DVMRP (Distance Vector Multicast Routing Protocol)

 

DVMRP – Multicast Intra-Domain Routing Protocol

DVMRP (Distance Vector Multicast Routing Protocol) is a multicast routing protocol used within an autonomous system (intra-domain). It is an extension of the RIP distance-vector routing protocol and uses the source-based tree approach.

DVMRP builds the multicast delivery tree in three steps:

1. Reverse Path Forwarding (RPF)

• The router checks the interface through which the shortest path back to the source lies.
• It accepts the multicast packet only if it arrives through this interface.
• Packets arriving through other interfaces are discarded.
• This prevents routing loops and duplicate packets.

2. Reverse Path Broadcasting (RPB)

• RPF alone can cause a LAN connected to multiple routers to receive duplicate copies.
• RPB selects one parent router for each network.
• Only the designated parent router forwards the packet to that network.
• This creates a shortest-path broadcast tree, with the source as the root and networks as leaves.

3. Reverse Path Multicasting (RPM)

• RPB broadcasts the packet to all networks, which is inefficient.
• RPM removes (prunes) branches that do not lead to any member of the multicast group.
• Group membership information is obtained using IGMP.
• Thus, the final tree contains only the branches needed to reach active multicast members.

Simple Flow

DVMRP
  |
  +---- RPF
  |      ↓
  |  Accept packet only
  |  from correct interface
  |
  +---- RPB
  |      ↓
  |  Select parent router
  |  for each network
  |
  +---- RPM
         ↓
     Prune branches
     with no members
         ↓
   Multicast delivery tree

In simple terms

RPF prevents loops → RPB prevents duplicate delivery → RPM removes unnecessary branches.

So, DVMRP efficiently converts the network into a source-based multicast tree containing only the paths needed to reach multicast group members.