Cisco NX-OS supports multicasting with Protocol Independent Multicast (PIM) sparse mode. PIM is IP routing protocol independent and can leverage whichever unicast routing protocols are used to populate the unicast routing table. In PIM sparse mode, multicast traffic is sent only to locations of the network that specifically request it. PIM dense mode is not supported by Cisco NX-OS.
In this publication, the term “PIM” is used for PIM sparse mode version 2.
To access multicast commands, you must enable the PIM or PIM6 feature. Multicast is enabled only after you enable PIM or PIM6 on an interface of each router in a domain. You configure PIM for an IPv4 network and PIM6 for an IPv6 network. By default, IGMP and MLD are running on the system.
PIM, which is used between multicast-capable routers, advertises group membership across a routing domain by constructing multicast distribution trees. PIM builds shared distribution trees on which packets from multiple sources are forwarded, as well as source distribution trees, on which packets from a single source are forwarded.
The distribution trees change automatically to reflect the topology changes due to link or router failures. PIM dynamically tracks both multicast-capable sources and receivers, although the source state is not created in Bidir mode.
The router uses the unicast routing table and RPF routes for multicast to create multicast routing information. In Bidir mode, additional routing information is created.
In this publication, “PIM for IPv4” and “PIM6 for IPv6” refer to the Cisco NX-OS implementation of PIM sparse mode. A PIM domain can include both an IPv4 and an IPv6 network.
The figure below shows two PIM domains in an IPv4 network.
Figure 6. PIM Domains in an IPv4 Network
The lines with arrows show the path of the multicast data through the network. The multicast data originates from the sources at hosts A and D.
The dashed line connects routers B and F, which are Multicast Source Discovery Protocol (MSDP) peers. MSDP supports the discovery of multicast sources in other PIM domains.
Hosts B and C receive multicast data by using the Internet Group Management Protocol (IGMP) to advertise requests to join a multicast group.
Routers A, C, and D are designated routers (DRs). When more than one router is connected to a LAN segment, such as C and E, the PIM software chooses one router to be the DR so that only one router is responsible for putting multicast data on the segment
Router B is the rendezvous point (RP) for one PIM domain and router F is the RP for the other PIM domain. The RP provides a common point for connecting sources and receivers within a PIM domain.
This figure shows two PIM6 domains in an IPv6 network. In an IPv6 network, receivers that want to receive multicast data use the Multicast Listener Discovery (MLD) protocol to advertise requests to join a multicast group. MSDP, which allows for discovery of multicast sources in other PIM domains, is not supported for IPv6. You can configure IPv6 peers and use Source-Specific Multicast (SSM) and Multiprotocol BGP (MBGP) to forward multicast data between PIM6 domains.
Figure 7. PIM6 Domains in an IPv6 Network
PIM supports three multicast modes for connecting sources and receivers:
Any source multicast (ASM)
Source-specific multicast (SSM)
Bidirectional shared trees (Bidir)
Cisco NX-OS supports a combination of these modes for different ranges of multicast groups. You can also define RPF routes for multicast.