Selected node multicast, why not use link local instead?

Just finished watching the IPv6 videos from Keith's CCNA course, and I feel like I have a decent handle on NDP but this seems odd to me:

 

After a host recieves a router advertisement which is sourced by the downstream router with its link local address for that segment, why doesn't the host just use the router's link local address in the neighbor solicitation that follows? It's more specific, and to do otherwise the host has to break the router's link local down and convert it to create the selected node multicast address. 

 

I'm sure there's a good reason, but it seems like an unnecessary additional step. Why would we prefer multicast in this scenario over unicast?

 

Thanks!

Comments

  • Great question. IPv6 is very different from IPv4. IPv4 was all about unicast and in the days of creation of IP routing, unicast was the primary need to accomplish the desired task. In this day in age, with all the routes going on, multicast is becoming more and more necessary. I am finalizing my CCNA but I notice that Multicast is a HUGE part of cisco networking and i can imagine for a good purpose.

  • leminhleminh ✭✭

    Hi bdseeliger,
    The host doesnt need to use neighbor solicitation to router because Router Advertisement packets contain link-layer addresses for the router.

    There is no need for the node receiving a Router Advertisement to send out an additional ARP request (as an IPv4 node would have to do) to get the link-layer address for the router interface.

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