router, mutlicasting

suppose a router is multicast enabled .Suppose it receives a packet
it could be  a mulicastpacket encapsulated within a  normal ip packet(thru tunneling)
or it could be  normal packet.
how does the multicast capab;e router diferentiate.
does it do a decapsulation every time it receives a packet

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The router doesn't care.  If the router can "see" the IP Multicast header of a tunneled datagram, then it's the endpoint of either a GRE or DVMRP tunnel.  The actions taken by a router configured as a tunnel endpoint are completely independent of the payload of the tunneled datagram.  Once the tunnel processing reveals the original IP header, the router hopefully can act on the datagram appropriately.   If there were routers between the two tunnel endpoints that had the capabilities to process the IP multicast datagram, that wouldn't have mattered since those routers would never get a chance to do so.  It is up to the designers of the network to ensure datagrams enter and exit tunnels (are tunnel encapsulated and decapsulated) at the appropriate points in the network to ensure proper IP multicast routing and forwarding takes place.


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