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Additionally, the network where the receivers are located is an Ethernet switched LAN, in which two layer 2 multicast features, must be considered: IGMP Snooping and Multicast VLAN.IGMPVersionThe first decision to be made is which version of IGMP will be implemented: IGMPv2 or IGMPv3.IGMPv3 should be used when source-specific multicast is required. This version matches adequately with PIM-SSM.If source-specific multicast is not required, then, IGMPv2 is simpler to implement and maintain. Additionally, not all layer 2 switches support IGMPv3 snooping.IGMP QuerierIf there are multiple IPv4 gateways in a receivers’ subnet, two or more routers can be enabled with IGMP.In any case, only one IGMP querier (Designated Router) can be active at any time. Anyway, there is no configuration necessary for this feature to work. If there is a need to force one of these routers to become the querier, it has to be configured the smallest IP address in the subnet.
HP Network Design Principles3 – 2Rev. 10.41IGMP SnoopingIGMP assumes that both queries and reports are received by all stations in the subnet – it is a single Ethernet collision domain. So by default, Ethernet switches are designed to treat multicast traffic as if it were broadcast.To avoid multicast traffic to be flooded, Ethernet switches include the IGMP snooping feature. It is recommended to implement everywhere.Multicast VLANWhen multiple VLANs are transported between a layer 2 switch and the IGMP querier and there are receivers for a multicast group in more than one VLAN, multiple copies of the multicast traffic are forwarded by the querier.To avoid this, a Multicast VLAN can be configured in the Layer 2 switch. This feature takes IGMP reports from a receiver’s VLAN and moves it to the Multicast vlan. In this way, the IGMP querier forwards a single multicast flow into this special VLAN. The layer 2 switch, then, copies this traffic into each one of the receiver’s VLANs.There are two types of Multicast VLANs:1.port-based multicast VLANs2.subnet-based multicast VLANsBecause the end result is the same, any one of these options can be selected.Multicast Routing: PIMThe main advantage of PIM is that it uses the underlying IPv4 unicast routing infrastructure to build the multicast distribution tree. This feature is called Reverse Path Forwarding or RPF. When planning multicast routing the first step is to select the version of PIM to be used, also called PIM Mode: dense or sparse.PIM-DM or dense mode works well in LAN-only multicast applications and PIM-SM is better for large multi-site networks. PIM-SM also allows for the configuration of multiple multicast domains, which can be useful for networks with many remote sites. In this last case, PIM-SM domains can be connected with MSDP (Multicast Source Discovery Protocol). MSDP build a network of RPs and allows for traffic to be forwarded from one domain to another while still having local RPs.
Multicast DesignRev. 10.413 – 3If PIM-DM is chosen, there is need for further planning.