MDS212 Datasheet, PDF (60/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
17.2 Multicast Packet Forwarding
For multicast packet forwarding, the destination device must determine the proper set of ports to transmit the
packet based on the VLAN Index and Hash Key, generated by the source Search Engine. Two functions are
needed to distribute multicast packets to the appropriate destination ports in a Trunk Group:
1. Selecting a Forwarding Port per Trunk Group
Only one port per Trunk Group will be used to forward multicast packets. This can be done with a VLAN INDEX
Table and a Forwarding Port MASK Table set up by CPU.
2. Blocking Multicast Packet Back to the Source Trunk
For multicast forwarding that includes ports in Trunk Groups in the same device as source port, all ports in the same
Trunk Group at the receiving port must be excluded. Otherwise, this multicast packet will be looped back to the
same source Trunk Group. This is achieved through a Trunk Group ID Register that contains 36 bits (36=12x3).
17.2.1 Select One Forwarding Port Per Trunk Group
To forward multicast frames, the Frame Engine retrieves the VLAN member ports from one of the 256 entries in the
VLAN Port Mapping Table (VMAP) as described in the VLAN section. By using the Hash Key and the Forwarding
Port Mask table, the Frame Engine can obtain the corresponding FP Mask. The final forwarding ports can then be
determined by the logical AND of the FP Mask and the VLAN Member Port bit map. The Forwarding Port- Mask
Table must be set by the CPU to THKM[0:7] registers beforehand. The format of this table and the method of setting
it up are shown below.
Forwarding Port Mask Registers
12 bits
3-bit Hash Key
8 entries in table
CPU sets up this table as follows:
1. Set up one entry of the register at a time until table is
exhausted.
2. Set all bits not in any Trunk Group to 1.
3. Set all bits in the Trunk Groups to 0.
4. Pick one forwarding port per Trunk Group and turn the
corresponding bit to 1.
(Each Hash Key may have different forwarding port, the
rule to pick forwarding port is up to CPU.)
VLAN member Port
AND
FP Mask
Forwarding ports
Figure 22 - Forwarding Port Mask Table
Two restrictions exist in setting up tables:
1. When setting up the VLAN Port Mapping Table, all the ports in the Trunk Group must be set to 1, if the VLAN has
ports in any Trunk Group.
2. When setting up the Forwarding Port Mask Table, the CPU software picks only one forwarding port per Trunk
Group.
60
Zarlink Semiconductor Inc.