Pim Sparse Mode; Multicast Source Discovery Protocol (Msdp) - Cisco Catalyst 3850 series Configuration Manual

Protocol Independent Multicast Overview
Dense mode is not often used and its use is not recommended. For this reason it is not specified in the
Note
configuration tasks in related modules.

PIM Sparse Mode

PIM sparse mode (PIM-SM) uses a pull model to deliver multicast traffic. Only network segments with active
receivers that have explicitly requested the data will receive the traffic.
Unlike dense mode interfaces, sparse mode interfaces are added to the multicast routing table only when
periodic Join messages are received from downstream routers, or when a directly connected member is on
the interface. When forwarding from a LAN, sparse mode operation occurs if an RP is known for the group.
If so, the packets are encapsulated and sent toward the RP. When no RP is known, the packet is flooded in a
dense mode fashion. If the multicast traffic from a specific source is sufficient, the first hop router of the
receiver may send Join messages toward the source to build a source-based distribution tree.
PIM-SM distributes information about active sources by forwarding data packets on the shared tree. Because
PIM-SM uses shared trees (at least, initially), it requires the use of a rendezvous point (RP). The RP must be
administratively configured in the network. See the
information.
In sparse mode, a router assumes that other routers do not want to forward multicast packets for a group,
unless there is an explicit request for the traffic. When hosts join a multicast group, the directly connected
routers send PIM Join messages toward the RP. The RP keeps track of multicast groups. Hosts that send
multicast packets are registered with the RP by the first hop router of that host. The RP then sends Join
messages toward the source. At this point, packets are forwarded on a shared distribution tree. If the multicast
traffic from a specific source is sufficient, the first hop router of the host may send Join messages toward the
source to build a source-based distribution tree.
Sources register with the RP and then data is forwarded down the shared tree to the receivers. The edge routers
learn about a particular source when they receive data packets on the shared tree from that source through the
RP. The edge router then sends PIM (S,G) Join messages toward that source. Each router along the reverse
path compares the unicast routing metric of the RP address to the metric of the source address. If the metric
for the source address is better, it will forward a PIM (S,G) Join message toward the source. If the metric for
the RP is the same or better, then the PIM (S,G) Join message will be sent in the same direction as the RP. In
this case, the shared tree and the source tree would be considered congruent.
If the shared tree is not an optimal path between the source and the receiver, the routers dynamically create
a source tree and stop traffic from flowing down the shared tree. This behavior is the default behavior in
software. Network administrators can force traffic to stay on the shared tree by using the ip pim spt-threshold
infinity command.
PIM-SM scales well to a network of any size, including those with WAN links. The explicit join mechanism
prevents unwanted traffic from flooding the WAN links.

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.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
126
Rendezvous Points, on page 129
Configuring PIM
section for more
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