TCP/IPv6 Configuration and Management Manual (G06.24+)

IPv6 Fundamentals
HP NonStop TCP/IPv6 Configuration and Management Manual—524523-008
A-13
Neighbor Discovery Protocol
If a node determines that its tentative link-local address is not unique,
autoconfiguration stops and manual configuration of the interface is required. To
simplify recovery in this case, an administrator should supply an alternate interface
identifier that overrides the default identifier in such a way that the autoconfiguration
mechanism can then be applied using the new (presumably unique) interface identifier.
Alternatively, link-local and other addresses will need to be configured manually.
After a node ascertains that its tentative link-local address is unique, the node assigns
the link-local address to the interface. At this point, the node has IP-level connectivity
with neighboring nodes. The remaining autoconfiguration steps are performed only by
hosts; the (auto)configuration of routers is beyond the scope of this subsection.
The next phase of autoconfiguration involves obtaining a router advertisement or
determining that no routers are present. If routers are present, they send router
advertisements that specify what sort of autoconfiguration a host should do.
Routers send router advertisements periodically, but the delay between successive
advertisements is generally longer than a host performing autoconfiguration wants to
wait. To obtain an advertisement quickly, a host sends one or more router solicitations
to the all-routers multicast group.
Because routers generate router advertisements periodically, hosts continually receive
new advertisements. Hosts process the information contained in each advertisement,
adding to and refreshing information received in previous advertisements.
For safety, all addresses must be tested for uniqueness prior to their assignment to an
interface. In the case of addresses created through stateless autoconfiguration,
however, the uniqueness of an address is determined primarily by the portion of the
address formed from an interface identifier. Therefore, if a node has already verified
the uniqueness of a link-local address, additional addresses created from the same
interface identifier need not be tested individually. In contrast, all addresses obtained
manually should be tested for uniqueness individually.
To speed the autoconfiguration process, a host can generate its link-local address (and
verify its uniqueness) in parallel with waiting for a router advertisement. Because a
router might delay responding to a router solicitation for a few seconds, the total time
needed to complete autoconfiguration can be significantly longer if the two steps are
done serially.
Neighbor Discovery Protocol
This protocol solves a set of problems related to the interaction between nodes
attached to the same link. It defines mechanisms for solving each of these problems:
•
Router discovery
How hosts locate routers that reside on an attached link.
•
Prefix discovery