A multicast distribution tree represents the path that multicast data takes between the routers that connect sources and receivers. The multicast software builds different types of trees to support different multicast methods.

Because multicast traffic is destined for an arbitrary group of hosts, the router uses reverse path forwarding (RPF) to route data to active receivers for the group. When receivers join a group, a path is formed toward the source (SSM mode) or the RP (ASM or Bidir mode). The path from a source to a receiver flows in the reverse direction from the path that was created when the receiver joined the group.

For each incoming multicast packet, the router performs an RPF check. If the packet arrives on the interface leading to the source, the packet is forwarded out each interface in the outgoing interface (OIF) list for the group. Otherwise, the router drops the packet.

Note	In Bidir mode, if a packet arrives on a non-RPF interface and the interface was elected as the designated forwarder (DF), then the packet is also forwarded in the upstream direction toward the RP.

This figure shows an example of RPF checks on packets coming in from different interfaces. The packet that arrives on E0 fails the RPF check because the unicast route table lists the source of the network on interface E1. The packet that arrives on E1 passes the RPF check because the unicast route table lists the source of that network on interface E1.

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.

Note	In this publication, the term “PIM” is used for PIM sparse mode version 2.

To access multicast commands, you must enable the PIM feature. Multicast is enabled only after you enable PIM on an interface of each router in a domain. You can configure PIM for an IPv4 network. By default, IGMP is 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 multicast routing information is created.

Note	In this publication, “PIM for IPv4” refers to the Cisco NX-OS implementation of PIM sparse mode.

This figure shows two 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 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.

PIM supports these 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.

By default, the Internet Group Management Protocol (IGMP) for PIM is running on the system.

IGMP is used by hosts that want to receive multicast data to request membership in multicast groups. Once the group membership is established, multicast data for the group is directed to the LAN segment of the requesting host.

You can configure IGMPv2 or IGMPv3 on an interface. You have to configure IGMPv3 with (S, G) to support SSM mode. By default, the software enables IGMPv2.

IGMP snooping is a feature that limits multicast traffic on VLANs to the subset of ports that have known receivers. By examining (snooping) IGMP membership report messages from interested hosts, multicast traffic is sent only to VLAN ports that interested hosts reside on. By default, IGMP snooping is running on the system.

Cisco NX-OS provides several methods that allow multicast traffic to flow between PIM domains.

The Cisco NX-OS IPv4 Multicast Routing Information Base (MRIB) is a repository for route information that is generated by multicast protocols such as PIM and IGMP. The MRIB does not affect the route information itself. The MRIB maintains independent route information for each virtual routing and forwarding (VRF) instance in a virtual device context (VDC).

Similar to the MRIB for IPv4 routing information, the M6RIB maintains IPv6 routing information that is generated by protocols such as PIM6 and MLD.

Beginning with Cisco NX-OS Release 10.2(1), Global Boundary Multicast configuration is supported.

You need to configure the {ip | ipv6} multicast group-range prefix-list <prefix-list-name> command in VRF configuration mode to define a global range of IP multicast groups and channels to be permitted or denied for the global multicast boundary. This command is used to disable multicast protocol actions and traffic forwarding for unauthorized groups or channels for all interfaces on a router. The prefix-list configures the boundary. A sample configuration is provided below:

vrf context enterprise
ip multicast group-range prefix-list test

The major components of the Cisco NX-OS multicast software architecture are as follows:

• The Multicast FIB (MFIB and M6FIB) Distribution (MFDM) API defines an interface between the multicast Layer 2 and Layer 3 control plane modules, including the MRIB and M6FIB, and the platform forwarding plane. The control plane modules send the Layer 3 route update using the MFDM API.

  When the Real-time/flex statistics is enabled with the configuration of hardware profile multicast flex-stats-enable command, the MFDM process sends the real-time packet statistics to MRIB.

  Note	The Bytes value is always lower in MRIB output compared to MFDM output although the packets value is roughly the same, as MFDM strips the outer header and sends only the IP datagram to MRIB.

• The multicast FIB distribution process distributes the multicast update messages to all the relevant modules and the standby supervisor. It runs only on the supervisor.

• The Layer 2 multicast client process sets up the Layer 2 multicast hardware forwarding path. It runs on both the supervisor and the modules.

• The unicast and multicast FIB process manages the Layer 3 hardware forwarding path. It runs on both the supervisor and the modules.

  The following figure shows the Cisco NX-OS multicast software architecture.

  

A virtual port channel (vPC) allows a single device to use a port channel across two upstream switches. When you configure a vPC, the following multicast features might be affected:

• PIM—Cisco NX-OS software for the Cisco Nexus 9000 Series switches does not support PIM Bidir on a vPC.

• IGMP snooping—You should configure the vPC peers identically.

It is recommended to configure a snooping querier on a L2 device with lower IP address to force the L2 device as the querier. This will be useful in handling the scenario where multi chassis EtherChannel trunk (MCT) is down.