In the figure, the output of the show ipv6 interface brief command confirms that the correct global IPv6 addresses have been successfully configured and that the interfaces are enabled. Also, notice that each interface automatically generated a link-local address, as highlighted in the figure.
Link-local addresses are automatically created when an IPv6 global unicast address is assigned to the interface. Global unicast addresses are not required on an interface; however, IPv6 link-local addresses are.
Unless configured manually, Cisco routers create the link-local address using FE80::/10 prefix and the EUI-64 process. EUI-64 involves using the 48-bit Ethernet MAC address, inserting FFFE in the middle and flipping the seventh bit. For serial interfaces, Cisco uses the MAC address of an Ethernet interface. Notice in the figure that all three interfaces are using the same link-local address.