Information Technology Reference
In-Depth Information
In a router, it is not possible for (S,G) entries to exist without a (*,G). However, it is possible
for (*,G) entries to exist without an (S,G). Traffic is forwarded using a matching (S,G) entry
if it exists, because it is more specific than the (*,G) entry.
Shared Tree
The
shared tree
is a multicast distribution tree that is not specific to a single source. It is
used by any source that does not have its own source-specific distribution tree. To build
a shared tree, there must be a device that is the shared root for the tree, as shown in
Figure 11-4.
Figure 11-4
Shared Tree Distribution Tree
Source
Receiver
Root
Source
Receiver
The shared tree is built from the receivers to the root. However, the traffic from the sources
must arrive at the shared root before it can be forwarded down the shared tree. This topic is
covered in greater detail during the discussion of Protocol-Independent Multicast Sparse
Mode (PIM-SM). Traffic arrives at the router inbound from the interface that faces back
toward the shared root. Traffic is delivered to the rest of the MDT ports other than the arrival
interface.
