Two down, two to go. Next, we'll explore the router
solicitation and router advertisement messages.
IPv6 hosts tend to send RS messages when their interfaces first come
online just after DAD completes. In the previous NS and NA demo, we
quickly observed the RS being sent, but didn't dig in.
Let's explore it now. The RS is needed when hosts are trying to perform SLAAC
to dynamically determine their routable IPv6 addresses.
This is what our Globomantics client is using here.
Similar to the NS and NA demo, I'll add a link‑local address of
FE80::4 to our client with the ICMPv6 debugging enabled.
Let's ignore the initial chatter of DAD and any unsolicited NAs
and instead wait for the host to send an RS.
Again, lots of debug. Let me scroll up so we can see where
the RS got sent. First, we see the client send an RS, or
rather, unceremonious event.
When the router replies with an RA, we are flooded with information.
When the RA is received, the client moves quickly to consume all the data.
One interesting detail is that the client gleans the MAC
address of the router without the NS/NA process.
The RA contains a source MAC option, which we'll see in the packet capture, and
this helps reduce some ND chatter when the hosts start up.
I'll scroll down so we can see the rest. You'll see that the client
stores this router as the best one it knows about.
We'll discuss router preference options in the RA packet capture.
In short,
the client keeps a list of all routers it knows about and can
choose the best one based on preference.
Most interestingly,
the client consumes the routable prefix and uses SLAAC to
derive its IPv6 address of FC00:10:1:4::4.
As you may have guessed,
the client sends a DAD NS to ensure that this address is unique.
Since IPv6 speakers often have multiple addresses, each one must be
individually checked for duplication.
The DAD process is identical regardless of the address type.
This new IPv6 address has been automatically configured, as shown in
the logs. Last, let's check our list of routers.
I'll scroll down to enter a show command. Look at all the
information the client now has at its disposal.
We've got the router's link‑local address,
hop limit, MTU, a routable prefix, and more. Let's explore the RS and RA in greater detail next.