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

Chapter 277
DVMRP Route Tables

To build a unicast route table, the upstream DVMRP router is dependent on other downstream routers for information. The information that is sent to other DVMRP routers is called a route report. The metrics in the route reports are the most important fields of the report. This not only builds the source subnetwork table (indicating the source subnetworks and their reachability), but also allows for the building of a forwarding table that indicates to the router which downstream routers are depending on that upstream router for forwarding multicast datagrams to them. Upstream routers send route reports to their downstream neighbors indicating source subnets and their metrics. Like RIP, the metric to a source subnet is the cumulative cost of all the incoming interfaces so far. The route reports will be sent to a DVMRP’s neighbor router. Contained in this list are source subnets and metrics (in the range of 1–31).

If a downstream router wishes to indicate to an upstream router that it is dependent on it for receiving multicast datagrams for a particular source subnet, that downstream router will echo the route back to the upstream neighbor with a metric higher than 32. Infinity for DVMRP is considered to be 32. Therefore, the downstream neighbor will add 32 to the incoming metric and echo this back to the upstream router. This relies on a technique known as poison reverse. When the upstream router receives this update, and sees the metric for the source subnetwork in the range between infinity to twice infinity, then the upstream router will add the downstream router to a list of dependent routers for that source. The value of infinity is 32 and indicates that a source network is not reachable. The range of metrics may be between 1–63. The original metric of the source is 1–31, 32 means not reachable, and 33–63 is the poison reverse metric of a downstream router telling its upstream router that it wants to be added to its table for multicast datagrams of a given source.

Why not just use the existing unicast routing table like a RIP2 table? The reason is that not all routers will be running DVMRP and multicast routers must be able to interact with nonmulticast routers. In order to accomplish this, we must build tunnels across nonmulticast routers. With tunnels, we effectively force the path that the multicast datagram will take. The tunnel may take one route, but a regular unicast packet may take another route path. With this, a router’s unicast table may not coincide with a DVMRP router’s unicast table. Therefore, we use the unicast information in DVMRP exclusively to determine the shortest route back to the source subnet of a multicast datagram.