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So now that we know a bit
about both IS-IS and IPv6, now

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it's time to jump
in and take a look

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at what had to be modified in
IS-IS in order to support IPv6.

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And well, really therefore
what the current requirements

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are in order for us to support
IPv6 in our environment.

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So let's jump in and take
a look at those details.

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So as it's implied since
we're discussing it,

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IS-IS, of course, can support
not only IPv4 but also IPv6,

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and of course, we can do
both at the same time.

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Technically, we can also
support CLNS at the same time.

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But that's not really a
topic of concern right now.

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Now, in order to do this, we
did add two new type length

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values to IS-IS.

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And this, by the way, is one
of the big advantages to IS-IS

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when we talk about
its extensibility

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that it's real easy
to add new protocols,

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we just have to add new TLVs.

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Well, the first
TLV that we added

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is TLV 236 which is
called IPv6 Reachability.

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And as the name implies, this
is going to handle IPv6 network

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reachability specifically.

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This is the equivalent
to some of the TLVs

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we spoke of earlier,
TLV 128, and TLV 130,

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that we talked about for IPv4.

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The second TLV is TLV 232.

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This one is specifically
for IPv6 interface addresses

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really meant for
hellos that require

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that link local address.

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It's also for LSPs that
need the global addresses.

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And this is the equivalent
of TLV 132, again, for IPv4.

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Now, TLV 129, which is what
carries the network layer

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protocol identifiers or NLPIDs,
this had to be updated as well.

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Now, this is not, of
course, a new TLV,

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we're just modifying this
one to include a new value

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142 indicating IPv6 support.

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We've always had 204 for IPv4.

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So we're simply
adding a new value

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to this field to indicate that
we're now also carrying IPv6.

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Something else that has
to be considered, not just

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the changes to the TLVs
themselves but also

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whether we're going to run
single or multi-topology.

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Single topology requires
that IPv4 and IPv6

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follow the same paths.

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It also requires
that all of the links

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have to support IPv4 and IPv6.

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We can't have some links
that don't support IPv6

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because if those were to get
chosen as the best path well,

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then it would actually
break the IPv6 pathway.

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This is because, of course,
we're using a single topology.

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That also implies that
there's only one SPF instance.

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If there's only
one topology, we're

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only going to run SPF once.

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So related to the
point up here, this

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is not going to
be suitable at all

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for a partial implementation
because, again, all the links

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have to carry both protocols.

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So that means I would
have to turn on IPv6

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on all the links at once.

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So, again, not going
to be good for partial.

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Another issue with this is
those level 1 adjacencies

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they will not even form if
the protocols don't match.

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Remember we saw previously that
which protocols are supported

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are carried in the TLVs
those have to match.

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Now, that said you
can disable this.

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So if needed, you can
disable this check.

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Now, again, this is not going
to be a good strategy long-term.

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As you can see by
our next point here,

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this is usually done
during transitioning.

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So that would be when you had
this partial implementation

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and we're not actually expecting
IPv6 to be working yet,

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we're still in the
middle of configuring it.

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But we don't want to break IPv4
for while we're rolling out

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IPv6.

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So I want to make sure we're
very clear on this option.

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If we do this, the
idea is we can roll out

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IPv6 support on our links
without breaking IPv4

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in the meantime.

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However, we also
can't expect IPv6

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to be functional until
all the links are

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up and running with IPv6.

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So at that point, once we're
done with that transition,

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you would usually want to
turn this check back on.

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Now, we can see here that we
have the two commands you would

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need to turn this off just
outlined here in our last two

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points.

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On iOS XE it's no
adjacency-check.

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And, of course, on iOS XR
it's adjacency-check disable

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under the IPv6 address family.

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Now, level 2 adjacencies
these will still form.

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Now, the backbone still
must be contiguous.

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That, of course,
still applies and that

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would include for IPv6.

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Now, if you did have
routers in the backbone that

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do not support IPv6, then
there could definitely

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be a black hole.

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We could try
forwarding IPv6 traffic

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and not have the destination.

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So this means that all
of the level 2 routers

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would have to support IPv6.

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And 1 final note
here, this design,

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meaning single topology,
would require wide metrics

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everywhere.

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And this, again, will be
going back to that concept

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that everything must support
IPv6 in single topology.

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Now, of course,
to contrast this,

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we have multitopology IS-IS and
this is outlined in RFC 5120.

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This would, of course,
allow interfaces to not have

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to have the same protocols.

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So IPv4 and IPv6 can
actually be operated

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completely independently.

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Two SPFs will be formed, of
course, one for each protocol.

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We're going to have two
different topologies.

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We're going to have
to execute SPF twice.

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Now, keep in mind that
wide metrics are still

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going to be needed, of course,
at least on the devices that

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are going to support IPv6, which
ultimately is probably going

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to be your whole topology.

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But at least in
the short term, it

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has to have wide
metrics to support IPv6.

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The other thing is
that all of the devices

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have to understand that
we're running multi-topology.

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If one device is
running multi-topology

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and others are not,
multi-topology routers

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will see single topology routers
as unable to support IPv6.

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So they would not try to
establish an IPv6 adjacency.

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Now, if you're transitioning
from single topology

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to multitopology, there
is a migration mode

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to help with this transition.

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And during that time, it
will support both TLVs types

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for single topology
and multi-topology,

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and this device would be
able to form neighborships

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with either type of device.

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In this video, we took
a look at the things

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that had to change in IS-IS
in order to support IPv6.

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Specifically, we
looked at the changes

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that had to happen
with new TLVs as well

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as the additional
information that we

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carry that we support IPv6.

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And then we talked about
single versus multitopology

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and how we either have to
have both protocols supported

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in the entire environment
for single topology

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or with multitopology we
can roll out IPv6 just

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where it's needed.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.

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