DELL N3000 User Manual

Managing IPv4 and IPv6 Multicast

The Protocol Independent Multicast protocol is a simple, protocol-

independent multicast routing protocol. PIM uses an existing unicast routing 

table and a Join/Prune/Graft mechanism to build a tree. Dell Networking 

series switches support two types of PIM: sparse mode (PIM-SM) and dense 

mode (PIM-DM). 
PIM-SM is most effective in networks with a sparse population of multicast 

receivers. In contrast, PIM-DM is most effective in networks with densely 

populated multicast receivers. In other words, PIM-DM can be used if the 

majority of network hosts request to receive a multicast stream, while PIM-

SM might be a better choice in networks in which a small percentage of 

network hosts, located throughout the network, wish to receive the multicast 

stream.

PIM-SM is used to efficiently route multicast traffic to multicast groups that 

may span wide area networks and where bandwidth is constrained. PIM-SM 

uses shared trees by default and implements source-based trees for efficiency. 

PIM-SM assumes that no hosts want the multicast traffic unless they 

specifically ask for it. It initially creates a shared distribution tree centered on 

a defined “rendezvous point” (RP) through which source traffic is relayed to 

the ultimate receiver. Multicast traffic sources first send the multicast data to 

the RP, which in turn sends the data down the shared tree to the receivers. 

Shared trees centered on an RP do not necessarily provide the shortest or 

most optimal path. In such cases, a Dell Networking PIM-SM router adjacent 

to the host switches to the shortest path upon seeing the very first multicast 

data packet.
Many IP multicast applications, such as those that handle real-time 

dissemination of financial information, require high performance. Multicast 

group membership management (IGMP), unicast routing protocols (OSPF, 

RIP), and multicast routing protocols are all required to enable end-to-end 

multicast capabilities. The RP is a critical function for PIM-SM deployments. 

RP redundancy is always recommended. In a shared-tree model, multicast 

traffic from the multicast source is routed via the RP. If the RP goes down, the 

multicast receivers do not receive traffic until the RP comes up again. In 

general, more than one RP is configured (for a group range) to provide RP 

redundancy. The PIM-SM router acting as a BSR advertises the list of