in their local RP cache. The routers and devices select the same RP for a given group because they all use a
common RP hashing algorithm.
Related Topics
Configuring Candidate BSRs (CLI), on page 159
Configuring PIMv2 BSR, on page 154
Example: Configuring Candidate BSRs, on page 184
Restrictions for Configuring Auto-RP and BSR, on page 123
PIM Domain Border
As IP multicast becomes more widespread, the chance of one PIMv2 domain bordering another PIMv2 domain
increases. Because two domains probably do not share the same set of RPs, BSR, candidate RPs, and candidate
BSRs, you need to constrain PIMv2 BSR messages from flowing into or out of the domain. Allowing messages
to leak across the domain borders could adversely affect the normal BSR election mechanism and elect a
single BSR across all bordering domains and comingle candidate RP advertisements, resulting in the election
of RPs in the wrong domain.
Related Topics
Defining the PIM Domain Border (CLI), on page 155
Multicast Forwarding
Forwarding of multicast traffic is accomplished by multicast-capable routers. These routers create distribution
trees that control the path that IP multicast traffic takes through the network in order to deliver traffic to all
receivers.
Multicast traffic flows from the source to the multicast group over a distribution tree that connects all of the
sources to all of the receivers in the group. This tree may be shared by all sources (a shared tree) or a separate
distribution tree can be built for each source (a source tree). The shared tree may be one-way or bidirectional.
Before describing the structure of source and shared trees, it is helpful to explain the notations that are used
in multicast routing tables. These notations include the following:
•
(S,G) = (unicast source for the multicast group G, multicast group G)
•
(*,G) = (any source for the multicast group G, multicast group G)
The notation of (S,G), pronounced “S comma G,” enumerates a shortest path tree where S is the IP address
of the source and G is the multicast group address.
Shared trees are (*,G) and the source trees are (S,G) and always routed at the sources.
Multicast Distribution Source Tree
The simplest form of a multicast distribution tree is a source tree. A source tree has its root at the source host
and has branches forming a spanning tree through the network to the receivers. Because this tree uses the
shortest path through the network, it is also referred to as a shortest path tree (SPT).
The figure shows an example of an SPT for group 224.1.1.1 rooted at the source, Host A, and connecting two
receivers, Hosts B and C.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
134 OL-32598-01
Configuring PIM
Multicast Forwarding
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