So now that we know a bit about both IS-IS and IPv6, now
it's time to jump in and take a look
at what had to be modified in IS-IS in order to support IPv6.
And well, really therefore what the current requirements
are in order for us to support IPv6 in our environment.
So let's jump in and take a look at those details.
So as it's implied since we're discussing it,
IS-IS, of course, can support not only IPv4 but also IPv6,
and of course, we can do both at the same time.
Technically, we can also support CLNS at the same time.
But that's not really a topic of concern right now.
Now, in order to do this, we did add two new type length
values to IS-IS.
And this, by the way, is one of the big advantages to IS-IS
when we talk about its extensibility
that it's real easy to add new protocols,
we just have to add new TLVs.
Well, the first TLV that we added
is TLV 236 which is called IPv6 Reachability.
And as the name implies, this is going to handle IPv6 network
reachability specifically.
This is the equivalent to some of the TLVs
we spoke of earlier, TLV 128, and TLV 130,
that we talked about for IPv4.
The second TLV is TLV 232.
This one is specifically for IPv6 interface addresses
really meant for hellos that require
that link local address.
It's also for LSPs that need the global addresses.
And this is the equivalent of TLV 132, again, for IPv4.
Now, TLV 129, which is what carries the network layer
protocol identifiers or NLPIDs, this had to be updated as well.
Now, this is not, of course, a new TLV,
we're just modifying this one to include a new value
142 indicating IPv6 support.
We've always had 204 for IPv4.
So we're simply adding a new value
to this field to indicate that we're now also carrying IPv6.
Something else that has to be considered, not just
the changes to the TLVs themselves but also
whether we're going to run single or multi-topology.
Single topology requires that IPv4 and IPv6
follow the same paths.
It also requires that all of the links
have to support IPv4 and IPv6.
We can't have some links that don't support IPv6
because if those were to get chosen as the best path well,
then it would actually break the IPv6 pathway.
This is because, of course, we're using a single topology.
That also implies that there's only one SPF instance.
If there's only one topology, we're
only going to run SPF once.
So related to the point up here, this
is not going to be suitable at all
for a partial implementation because, again, all the links
have to carry both protocols.
So that means I would have to turn on IPv6
on all the links at once.
So, again, not going to be good for partial.
Another issue with this is those level 1 adjacencies
they will not even form if the protocols don't match.
Remember we saw previously that which protocols are supported
are carried in the TLVs those have to match.
Now, that said you can disable this.
So if needed, you can disable this check.
Now, again, this is not going to be a good strategy long-term.
As you can see by our next point here,
this is usually done during transitioning.
So that would be when you had this partial implementation
and we're not actually expecting IPv6 to be working yet,
we're still in the middle of configuring it.
But we don't want to break IPv4 for while we're rolling out
IPv6.
So I want to make sure we're very clear on this option.
If we do this, the idea is we can roll out
IPv6 support on our links without breaking IPv4
in the meantime.
However, we also can't expect IPv6
to be functional until all the links are
up and running with IPv6.
So at that point, once we're done with that transition,
you would usually want to turn this check back on.
Now, we can see here that we have the two commands you would
need to turn this off just outlined here in our last two
points.
On iOS XE it's no adjacency-check.
And, of course, on iOS XR it's adjacency-check disable
under the IPv6 address family.
Now, level 2 adjacencies these will still form.
Now, the backbone still must be contiguous.
That, of course, still applies and that
would include for IPv6.
Now, if you did have routers in the backbone that
do not support IPv6, then there could definitely
be a black hole.
We could try forwarding IPv6 traffic
and not have the destination.
So this means that all of the level 2 routers
would have to support IPv6.
And 1 final note here, this design,
meaning single topology, would require wide metrics
everywhere.
And this, again, will be going back to that concept
that everything must support IPv6 in single topology.
Now, of course, to contrast this,
we have multitopology IS-IS and this is outlined in RFC 5120.
This would, of course, allow interfaces to not have
to have the same protocols.
So IPv4 and IPv6 can actually be operated
completely independently.
Two SPFs will be formed, of course, one for each protocol.
We're going to have two different topologies.
We're going to have to execute SPF twice.
Now, keep in mind that wide metrics are still
going to be needed, of course, at least on the devices that
are going to support IPv6, which ultimately is probably going
to be your whole topology.
But at least in the short term, it
has to have wide metrics to support IPv6.
The other thing is that all of the devices
have to understand that we're running multi-topology.
If one device is running multi-topology
and others are not, multi-topology routers
will see single topology routers as unable to support IPv6.
So they would not try to establish an IPv6 adjacency.
Now, if you're transitioning from single topology
to multitopology, there is a migration mode
to help with this transition.
And during that time, it will support both TLVs types
for single topology and multi-topology,
and this device would be able to form neighborships
with either type of device.
In this video, we took a look at the things
that had to change in IS-IS in order to support IPv6.
Specifically, we looked at the changes
that had to happen with new TLVs as well
as the additional information that we
carry that we support IPv6.
And then we talked about single versus multitopology
and how we either have to have both protocols supported
in the entire environment for single topology
or with multitopology we can roll out IPv6 just
where it's needed.
I hope this has been informative for you.
And I'd like to thank you for viewing.