
Customizing DVMRP
117355-A Rev. A 4-7
Shortest-Path Trees
Route information used by DVMRP is independent of any other routing
information used by the router -- for example, routes provided by RIP. The
purpose of this routing information is to create a shortest-path tree entry in the
routing table for the propagation of multicast datagrams.
The shortest-path tree entry indicates the interface that provides the shortest path
to the network that is the source of the multicast datagram.
In F
igure 4-1 on page 4-3, for example, the routing table on Router D includes an
entry describing the shortest-path route to the network connected to Router A. The
entry indicates that Interface d1 provides the shortest path to that source network.
A shortest-path tree also indicates those interfaces that are on the shortest path to
that source network from a neighboring router.
Router E considers the network connecting it to Router D to be on the shortest
path to the source network connected to router A. Router D has an interface (d3)
that is part of the shortest path from Router E from the network connected to
Router A.
If neighboring routers have the same metric to a given source network, the router
with the lower IP address is responsible for propagating multicast traffic
originating from that source network onto the network or tunnel that is common to
these neighboring routers.
A network is considered a leaf network if it has no dependent downstream
neighbors for a source. In F
igure 4-1, the network connected to Router E (Net 5)
is a leaf network.
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