D-Link DGS-3308FG Product Manual - Page 58

8-port Gigabit Ethernet Switch User's Guide Multicast Routing Protocols This section gives a brief review of two multicast routing protocols - the Distance Vector Multicast Routing Protocol (DVMRP) and the Protocol Independent Multicast - Dense Mode (PIM-DM). Distance Vector Multicast Routing Protocol (DVMRP) DVMRP was derived from the Routing Information Protocol (RIP). The main difference is the RIP forwards unicast packets based on information about the next-hop (next router) on the path to its destination, but DVMRP constructs delivery trees based on previous-hop (last router) toward the multicast source. DVMRP uses the RPM algorithm. The first multicast packet receive from a given source is flooded to all ports (except the one on which it was received). Prune messages are then used to identify links which do not have group members. These links are then pruned from the delivery tree. A new message is added that allows a previously pruned link to be grafted back onto the multicast delivery tree, to accommodate new group members. Graft messages are forwarded only one hop back toward the multicast source. If there is more than one router on a given subnetwork, the router closest to the multicast source is elected to forward that source's multicast messages. All other routers will drop multicast messages from this source. DVMRP supports tunnel interfaces (that is, interfaces connecting two multicast routers through one or more multicastunaware routers). Each tunnel interface must be configured with the IP address of the local router's tunnel interface and the IP address of the remote router's tunnel interface. Protocol-Independent Multicast - Dense Mode The Protocol-Independent Multicast - Dense Mode (PIM-DM) routing protocol also uses the RPM algorithm for constructing distribution trees, but PIM-DM requires the presence of a unicast routing protocol for finding routes back to the multicast source. PIM-DM is, however, independent of the mechanisms used by the unicast routing protocol. It floods multicast messages until it receives prune messages and also uses graft messages similar to DVMRP. Routing Protocols Routing Information Protocol (RIP) The RIP protocol is a straightforward implementation of distance-vector routing. It partitions participants into active and passive. Active participants advertise their routes to others; passive participants listen to RIP messages and use them to update their routing table, but do not advertise. Only a router can run RIP in active mode; a host must use passive mode. A router running RIP in active mode broadcasts a routing update message every 30 seconds. The update contains a set of pairs, where each pair contains an IP network address and an integer distance to that network. RIP uses a hop count metric to measure distances. The update contains information taken from the router's current routing database. Each update contains a set of pairs, where each pair contains an IP network address and and integer distance to that network. RIP uses a hop count metric to measure distances. In the RIP metric, a router is defined to be one hop from a directly connected network, two hops from a network that is reachable through one other router, and so on. Thus, the number of hops, or hop count, along a path from a given source to a given destination refers to the number of routers that a datagram encounters along a path. Both active and passive RIP participants listen to all broadcast messages, and update their tables according to the distance-vector algorithm described earlier. 48