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 puts the RPF interface into the OIF list of the MRIB but not in the OIF list of the MFIB.
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 either 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, the packet is also forwarded in the upstream direction toward the RP.
Figure 1-17 shows an example of RPF checks on packets coming in from different interfaces. The packet that arrives on E2/1 fails the RPF check because the unicast route table lists the source of the network on interface E3/1. The packet that arrives on E3/1 passes the RPF check because the unicast route table lists the source of that network on interface E3/1.
Table 1-19 lists IGMP/MLD default parameters; you can alter these parameters as necessary.
Table 1-19 Default IGMP/MLD Parameters
Table 1-20 lists the default PIM/PIM6 parameters.
Table 1-20 Default PIM/PIM6 Parameters
Note
No license is required for IGMP/MLD.
Because PIMv2 required a license, Table 1-21 shows the required NX-OS feature licenses. For more information, visit the Cisco NX-OS Licensing Guide.
Table 1-21 Feature-Based Licenses for Cisco NX-OS
PIM and PIM6 have the following configuration guidelines and limitations:
Cisco NX-OS PIM and PIM6 do not interoperate with any version of PIM dense mode or PIM sparse mode version 1.
Do not configure both Auto-RP and BSR protocols in the same network.
Configure candidate RP intervals to a minimum of 15 seconds.
If a device is configured with a BSR policy that should prevent it from being elected as the BSR, the device ignores the policy. This behavior results in the following undesirable conditions:
If a device receives a BSM that is permitted by the policy, the device, which incorrectly elected itself as the BSR, drops that BSM so that routers downstream fail to receive it. Downstream devices correctly filter the BSM from the incorrect BSR so that these devices do not receive RP information.
A BSM received by a BSR from a different device sends a new BSM but ensures that downstream devices do not receive the correct BSM.
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