These two messages are critical for proper IPv6 operation in any network.
We'll explore those from the command line next. Situation: You and I just pulled
an all‑nighter migrating the rest of the network to IPv6, and our last step
involves converting the Globomantics client machines.
I'll configure a static link‑local address on the
client just to keep things simple.
I'll also be enabling IPv6 ND and ICMP debugging so we
can follow what happens in real time.
This is more output than we are used to, so I'll scroll up first.
In the previous clip, I mentioned that the DAD NS is always the first
message sent by any device when an interface comes up. The ND process
constructs the NS ,and then the ICMP process puts it on the wire. After
exactly 1 second, the IPv6 process declares FE80::4 unique, so the client
can use it as its link‑local address.
Okay, time to scroll back down.
You'll notice other ND messages here,
like the client sending an unsolicited router advertisement or sending a
router solicitation. We'll cover these later since I don't want to
overcomplicate explaining the process. With DAD completed, let's try to
ping the Globomantics router at FE80::1 with one packet. Because this is a
link‑local target, Cisco IOS asks us to specify the outgoing interface
since this target could be adjacent to the client on multiple network
interfaces. Again,
this is a lot of debug, so I'll quickly scroll up so we can examine it.
Now we see another NS message sent from the client to the router, much
like an ARP request, to discover the router's MAC address. This message
is sent towards the solicited node address associated with FE80::1,
which is FF02::1:FF00:1.
That's a mouthful, and we'll revisit it in the NS packet capture.
The router receives the NS and looks inside,
sees that the packet is for itself, and replies with
a neighbor advertisement, or NA.
This NA is similar to an ARP reply.
The message is sent from the Globomantics router to the client as
link‑local unicast from FE80::1 to FE80::4.
Because this was a solicited NA, there is little reason to
multicast it out to the entire LAN segment.
This NA message is the building block of IP MAC resolution for
IPv6 over Ethernet. R1's MAC is 0000.a616.0001.
Let's double‑check that by running a show command.
I'll scroll down first. Quickly checking the ND cache on the
router, the client should have learned R1's MAC address with a
binding to FE80::1. Looks good to me. Let's dig into these packets more.