MDS212 Datasheet, PDF (59/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
The MDS212 has a programmable option to recognize IP multicast (IPMC) frames. By default, IPMC frames are
treated equally with Layer 2 multicast frames. This option gives IPMC privilege, in terms of buffer allocation, over
regular Layer 2 multicast frames. In a broadcast storm, Layer 2 multicast frames are discarded before IPMC ones.
The system has the flexibility to recognize a programmable IPMC MAC address signature, set by registers
IPMCAS0, IPMCAS1, IPMCMSK0, and IPCMMSK1. If a programmable option, DCR2, bit 26, is turned on, the
system reserves space in the MC forwarding FIFO for IPMC frames. This ensures that Layer 2 multicast frames do
not block IPMC ones
17.0 Port Trunking
Port trunking groups a set of 8 MDS212 10/100Mbps physical ports into one logical link; however, all ports in the
trunk group must be within the same access device, and each port can only belong to one trunk group. All ports in
the Trunk group must belong to the same VLAN and share the same MAC Address. Each system can support up to
4 groups.
Load distribution for unicast and multicast traffic is done based on a hash key, a hash function of the Source
Address and the Destination Address.
17.1 Unicast Packet Forwarding
A trunked port will need to have its ECR1 MAC Port Configuration Register set by CPU software to contain its
associated Trunk Group ID. Later on, when a new source MAC Address is learned through that port, the Trunk
Group ID will be recorded in the MCT entry by either the Search Engine or the microcode in the HISC. The Trunk
Group ID will be used for forwarding decision when the destination MCT entry of a received packet is found by the
Search Engine, if the status field indicates that the address found is on a Trunk Group.
The Trunk Group ID is used by the Search Engine, along with the “hash key” (3 bits result of a hash operation
between source address and destination MAC address), to access a Trunk Port Mapping Table entry in the internal
RAM. Each entry in this table contains the device and port IDs for the physical port used to transmit this packet.
Software needs to set these entries, using TPMXR and TPMTD registers, to distribute the traffic load across the
ports in the Trunk Group. If the source MAC Address of an incoming packet is on a Trunk Group (based on the MCT
information), the receiving port’s TGID will be compared against the Trunk Group ID in the source MCT to decide
whether the source MAC address has moved to another Trunk Group or not.
TG provided by
Search Eng
Dev
ID
(1bit)
Port
ID
(4bit)
TG Hash Key
(2bits) (3bits)
.
.
Hash Key=
.
.
.
.
Figure 21 - Port Mapping Table
The Trunk Port Mapping Table is 32 entries deep (4 groups * 8 hash entries), and each entry is 5 bits wide (1-bit
device ID, 4-bit port ID), as show in Figure 21.
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