MOSPF (Multicast Open Shortest Path First) definition

MOSPF (Multicast Open Shortest Path First) is an extension to the OSPF (Open Shortest Path First) protocol that facilitates interoperation between unicast and multicast routers. MOSPF is becoming popular for proprietary network multicasting and may eventually supersede RIP (Routing Information Protocol).

Here's a brief explanation of how MOSPF works: Multicast information goes out in OSPF link state advertisements (LSA). That information allows a MOSPF router to identify active multicast groups and the associated local area networks (LANs). MOSPF creates a distribution tree for each multicast source and group and another tree for active sources sending to the group. The current state of the tree is cached. Each time a link state changes or the cache times out, the the tree must be recomputed to accomodate new changes.

MOSPF uses both source and destination to send a datagram, based on information in the OSPF link state database about the autonomous system's topology. A group-membership-LSA makes it possible to identifiy the location of each group member. The shortest path for the datagram is calculated from that information.

MOSPF was designed to be backwards-compatible with non-multicast OSPF routers for forwarding regular unicast traffic.

This was first published in July 2008

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