MOSPF (Multicast Open Shortest Path First)

 

MOSPF – Multicast Link-State Routing

MOSPF (Multicast Open Shortest Path First) is an extension of OSPF for multicast routing. Like DVMRP, it uses the source-based tree approach, but it uses link-state information and Dijkstra's algorithm.

Four main steps

1. Create a shortest-path tree
   • The router uses its Link-State Database (LSDB) to know the complete network topology.
   • Using Dijkstra's algorithm, it creates a shortest-path tree with the source S as the root.
2. Create a subtree
   • The router identifies its position in this source-rooted tree.
   • It considers the portion of the tree from the source toward itself and the networks beyond it.
3. Prune the tree
   • Initially, the subtree represents a broadcast tree.
   • Using IGMP group-membership information, MOSPF removes branches that do not lead to members of multicast group G.
   • MOSPF also floods membership information to other routers using a special link-state update.
4. Forward the multicast packet
• The router forwards the packet only through interfaces belonging to the remaining multicast tree.
• Therefore, networks without group members do not receive unnecessary copies.

Simple Flow

          MOSPF
            |
      LSDB + Dijkstra
            ↓
   Source-based shortest
       path tree
            ↓
     Find router's subtree
            ↓
   IGMP membership information
            ↓
       Prune branches
      without members
            ↓
     Multicast tree
            ↓
    Forward to required
     group members only

DVMRP vs MOSPF

DVMRP	MOSPF
Extension of RIP	Extension of OSPF
Distance-vector based	Link-state based
Uses RPF, RPB, RPM	Uses LSDB + Dijkstra
Source-based tree	Source-based tree
Uses IGMP for membership	Uses IGMP + link-state membership updates

In short:

MOSPF uses the complete network topology from the LSDB and Dijkstra's algorithm to build a source-based shortest-path tree, then prunes branches without multicast members to obtain the multicast tree.