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.
Note
In this book, we use PIMv2 for IPv4 and PIM6 for IPv6 to refer to the Cisco NX-OS implementation of PIM sparse mode. A PIM domain can include both an IPv4 and an IPv6 network. Figure 1-15 shows two PIM domains in an IPv4 network.
Figure 1-15 PIM Domains in an IPv4 Network
Figure 1-15 shows the following PIM elements:
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 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.
Figure 1-16 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 1-16 PIM6 Domains in an IPv6 Network
Cisco NX-OS supports a combination of three modes for different ranges of multicast groups. You can also define RPF routes for multicast.
Any-source multicast (ASM) is a PIM tree building mode that uses shared trees to discover new sources and receivers as well as source trees to form shortest paths from receivers to sources. The shared tree uses a network node as the root, called the rendezvous point. The source tree is rooted at first hop routers, directly attached to each source that is an active sender. The ASM mode requires an RP for a group range. An RP can be configured statically or learned dynamically by the Auto-RP or BSR group-to-RP discovery protocols. If an RP is learned and is not known to be a Bidir RP, the group operates in ASM mode.
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