- [Instructor] ICMP version six is used extensively
for various services in IPv6,
especially the Neighbor Discovery Protocol, or NDP,
which is also RFC 4861.
IP version four's ARP mechanism is replaced by NDP's
Neighbor Solicitation and Neighbor
Advertisement messages.
Neighbor Solicitation acts like an IPv4 ARP request
asking for the MAC address associated
with a unicast IPv6 address.
The message is sent to the solicited node
multicast address associated with the target address.
Neighbor Advertisements act like an IPv4 ARP reply,
supplying the MAC for a given IPv6 address
to the requesting host.
Neighbor Advertisements can also be sent
without being requested.
These are referred to as
unsolicited Neighbor Advertisements.
These NAs are sent to IPv6 hosts local
scope multicast address of FF02::1.
These NDP entries and stored in the NDP neighbor table
and can be viewed with a show IPv6 neighbors command.
NDP has a couple more message types that can help hosts
find routers to use for default gateways on the LAN.
First is Router Solicitation.
This is a message sent to FF02::2
which is the all IPv6 routers link-local multicast address.
The next is Router Advertisement messages.
This is sent in response to a Router Solicitation
message listing the link-local address of the router.
It can also be sent unsolicited to FF02::1
all IPv6 host address.
More than one prefix and default router can be sent
in a Router Advertisement message.
Remember that while IPv6 doesn't use broadcast
any longer, it does heavily utilize multicast.
FF02::1 effectively replaces broadcast traffic
as it will send to all nodes on the segment.
While IPv6 can use Dynamic Host Configuration
Protocol to find an address, there are other methods
that can be employed to determine
a unique address for a client to use.
The Stateless Address Autoconfiguration, or SLAAC method,
employs a three-step process as follows.
First, employ Neighbor Discovery Protocol
Router Solicitation and Router Advertisement messages
to determine what IPv6 prefix is being used on the LAN.
Second, take the prefix and generate the interface ID
using EUI-64, or a random value.
Third, use Duplicate Address Detection, or DAD,
to determine the address is unique.
The DAD process is used not only when assigning
an address, but anytime an IPv6 interface is activated.
It's actually a fairly clever process.
The device will send out a Neighbor Solicitation message
listing the address it wants to,
or is statically configured to use.
If nothing replies back with a Neighbor Advertisement,
then it's safe to assume no other device on the segment
is using this address.
The Neighbor Discovery Protocol is instrumental
in the function of IPv6,
so be sure to commit its operation to memory.