Users Guide

PIM Sparse-Mode (PIM-SM)

Protocol-independent multicast sparse-mode (PIM-SM) is a multicast protocol that forwards multicast traffic to a subnet only

after a request using a PIM Join message; this behavior is the opposite of PIM-Dense mode, which forwards multicast traffic to

all subnets until a request to stop.

Implementation Information

The following information is necessary for implementing PIM-SM.

• The Dell Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-new-05.

• The platform supports a maximum of 95 PIM interfaces and 2000 multicast entries including (*,G), and (S,G) entries. The

maximum number of PIM neighbors is the same as the maximum number of PIM-SM interfaces.

• The SPT-Threshold is zero, which means that the last-hop designated router (DR) joins the shortest path tree (SPT) to the

source after receiving the first multicast packet.

• Dell Networking OS reduces the number of control messages sent between multicast routers by bundling Join and Prune

requests in the same message.

• Dell Networking OS supports PIM-SM on physical, virtual local area network (VLAN), and port-channel interfaces.

NOTE: Multicast routing is supported across default and non-default VRFs.

Protocol Overview

PIM-SM initially uses unidirectional shared trees to forward multicast traffic; that is, all multicast traffic must flow only from the

rendezvous point (RP) to the receivers.

After a receiver receives traffic from the RP, PM-SM switches to SPT to forward multicast traffic. Every multicast group has an

RP and a unidirectional shared tree (group-specific shared tree).

Requesting Multicast Traffic

A host requesting multicast traffic for a particular group sends an Internet group management protocol (IGMP) Join message to

its gateway router.

The gateway router is then responsible for joining the shared tree to the RP (RPT) so that the host can receive the requested

traffic.

1. After receiving an IGMP Join message, the receiver gateway router (last-hop DR) creates a (*,G) entry in its multicast

routing table for the requested group. The interface on which the join message was received becomes the outgoing

interface associated with the (*,G) entry.

2. The last-hop DR sends a PIM Join message to the RP. All routers along the way, including the RP, create an (*,G) entry in

their multicast routing table, and the interface on which the message was received becomes the outgoing interface

associated with the (*,G) entry. This process constructs an RPT branch to the RP.

3. If a host on the same subnet as another multicast receiver sends an IGMP report for the same multicast group, the

gateway takes no action. If a router between the host and the RP receives a PIM Join message for which it already has a

(*,G) entry, the interface on which the message was received is added to the outgoing interface list associated with the

(*,G) entry, and the message is not (and does not need to be) forwarded towards the RP.

PIM Sparse-Mode (PIM-SM) 569