the destination MAC address. When a match is found, the Search Engine will check to
ensure that the destination address is not to be filtered before sending a search result message back to the Frame
Engine to start the packet forwarding process. At the same time, a search for the MCT that matches the source
MAC address is also performed. If no match is found for the source address, then the source MAC address needs
to be learned.
6.1.2 VLAN Aware
When VLANs are enabled and configured, the Search Engine will begin searching for the destination MCT and the
source MCT. If a matching MCT is found for the source address, then no learning is required, and the Search
Engine will check the VLAN membership of the source port. If the source port is a member of the VLAN, and the
destination port is also a member of the VLAN, then a normal response message will be passed to the Frame
Engine. If the source port is not a valid member of the VLAN, or the destination port is not a member of the VLAN,
then the Search Engine will decide to forward the packet or drop the packet depending upon a user defined
configuration. Then it will send a message to the HISC to allow the HISC to resolve the issue.
6.2 Address and VLAN Learning
Address learning can be performed by either the HISC or the Search Engine and can be enabled or disabled. The
global learning control is set in the Device Configuration Register (DCR2). The Global Learning Disable (GLN) bit
controls whether learning is active or disabled, and can be set during initial power up configuration, or by an
external CPU before it begins modifying the SDM. It is necessary for an external CPU to disable learning before
updating or modifying MCT entries. This prevents the internal learning process from modifying MCT entries without
the CPU’s knowledge.
When learning is globally enabled, by the Search Engine not finding a match to a source address search, it can
create a new MCT with the necessary information, and then notify the HISC that a new address has been learned.
If the Search Engine request queue becomes 3/4th full, the Search Engine will ignore address learning until the
request queue is less full. In that case, packets are forwarded as usual, and a message is sent to the HISC
requesting that the HISC learn the new address. If the Search Engine request queue is too full, and the HISC
request queue is full, then no learning will take place.
When two MDS212 chips are connected, and configured to operate with synchronized MCT entries, the HISC
processor has the ability to send a request to the Search Engine, instructing it to learn a new address received from
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