What happens when a router is configured with the ipv6 unicast-routing global command?

When a router is configured with the ipv6 unicast-routing global command, it enables the router to perform IPv6 routing. This command is essential for a router to participate in an IPv6 network, allowing it to forward IPv6 packets between interfaces.

Enabling ipv6 unicast-routing specifically allows the router to utilize neighboring discovery protocols, such as ICMPv6. This includes the ability to send ICMPv6 Router Advertisement messages. These advertisements are crucial in IPv6 networks as they allow routers to inform devices on the local network about their presence and the parameters for using the network. This is part of the Neighbor Discovery Protocol which helps in the configuration of nodes within the same link.

The other options either reference concepts that do not occur as a direct consequence of configuring ipv6 unicast-routing or deal with IPv4 rather than IPv6 functions. Therefore, stating that the router interfaces begin sending ICMPv6 Router Advertisement messages is correct and reflects the effect of enabling IPv6 unicast routing.