Posts Tagged ‘ip mroute’


In this short article we’ll take a look at Cisco IOS static multicast routes (mroutes) and the way they are used for RPF information selection. Multicast routing using PIM is not based on propagation of any type of multicast routes – the process that was used say, in DVMRP. Instead, router performs RPF checks based on the contents of unicast routing table, populated by regular routing protocols. The RPF checks could be classified as either data-plane or control-plane. Data-plane RPF check applies when router receives a multicast packet, to validate if the interface and upstream neighbor sending the packet match RPF information. For data-plane multicast, the packet must be received from an active PIM neighbor on the interface that is on the shortest path to the packet source IP address, or RPF check would fail. Control-plane RPF check is performed when originating/receiving control-plane messages, such as sending PIM Join or receiving MSDP SA message. For example, PIM needs to know where to send the Join message for a particular (S,G) or (*,G) tree, and this is done based on RPF lookup for the source IP or RP address. Effectively for PIM, RPF check influences the actual multicast path selection in the “reversed way”: it carves the route that PIM Join message would take and thus affects the tree construction. In both control and data-plane RPF check cases, the process is similar, and based on looking through all available RPF sources.

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