System information

IP Multicast
Book Title
7-102
Multicast Open Shortest Path First
Multicast Open Shortest Path First (MOSPF) is an extension to OSPF, which is a unicast routing
protocol that requires each router in a network to be aware of all 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 states. MOSPF calculates the routes for each source/multicast
group pair when the router receives traffic for that pair. These routes are cached until a topology
change occurs, which requires MOSPF to recalculate the topology.
MOSPF works only in internetworks that are using OSPF and is best suited for environments in
which relatively few source/group pairs are active at any one time. MOSPF performance degrades
in environments that have many active source/group pairs and in environments in which links are
unstable.
Protocol Independent Multicast
Multicast traffic tends to fall into one of two categories: traffic that is intended for almost all LANs
(known as dense) and traffic that is intended for relatively few LANs (known as sparse). Protocol
Independent Multicast (PIM) is an Internet draft (under discussion by the IETF Multicast Routing
Working Group) that has two modes of behavior for the two traffic types: dense mode (DM) and
sparse mode (SM). A router that is running PIM can use dense mode from some multicast groups
and sparse mode for other multicast groups:
• Dense mode—In dense mode, PIM uses reverse path flooding and is similar to DVMRP. One
significant difference between PIM and DVMRP is that PIM does not require a particular unicast
protocol to determine which interface leads back to the source of a data stream. Instead, PIM uses
whatever unicast protocol the internetwork is using.
• Sparse mode—In sparse mode, PIM is optimized for environments in which there are many data
streams but each data stream goes to a relatively small number of the LANs in the internetwork.
For this type of traffic, reverse path flooding wastes bandwidth.
PIM-SM works by defining a rendezvous point. When a sender wants to send data, it first sends to
the rendezvous point. When a host wants to receive data, it registers with the rendezvous point. Once
the data stream begins to flow from the sender, to the rendezvous point, and to the receiver, the
routers in the path optimize the path automatically to remove any unnecessary hops, including the
rendezvous point.
Comparison of Multicast Routing Protocols
Table 7-2 compares the characteristics of the routing protocols when handling multicast traffic.
Table 7-2 Comparison of Multicast Routing Protocols
Protocol
Unicast Protocol
Requirements Flooding Algorithm Environment
DVMRP RIP Reverse path flooding Small
MOSPF OSPF SPF Few senders, stable links
PIM-DM Any RPF Dense distribution pattern
PIM-SM Any None Sparse distribution pattern