Managing IPv4 and IPv6 Multicast 1135To minimize the repeated flooding of datagrams and subsequent pruningassociated with a particular source-group (S,G) pair, PIM-DM uses a StateRefresh message. This message is sent by the router(s) directly connected tothe source and is propagated throughout the network. When received by arouter on its RPF interface, the State Refresh message causes an existingprune state to be refreshed. State Refresh messages are generated periodicallyby the router directly attached to the source.What Is DVMRP?DVMRP is an interior gateway protocol that is suitable for routing multicasttraffic within an autonomous system (AS). DVMRP should not be usedbetween different autonomous systems due to limitations with hop count andscalability.DVMRP exchanges probe packets with all its DVMRP-enabled routers, itestablishes two-way neighboring relationships, and it builds a neighbor table.DVMRP exchanges report packets and creates a unicast topology table, withwhich it builds the multicast routing table. This table is used to route themulticast packets. Since every DVMRP router uses the same unicast routingprotocol, routing loops are avoided.Understanding DVMRP Multicast Packet RoutingDVMRP is based on RIP; it forwards multicast datagrams to other routers inthe AS and constructs a forwarding table based on information it learns inresponse. More specifically, it uses this sequence.• A new multicast packet is forwarded to the entire multicast network, withrespect to the time-to-live (TTL) of the packet.• The TTL restricts the area to be flooded by the message.• All routers that do not have members on directly-attached subnetworkssend backPrune messages to the upstream router.NOTE: In addition to DVMRP, the switch supports the Protocol-IndependentMulticast (PIM) sparse-mode (PIM-SM) and dense-mode (PIM-SM) routingprotocol. Only one multicast routing protocol can be operational on the switch atany time. If you enable DVMRP, PIM must be disabled. Similarly, if PIM is enabled,DVMRP must be disabled.
