MDS213 Datasheet, PDF (61/120 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps + 1Gbps Ethernet Switch

MDS213
Data Sheet
frame buffers. The MDS213 features buffer management functions that prevent a single type of traffic from
depleting all frame buffers. The buffer manager limits the number of frames each destination port can store, thereby
preventing congested ports from occupying all the buffers and blocking incoming frames.
The buffer manager examines the destination port of every frame stored, and increments a counter associated with
this destination port. These buffer counters keep track of the number of buffers occupied by frames destined to
each port. If the counter reaches a threshold, incoming frames destined for the associated port will be dropped.
This threshold is programmable via register BCT and BCHL. Register BCT allows the user to program two
thresholds, one high and one low. The user specifies a threshold, high or low, for each port in register BCHL.
The buffer manager also prevents multicast frames from occupying all frame buffers. A programmable threshold,
register MBCR, limits the number of multicast frames stored in memory. In another word, buffers are reserved for
unicast frames.A multicast forwarding job points to a multicast frame in memory fetched and forwarded by the
Frame Engine across the XPipe to the remote device. The Frame Engine can only forward a handful of multicast
frames simultaneously across the XPipe. Excess multicast forwarding jobs are stored in an internal FIFO, called the
MC-Forwarding-FIFO. If the MC-Forwarding-FIFO is full, incoming multicast frames can no longer be forwarded to
the remote device.
The MDS213 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
frames. The system has the flexibility to recognized 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 frames.
17.0 Port Trunking
Port trunking groups a set of 8 MDS213 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. Gigabit ports cannot be trunked.
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.
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