Designated Routers; Designated Forwarders; Asm Switchover From Shared Tree To Source Tree - Cisco Nexus 7000 Series Configuration Manual

Designated Routers

Designated Routers
In PIM ASM and SSM modes, the software chooses a designated router (DR) from the routers on each network
segment. The DR is responsible for forwarding multicast data for specified groups and sources on that segment.
The DR for each LAN segment is determined as described in the Hello Messages.
In ASM mode, the DR is responsible for unicasting PIM register packets to the RP. When a DR receives an
IGMP membership report from a directly connected receiver, the shortest path is formed to the RP, which
may or may not go through the DR. The result is a shared tree that connects all sources transmitting on the
same multicast group to all receivers of that group.
In SSM mode, the DR triggers (*, G) or (S, G) PIM join messages toward the RP or the source. The path from
the receiver to the source is determined hop by hop. The source must be known to the receiver or the DR.
For information about configuring the DR priority, see the Configuring PIM or PIM6 Sparse Mode.

Designated Forwarders

In PIM Bidir mode, the software chooses a designated forwarder (DF) at RP discovery time from the routers
on each network segment. The DF is responsible for forwarding multicast data for specified groups on that
segment. The DF is elected based on the best metric from the network segment to the RP.
If the router receives a packet on the RPF interface toward the RP, the router forwards the packet out all
interfaces in the OIF-list. If a router receives a packet on an interface on which the router is the elected DF
for that LAN segment, the packet is forwarded out all interfaces in the OIF-list except the interface that it was
received on and also out the RPF interface toward the RP.
Note Cisco NX-OS does not support PIM Bidir mode on F2 modules.
Note Cisco NX-OS puts the RPF interface into the OIF-list of the MRIB, but not in the OIF-list of the MFIB.

ASM Switchover from Shared Tree to Source Tree

Note Cisco NX-OS puts the RPF interface into the OIF-list of the MRIB, but not in the OIF-list of the MFIB.
In ASM mode, the DR that is connected to a receiver switches over from the shared tree to the shortest-path
tree (SPT) to a source unless you configure the PIM parameter to use shared trees only. For information about
configuring the use of shared trees only, see the Configuring Shared Trees Only for ASM.
During the switchover, messages on the SPT and shared tree may overlap. These messages are different. The
shared tree messages are propagated upstream toward the RP, while SPT messages go toward the source.
For information about SPT switchovers, see the "Last-Hop Switchover" to the SPT section in RFC 4601.
Cisco Nexus 7000 Series NX-OS Multicast Routing Configuration Guide
70
Configuring PIM and PIM6