Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 276
DVMRP Tables

When an IP multicast datagram is received by a router running DVMRP, it first looks up the source network in its DVMRP unicast routing table . That’s right! In order to ensure that all DVMRP routers have a consistent view of the unicast path back to a source, a unicast routing table is propagated to all DVMRP routers as an integral part of the protocol. This is a separate unicast routing table for DVMRP. Actually, there are two tables used in multicast routing: a routing table and a forwarding table. The DVMRP routing table contains Source Subnets and From Gateways. It has the shortest-path source-rooted spanning tree to every participating source subnet in the internet. Compare this with entries from a typical IGP table such as RIP, which contains Destinations and Next-Hop Gateways. The forwarding table is created because the routing table is not aware of group memberships. The following table shows a simple DVMRP routing table.

A simple DVMRP routing table.

Source subnet: The subnetwork that contains a host source.

Subnet mask: The subnet mask of the Source Subnet.

From-gateway: The immediate upstream router that leads back to the source subnet.

Metric: hops (routers) to the source subnet.

TTL (Time to Live): Indicates how long the entry in the table stays before being removed.

A DVMRP forwarding table.

Any interface that is designated a leaf network, or any downstream routers for the source group will be included in the downstream ports. The upstream port is determined by the unicast routing table in that if this interface has the shortest route back to the source subnet of this group, it is registered as the upstream interface with that port designation. The forwarding table is created to represent the local router’s understanding of the shortest-path source-rooted delivery tree for each (source, group) pair.