building tunnels between DVMRP-capable machines...

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Smutek to uczucie, jak gdyby się tonęło, jak gdyby grzebano cię w ziemi.

The MBONE is used widely in the research
community to transmit the proceedings of various conferences and to permit desktop conferencing.
Designing Internetworks for Multimedia 13-25
Understanding Multicasting
Multicast OSPF
Multicast OSPF (MOSPF) is an extension of the OSPF unicast routing protocol and works only in
internetworks that use OSPF. OSPF works by having each router in a network understand all of the
available links in the network. Each OSPF router calculates routes from itself to all possible
destinations. MOSPF works by including multicast information in OSPF link-state advertisements
so that an MOSPF router learns which multicast groups are active on which LANs.
MOSPF builds a distribution tree for each source-group pair and computes a tree for active sources sending to the group. The tree state is cached and must be recomputed when a link state change
occurs or when the cache times out.
MOSPF works well in environments that have relatively few source-group pairs active at any given
time. It works less well in environments that have many active sources or in environments that have unstable links.
Protocol Independent Multicast
Unlike MOSPF, which is OSPF dependent, Protocol Independent Multicast (PIM) works with all
existing unicast routing protocols. Unlike DVMRP, which has inherent scaling problems, PIM solves potential scalability problems by supporting two different types of multipoint traffic distribution patterns: dense mode and sparse mode. Dense mode is most useful when the following conditions
occur:
• Senders and receivers are in close proximity to one another.
• There are few senders and many receivers.
• The volume of multicast traffic is high.
• The stream of multicast traffic is constant.
Dense-mode PIM uses reverse path forwarding and is similar to DVMRP. The most significant
difference between DVMRP and dense-mode PIM is that PIM works with whatever unicast protocol
is being used—it does not require any particular unicast protocol.
In dense mode, PIM floods the network and prunes back based on multicast group member
information. Dense mode is effective, for example, in a LAN TV multicast environment because it
is likely that there will be a group member on each subnet. Flooding the network is effective because little pruning is necessary. An example of PIM dense-mode operation is shown in Figure 13-15.
13-26
Cisco CCIE Fundamentals: Network Design
IP Multicast
Figure 13-15
PIM dense-mode operation.
Member of Group 1
Traffic
Prune
Prune
Prune
Member of Group 1
Member of Group 1
Sparse-mode PIM is most useful when the following conditions occur:
• There are few receivers in a group.
• Senders and receivers are separated by WAN links.
• The stream of multicast traffic is intermittent.
Sparse-mode PIM is optimized for environments where there are many multipoint data streams.
Each data stream goes to a relatively small number of the LANs in the internetwork. For these types of groups, reverse path forwarding would make inefficient use of the network bandwidth.
In sparse-mode, PIM assumes that no hosts want the multicast traffic unless they specifically ask for it. It works by defining a rendezvous point (RP). The RP is used by senders to a multicast group to announce their existence and by receivers of multicast packets to learn about new senders. When a sender wants to send data, it first sends the data to the RP. When a receiver wants to receive data, it registers with the RP. Once the data stream begins to flow from sender to RP to receiver, the routers in the path automatically optimize the path to remove any unnecessary hops. An example of PIM
sparse-mode operation is shown in Figure 13-16.
Figure 13-16
PIM sparse-mode operation.
Data
Sender
Rendezvous point
Data
Optimized
path
Receiver
Designing Internetworks for Multimedia 13-27
Network Designs for Multimedia Applications
Note The administrators of the MBONE plan to adopt PIM because it is more efficient than
DVMRP.
Simple Multicast Routing Protocol
Simple Multicast Routing Protocol (SMRP) is a transport layer multicast protocol standard for
multicast AppleTalk and IPX traffic.
Note Initial support for SMRP is provided by Cisco IOS Software Release 11.0 or later for
AppleTalk only.
With SMRP, a router on each local network segment is elected as the primary node. The primary
node handles requests from local devices to create multicast groups on that segment. When it wants to send multicast data, a device sends a Create Group Request packet to ask the primary node to
assign a group address. The primary node responds by sending to the requesting device a Create
Group Response packet that contains the assigned group address.
Devices that want to receive multicast data from this group send a Join Request packet to ask their local router to join the group. The local router forwards the Join Request to the primary node that created the group. The primary node responds by sending a Join Response.
Multicast data sent by the source is forwarded by router downstream interfaces toward receivers.
Receivers can join and leave a group at any time, and a sender can delete the group at any time. The routers ensure that multicast data is transmitted as efficiently as possible, without duplication, from senders to receivers.