Pim Modes; Pim Dm; Pim-Sm - Cisco Catalyst 3650 Configuration Manual

Configuring PIM
We recommend that you use PIMv2. The BSR function interoperates with Auto-RP on Cisco routers and
Note
multilayer devices.
When PIMv2 devices interoperate with PIMv1 devices, Auto-RP should have already been deployed. A PIMv2
BSR that is also an Auto-RP mapping agent automatically advertises the RP elected by Auto-RP. That is,
Auto-RP sets its single RP on every router or multilayer device in the group. Not all routers and devices in
the domain use the PIMv2 hash function to select multiple RPs.
Dense-mode groups in a mixed PIMv1 and PIMv2 region need no special configuration; they automatically
interoperate.
Sparse-mode groups in a mixed PIMv1 and PIMv2 region are possible because the Auto-RP feature in PIMv1
interoperates with the PIMv2 RP feature. Although all PIMv2 devices can also use PIMv1, we recommend
that the RPs be upgraded to PIMv2. To ease the transition to PIMv2, we recommend:
• Using Auto-RP throughout the region.
• Configuring sparse-dense mode throughout the region.
If Auto-RP is not already configured in the PIMv1 regions, configure Auto-RP.

PIM Modes

PIM can operate in dense mode (DM), sparse mode (SM), or in sparse-dense mode (PIM DM-SM), which
handles both sparse groups and dense groups at the same time.

PIM DM

PIM DM builds source-based multicast distribution trees. In dense mode, a PIM DM router or multilayer
device assumes that all other routers or multilayer devices forward multicast packets for a group. If a PIM
DM device receives a multicast packet and has no directly connected members or PIM neighbors present, a
prune message is sent back to the source to stop unwanted multicast traffic. Subsequent multicast packets are
not flooded to this router or device on this pruned branch because branches without receivers are pruned from
the distribution tree, leaving only branches that contain receivers.
When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM DM device
detects the new receiver and immediately sends a graft message up the distribution tree toward the source.
When the upstream PIM DM device receives the graft message, it immediately puts the interface on which
the graft was received into the forwarding state so that the multicast traffic begins flowing to the receiver.

PIM-SM

PIM-SM uses shared trees and shortest-path-trees (SPTs) to distribute multicast traffic to multicast receivers
in the network. In PIM-SM, a router or multilayer device assumes that other routers or devices do not forward
multicast packets for a group, unless there is an explicit request for the traffic (join message). When a host
joins a multicast group using IGMP, its directly connected PIM-SM device sends PIM join messages toward
the root, also known as the rendezvous point (RP). This join message travels router-by-router toward the root,
constructing a branch of the shared tree as it goes.
OL-29890-01
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
PIM Modes
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