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In RFC 6724 section 5 on "Source Address Selection" rule 2 says to "Prefer appropriate scope":
If Scope(SA) < Scope(SB): If Scope(SA) < Scope(D), then prefer SB and otherwise prefer SA. Similarly, if Scope(SB) < Scope(SA): If Scope(SB) < Scope(D), then prefer SA and otherwise prefer SB.
This rule does not seem to forbid selecting an address that has a smaller scope than the destination address.
Is this ever valid? If yes, can you give an example?
Sure, consider for example a LAN where global scope addresses are managed by DHCPv6 or manually and where the "A" bit is not set in router advertisements so hosts do not auto-configure global addresses (but do get the routes for on-link addresses). Now consider what happens when a host without a global address pings a global address on the same LAN.
