Summary
Now that you have completed this chapter, you should a have a firm understanding of the following concepts:
  • Because of its unique capability to send out a single data stream to multiple clients, the multicast method has become the transmission method of choice for most multimedia applications.
  • IP multicasting is the transmission of an IP data frame to a host group, which is identified by a single IP address. Because of the host group is identified by a single IP address rule, the IP multicast contains a specific combination of the destination MAC address and a destination IP address.
  • IGMP provides a way for clients to join and leave multicast groups.
  • CGMP allows the router to configure the multicast forwarding table in the switch to correspond with the current group membership.
  • For efficient transmission, designated routers construct a tree that connects all members of an IP multicast group. A distribution tree specifies a unique forwarding path between the subnet of the source and each subnet containing members of the multicast group.
  • Multicast routing protocols fall into two categories: dense mode (DM) and sparse mode (SM).
  • DM protocols assume that almost all routers in the network will need to distribute multicast traffic for each multicast group. SM protocols assume that relatively few routers in the network will be involved in each multicast. The hosts belonging to the group are widely dispersed, as might be the case for most multicasts in the Internet.
  • The DM protocols are most appropriate in LAN environments with densely clustered receivers and the bandwidth to tolerate flooding.
  • Because SM protocols assume that relatively few routers in each individual network will be involved in each multicast, these protocols are more appropriate in WAN environments.
  • PIM is used between routers so that they can track which multicast packets to forward to each other and to their directly connected LANs.
  • IGMP is used between hosts on a LAN and the router(s) on that LAN to keep track of which multicast groups the hosts are members.
  • CGMP is a protocol used on routers connected to Cisco Catalyst switches to perform tasks similar to those performed by IGMP.
  • On a multi-accessed LAN, one router is selected to poll the LAN for host group membership. However, all PIM routers on a single subnet receive replies from the host. The router selected to poll the LAN is called the designated router (DR). The DR is responsible for sending IGMP host query messages to all hosts on the LAN.
  • An interface can be configured to be in dense mode, sparse mode, or sparse-dense mode. The mode determines how the router populates its multicast routing table and how the router forwards multicast packets received from directly connected LANs. You must enable PIM in one of these modes for an interface to perform IP multicast routing.
  • In sparse-dense mode, the interface is treated as dense mode if no rendezvous point is detected; the interface is treated as sparse mode if a rendezvous point is detected.
  • With the auto-RP mechanism, an RP announce command must be entered on the router to act as an RP for a certain range of multicast group addresses. Multiple RPs can be used to serve different group ranges, or as backups of each other. An RP mapping agent is assigned to a router to receive the RP-announcement messages. The RP mapping agent then sends the consistent group-to-RP mappings to all designated routers. This method allows all designated routers to automatically discover which RP to use for attached receivers and senders.
  • Configuring CGMP on the switch allows IP multicast packets to be switched only to those ports that have IP multicast clients. Directing multicast traffic only to those user segments that have interested clients reduces the consumption of network bandwidth by not propagating IP multicast traffic throughout the broadcast domain. CGMP can only run on an interface if PIM is configured on the same interface.