Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 291
Multicast Open Shortest Path First (MOSPF)

Review RFC 1584. If you are not familiar with the OSPF (RFC 1583) protocol, please review the section on that protocol. There are assumptions about that protocol that are made here.

Modifications have been made to the OSPF routing protocol that have enabled the protocol to route IP multicast datagrams. There are three types of routing provided: intra-area routing, inter-area routing, and inter-autonomous system routing.

Intra-area routing is the most basic routing algorithm provided. It runs in a single OSPF area and supports the forwarding of multicast datagrams within a single area. This could be a single area in a multiple area of an OSPF autonomous system, or it could be a single autonomous system when there is only one area in the OSPF topology.

Inter-area routing is an OSPF topology that is split into several routing areas connected through a common area known as the backbone area. Decisions on forwarding multicast datagrams are still determined as in the intra-area routing; the information contained in the forwarding cache is used. The difference between the two is the method of forwarding group membership information and the method of constructing the inter-area multicast tree. Selected Area Border Routers (ABRs) are configured to perform a function known as inter-area multicast routers. These routers are responsible for the forwarding of group membership information and multicast datagrams between areas.

Multicast Open Shortest Path First (MOSPF)

•  An extension of OSPF [RFC 1584] March 1994.
•  A new LSA [group-membership-LSA] is used to describe the location of multicast destinations.
•  A multicast packet’s path is calculated using a shortest-path tree based on the IP datagram’s source and destination.
•  It is not based on a Broadcast and Prune
•  Multicast Hosts join/leave via IGMP [RFC 1112] facilities.
•  Branches of the tree not containing multicast members are pruned from the tree.
•  Vendor specific implementations can do “route pruning” for better member administration.
•  Three types of routing are provided:
•  Intra-area, inter-area, and inter-autonomous system routing

Inter-autonomous routing involves a source and destination path that is outside at least one autonomous system (AS). Selected Autonomous System Boundary Routers (ASBRs) are selected as inter-AS multicast forwarders. MOSPF makes the assumption that each inter-AS multicast forwarder is running a multicast routing protocol (such as PIM or DVMRP) that uses the RPF forwarding mechanism. This is the method used by MOSPF to leave its AS and route to another AS that could be running another routing protocol, or to get across the unicast Internet (since MSOPF does not support tunnels).


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