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When IPv6 is performing NDP and the message is Neighbor Solicitation and the observation point is between two routers, what would the IPv6 source address be? The one from the host or the one from the router?

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It depends.

Neighbor Discovery Protocol (NDP) serves various functions on a network. Multiple of those functions require sending a Neighbor Solicitation message, and the structure of that message, and the underlying IP Header are different depending on what specific function NDP is performing.

If NDP is performing Address Resolution (aka, IPv6's version of ARP), the Source IP address of the Neighbor Solicitation message will be the Link-Layer address of the sending host (typically starting with FE80).

If NDP is performing Duplicate Address Detection, the Source IP address of the Neighbor Solicitation message will be the unspecified address: ::

All this is outlined in section 4.3 of RFC 4861.

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The source address is the link local address of the sending host. NDP is not routed, so unless you're on the same subnet, you won't be able to observe it.

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