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.
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