MDS212 Datasheet, PDF (56/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
dynamic multicast group entries between switches and multicast clients and servers. RFC 1112 specifies the
protocols and behaviour for IP Multicasting.
The MDS212 supports up to 255 IP Multicast Groups and treats them as extensions of the VLAN operation. No
additional hardware is needed, since the IGMP operates on the hardware already provided for VLAN functionality.
IGMP packets are identified by the Frame Engine and are passed to the external CPU for processing, when the
destination MAC address is 01-00-5E-xx-xx-xx and the Protocol field value equals 2, or when the destination IP
address is 224.0.0.x. The external CPU then instructs the HISC to setup IP Multicast entries for the MAC
Addresses in the Switch Database Memory, the VLAN Table, and the MCT-VLAN Table. The HISC builds and
maintains an MCT-VLAN and a VLAN Table for IP Multicast Groups in the Frame Buffer Memory.
When an IP Multicast packet is received, it is identified by a specific class of Multicast Destination MAC addresses,
where the high-order bits indicate use of IGMP, and the low-order bits indicate the specific IGMP Group Identifier.
The MDS212 searches the MCT VLAN Association Table for destination MAC addresses, using the IGMP or the
IGMP Group Identifier stored in the MCT, to obtain port membership for the IP Multicast Group. The Search Engine
forwards the packet to each port associated with the IP Multicast Group. Where no address is found, the HISC
firmware updates the MCT-VLAN to include this address.
The Multicast Buffer Control Register (MBCR) allows the configuration of multicast frames to be forwarded, the
number of buffers reserved for receiving remote multicast frames, the number of multicast frames allowed, and the
multicast forwarding threshold.
15.2 IGMP AND IP Multicast Filtering
IP multicast filtering optimizes switched network performance by limiting multicast packets to only be forwarded to
ports containing multicast group membership instead of flooding all ports in a subnet (VLAN).
The Internet Group Management Protocol (IGMP) runs between hosts and their immediate neighbouring multicast
routers. The mechanism of the protocol allows a host to inform its local router that it wishes to receive transmissions
addressed to a specific multicast group. Routers, also, periodically query the LAN to determine if known group
members are still active.
Based on the group membership information, learned from the IGMP, a router is able to determine which (if any)
multicast traffic needs to be forwarded to each of its “leaf” subnetwork. Multicast routers use this information, in
conjunction with a multicast routing protocol, to support IP multicasting across the Internet.
The MDS212 based switch supports IP Multicast Filtering by passively snooping on the IGMP Query. The IGMP
Report packets are transferred between IP Multicast Routers and IP Multicast host groups to learn the IP Multicast
group members within each VLAN actively sending out IGMP Query messages soliciting IP Multicast group
members. They thus learn the location of multicast routers and member hosts in multicast groups within each
VLAN. Since IGMP is not concerned with the delivery of IP multicast packets across subnetworks, an external IP
multicast router will be needed if the IP multicast packets have to be routed across different IP subnetworks.
15.3 Implementation In MDS212
The MDS212 supports up to 255 IP Multicast Groups and treats them as an extension of the VLAN operation. No
additional hardware is needed, since IP Multicast Switching/Filtering already operates in hardware provided for
VLAN functionality.
IGMP packets are identified by the Search Engine and are passed to the external CPU for processing, when the
destination MAC address is 01-00-5E-xx-xx-xx and the Protocol field value equals 2, or when the destination IP
address is 224.0.0.x. The external CPU then instructs the HISC to setup an MCT entry for this IP Multicast Address
in the Switch Database Memory. If this is a new IP Multicast group, it sets up an entry in the VLAN Port Mapping
Table by itself,
Whenever an IP Multicast data packet (destination MAC = 01-00-5e-xx-xx-xx, and destination IP address is within
the range of 224.0.1.0 and 239.255.255.255) is received, the Search Engine will use the MCT table to look up the
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