Multicast Extensions to OSPF (MOSPF) provides enhancements to OSPF Version 2 to support IP multicast routing. The enhancements have been added in a backward-compatible fashion; routers running the multicast additions will interoperate with non-multicast OSPF routers when forwarding regular (unicast) IP data traffic.

MOSPF works by including multicast information in OSPF link state advertisements. 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 trees must be recomputed when a link state change occurs or when the cache times out.

MOSPF provides the ability to forward multicast datagrams from one IP network to another through internet routers. MOSPF forwards a multicast datagram on the basis of both the datagram"s source and destination. The OSPF link state database provides a complete description of the Autonomous System"s topology. By adding a new type of link state advertisement, the group-membership-LSA, the location of all multicast group members is pinpointed in the database. The path of a multicast datagram can then be calculated by building a shortest-path tree rooted at the datagram"s source. All branches not containing multicast members are pruned from the tree. These pruned shortest-path trees are initially built when the first datagram is received. The results of the shortest path calculation are then cached for use by subsequent datagrams having the same source and destination.

MOSPF is used internal to a single Autonomous System. When supporting IP multicast over the entire Internet, MOSPF would have to be used in concert with an inter-AS multicast routing protocol such as DVMRP.

Routers running MOSPF works only in internetworks that are using but can be intermixed with non-multicast OSPF routers. Both types of routers can interoperate when forwarding regular (unicast) IP data traffic. In MOSPF, just as in the base OSPF protocol, datagrams (multicast or unicast) are routed "as is" -- they are not further encapsulated or decapsulated as they transit the Autonomous System.

Protocol Structure

The MOSPF packet formats are the same as for OSPF Version 2. One additional option has been added to the Options field that appears in OSPF Hello packets, Database Description packets and all link state advertisements. This new option indicates a router"s/network"s multicast capability. The presence of this new option is ignored by all non-multicast routers.

• T-bit C describes the router's TOS capability.
• E-bit C AS external link advertisement are not flooded into/through OSPF sub areas. The E-bit ensures that all members of a stub area agree on that are's configuration.
• MC-bit C describes the multicast capability of the various pieces of the OSPF routing domain.

To support MOSPF, one of OSPF"s link state advertisements has been modified, and a new link state advertisement has been added. The format of the router-LSA has been modified to include a new flag indicating whether the router is a wild-card multicast receiver.

The rtype field in the router LSA:

• bit B - B is for border . When set, the router is an area border router. These routers forward unicast data traffic between OSPF areas.
• bit E - E is for external. When set, the router is an AS boundary router (). These routers forward unicast data traffic between Autonomous Systems.
• bit V - V is for virtual. When set, the router is an endpoint of an active virtual link which uses the described area as its Transit area.
• bit W - When set, the router is a wild-card multicast receiver. These routers receive all multicast datagrams, regardless of destination. Inter-area multicast forwarders and inter-AS multicast forwarders are sometimes wild-card multicast receivers.

A new link state advertisement, called the group-membership-LSA, has been added to pinpoint multicast group members in the link state database. This new advertisement is neither flooded nor processed by non-multicast routers.

Related Terms: IP, TCP, OSPF, IGMP

Sponsor Source: MOSPF is defined by IETF (http://www.ietf.org) in RFC 1584.

Reference:
http://www.javvin.com/protocol/rfc1584.pdf: Multicast Extensions to OSPF
http://www.javvin.com/protocol/rfc1585.pdf: MOSPF: Analysis and Experience