The RP keeps track of multicast receivers. It also registers sources through register messages received from
the source
’
s first-hop router (designated router [DR]) to complete the shared tree path from the source to the
receiver. When using a shared tree, sources must send their traffic to the RP so that the traffic reaches all
receivers.
Prune messages are sent up the distribution tree to prune multicast group traffic. This action permits branches
of the shared tree or SPT that were created with explicit join messages to be torn down when they are no
longer needed.
When the number of PIM-enabled interfaces exceeds the hardware capacity and PIM-SM is enabled with the
SPT threshold is set to
infinity
, the device does not create (source, group (S, G) ) entries in the multicast
routing table for the some directly connected interfaces if they are not already in the table. The device might
not correctly forward traffic from these interfaces.
Multicast Source Discovery Protocol (MSDP)
Multicast Source Discovery Protocol (MSDP) is used for inter-domain source discovery when PIM SM is
used. Each PIM administrative domain has its own RP. In order for the RP in one domain to signal new sources
to the RP in the other domain, MSDP is used.
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.
PIM Stub Routing
The PIM stub routing feature, available in all of the device software images, reduces resource usage by moving
routed traffic closer to the end user.
The PIM stub routing feature supports multicast routing between the distribution layer and the access layer.
It supports two types of PIM interfaces: uplink PIM interfaces and PIM passive interfaces. A routed interface
configured with the PIM passive mode does not pass or forward PIM control traffic, it only passes and forwards
IGMP traffic.
In a network using PIM stub routing, the only allowable route for IP traffic to the user is through a device
that is configured with PIM stub routing. PIM passive interfaces are connected to Layer 2 access domains,
such as VLANs, or to interfaces that are connected to other Layer 2 devices. Only directly connected multicast
(IGMP) receivers and sources are allowed in the Layer 2 access domains. The PIM passive interfaces do not
send or process any received PIM control packets.
When using PIM stub routing, you should configure the distribution and remote routers to use IP multicast
routing and configure only the device as a PIM stub router. The device does not route transit traffic between
distribution routers. You also need to configure a routed uplink port on the device. The device uplink port
cannot be used with SVIs. If you need PIM for an SVI uplink port, you should upgrade to the IP Services
feature set.
You must also configure EIGRP stub routing when configuring PIM stub routing on the device. For information
about this procedure, refer to the
Catalyst 3850 IP Routing Configuration Guide
.
The redundant PIM stub router topology is not supported. The redundant topology exists when there is more
than one PIM router forwarding multicast traffic to a single access domain. PIM messages are blocked, and
the PIM asset and designated router election mechanisms are not supported on the PIM passive interfaces.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
116
OL-29890-01
Configuring PIM
PIM Stub Routing
