n IPv6 address configured from this prefix remains in 'preferred' state (see Section 3.5.1) - in this case for 14400 seconds (for four hours). The Source Link-Layer Address option tells to nodes what is the link layer address of the router, and allows nodes to directly update their Neighbor Caches. If the Source Link-Layer Address Option were not included in the Router Advertisement, the nodes would have to perform Neighbor Discovery (and not to pick the link-layer address from Introduction to IPv6 the Ethernet header). The address might be omitted to allow router load balancing, and of course if a link type did not have link-layer addresses, this option would not be present. Initiating DAD for a Global IPv6 Address The unsolicited Router Advertisement received on packet number 7 had an interesting effect on the stack. With the information from the Router Advertisement, the stack was able to configure a global IPv6 address based on the 64-bit prefix received in the PIO of the Router Advertisement and the same IID that was used for the link-local address (see Section 3.5.1). The resulting global IPv6 address was 2001:1 14b8 : 138 : 42 ba27:ebff:fef1:8d0d, and as usual, it was initially in 'tentative' state. RFC 4862 says that DAD must be performed for all autoconfigured addresses. We can see the host stack initiating DAD for the global IPv6 address on packet number 8 illustrated in Figure 3.29. The Neighbor Solicitation message is essentially the same as the previous Neighbor Solicitation sent for the link-local address. Also this Neighbor Solicitation is sent from the unspecified IPv6 address of all zeros, because the DAD for the link-local address is still ongoing. Soliciting for Routers on the Link At packet 9, illustrated in Figure 3.30, the host stack sends an ICMPv6 Router Solicitation message. One might wonder why the Router Solicitation is sent even though a Router Advertisement had (luckily) already been received. The reason is that the host may want to learn whether there are other routers on the same li