Hosts can learn the network prefixes for all directly attached links from the router advertisements received from their routers. To determine whether another host is on a directly attached link or on a remote link, determine whether that host's IPv6 address is constructed from a network prefix of one of the directly attached links. If it is on a directly attached link, data can be sent directly to that host without going through a router; otherwise, data must be sent through a router using a default route or an indirect prefix route that can also be learned from router advertisements.

Router advertisements are not a replacement for dynamic routing protocols such as IPv6 OSPF and IPv6 RIP. If a host is not using a dynamic routing protocol, some limitations apply.

If the host has multiple interfaces attached to more than one link, the host must decide which interface to use when sending a packet to a host on a remote link. If there are multiple routers on the link attached to the interface, the host must decide to which router it should send the packet. To make these decisions, the host needs a route in its routing table. When both of the following criteria are true, only default routes are available for accessing a host on a remote link:

• Neither the IPv6 OSPF nor the IPv6 RIP dynamic routing protocol of OMPROUTE is being used.
• Adjacent routers are not including indirect prefix routes (using the Route Information option as described in RFC 4191) in their router advertisement messages.

When there are multiple default routers on the same physical link, the host might select a router that is not optimal. This selection might not be a serious problem, because that router can send an ICMP Redirect, which indicates that future packets should be sent to the optimal router. However, if the default routers are on multiple physical links, the results might be more serious. A router on one link is not able to redirect the host to use a different physical link. If the selected router cannot reach the destination, attempts to send data fail, even if the destination could be reached by a default router on another physical link. To resolve these limitations when you are not using a dynamic routing protocol, static routes might be needed to direct the traffic over the best interface and using the appropriate router.

If a dynamic routing protocol is not used, routes to VIPAs cannot be advertised. For this reason, use a network prefix defined as being on-link for the interfaces that are associated with the VIPA. In this way, routers and hosts perceive that the VIPA is on a physical interface and sends Neighbor Discovery messages (the IPv6 equivalent of an ARP request) to get the MAC address of the interface. This is not the best method for setting up VIPAs if a dynamic routing protocol is being used. It is better to associate VIPAs with interfaces on different LANs. Without a dynamic routing protocol, you can use a network prefix defined as being on-link for the associated interfaces or define static routes at all routers on the same links as the z/OS® system.

See Router advertisements for more information about how received router advertisements are processed.