When RP in a domain receives a PIM register message for a new source, with MSDP configured it sends a
new source-active (SA) message to all its MSDP peers in other domains. Each intermediate MSDP peer floods
this SA message away from the originating RP. The MSDP peers install this SA message in their MSDP
sa-cache. If the RPs in other domains have any join requests for the group in the SA message (indicated by
the presence of a (*,G) entry with non empty outgoing interface list), the domain is interested in the group,
and the RP triggers an (S,G) join toward the source.
Sparse-Dense Mode
If you configure either sparse mode or dense mode on an interface, then sparseness or denseness is applied
to the interface as a whole. However, some environments might require PIM to run in a single region in sparse
mode for some groups and in dense mode for other groups.
An alternative to enabling only dense mode or only sparse mode is to enable sparse-dense mode. In this case,
the interface is treated as dense mode if the group is in dense mode; the interface is treated in sparse mode if
the group is in sparse mode. You must have an RP if the interface is in sparse-dense mode and you want to
treat the group as a sparse group.
If you configure sparse-dense mode, the idea of sparseness or denseness is applied to the groups for which
the router is a member.
Another benefit of sparse-dense mode is that Auto-RP information can be distributed in a dense mode; yet,
multicast groups for user groups can be used in a sparse mode manner. Therefore there is no need to configure
a default RP at the leaf routers.
When an interface is treated in dense mode, it is populated in the outgoing interface list of a multicast routing
table when either of the following conditions is true:
•
Members or DVMRP neighbors are on the interface.
•
There are PIM neighbors and the group has not been pruned.
When an interface is treated in sparse mode, it is populated in the outgoing interface list of a multicast routing
table when either of the following conditions is true:
•
Members or DVMRP neighbors are on the interface.
•
An explicit Join message has been received by a PIM neighbor on the interface.
PIM Versions
PIMv2 includes these improvements over PIMv1:
•
A single, active rendezvous point (RP) exists per multicast group, with multiple backup RPs. This single
RP compares to multiple active RPs for the same group in PIMv1.
•
A bootstrap router (BSR) provides a fault-tolerant, automated RP discovery and distribution function
that enables routers and multilayer devices to dynamically learn the group-to-RP mappings.
•
Sparse mode and dense mode are properties of a group, as opposed to an interface.
We strongly recommend using sparse-dense mode as opposed to either sparse mode or
dense mode only.
Note
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
OL-32598-01 127
Configuring PIM
Protocol Independent Multicast Overview
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