With unicasts, a separate packet must be sent to each destination. In a shared environment, every network device on the segment sees the packet, but only the actual destination processes it. In a switched environment, only devices on the source and destination segments actually see the frame. When a broadcast packet is generated, everyone in the broadcast domain sees this packet and processes it. When a multicast frame is generated, everyone in the broadcast domain sees this packet, but only machines in the group, from zero to everyone, process it. It is sent with a best effort reliability. With Class D addresses, the first four high-order bits are 1110, providing addresses ranging from 224.0.0.0 through 239.255.255.255. To provide a Layer 2 MAC address, part of the corresponding IP multicast address is mapped to the end of a reserved range of MAC addresses: 28 bits. All messages sent to the group address 224.0.0.1 (the all-host group) have their TTL field in the IP packet set to 1. This ensures that the RP does not forward them to other segments. Every IGMPv1 RP on a segment sees the others' query messages. Through this process, the RP with the lowest IP address on the segment is elected as the active querier. An RP, with IGMPv1, discovers an end station has left a group by examining the responses to its queries. In IGMPv2, the end station generates a leave message. DM protocols initially flood the network with multicast traffic and then, based on the discovery of participating end stations, prune back the distribution tree to include only those segments with participating end stations. The RPs use IGMP to discover those end stations. SM protocols use join messages to construct a distribution tree, ensuring that only those segments with participating end stations have traffic forwarded to them by their connected RPs. Therefore, SM protocols scale much better and are more suited for large, geographically dispersed environments. Unlike DM protocols, SM protocols do not waste bandwidth by flooding multicasts everywhere. Traffic is not forwarded to a segment until an end station joins a multicast group. Typically, a rendezvous point is used to disseminate traffic. PIM can coexist with any type of unicast routing protocol, including RIP, IGRP, Enhanced IGRP, and even OSPF. PIM can simultaneously support both dense and sparse modes to more adequately meet multicasting requirements in any network. CGMP (recommended approach) allows Cisco switches to learn from Cisco's IGMP-enabled RPs about the list of end stations participating in the different multicast groups. The switches take this address information and appropriately update their CAM tables. This solution has very little overhead only a minimal amount of management traffic is relayed between the RP and the switch. Unlike IGMP snooping, CGMP has no affect on the Layer 2 switching speeds. ip multicast-routing enables multicasting on an RP. You must also enable multicasting on an interface with ip pim, which also enables IGMP. To view the multicast routing table, use show ip mroute.
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