Administrator Guide

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PIM Sparse-Mode (PIM-SM)
Protocol-independent multicast sparse-mode (PIM-SM) is supported on the MXL switch platform.
PIM-SM is a multicast protocol that forwards multicast trac to a subnet only after a request using a PIM Join message; this
behavior is the opposite of PIM-Dense mode, which forwards multicast trac to all subnets until a request to stop.
Implementation Information
Be aware of the following PIM-SM implementation information.
The Dell Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-new-05.
MXL supports a maximum of 31 PIM interfaces and 2K multicast entries including (*,G), and (S,G) entries. There is no limit on the
number of PIM neighbors MXL can have.
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 rst multicast packet.
The Dell Networking operating system (OS) reduces the number of control messages sent between multicast routers by
bundling Join and Prune requests in the same message.
The Dell Networking OS supports PIM-SM on physical, virtual local area network (VLAN), and port-channel interfaces.
The Dell Networking OS supports 2000 IPv6 multicast forwarding entries, with up to 128 PIM-source-specic multicast (SSM)
neighbors/interfaces.
IPv6 Multicast is not supported on synchronous optical network technologies (SONET) interfaces.
Protocol Overview
PIM-SM initially uses unidirectional shared trees to forward multicast trac; that is, all multicast trac must ow only from the
rendezvous point (RP) to the receivers.
After a receiver receives trac from the RP, PM-SM switches to SPT to forward multicast trac. Every multicast group has an RP
and a unidirectional shared tree (group-specic shared tree).
Requesting Multicast Trac
A host requesting multicast trac 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 trac.
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.
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PIM Sparse-Mode (PIM-SM)