[AUDIO LOGO]
So let's begin here by jumping in and just getting
an introduction or an overview of the IS-IS protocol.
Hopefully, some of this is a review for you.
But if not, it'll at least get us to a good foundation
where we can begin to talk about some
of the more detailed information about IS-IS.
So even if this is a review, it's
probably worth watching just to refresh yourself
on all this information and make sure
that we're all ready to go with some of our more advanced IS-IS
topics.
So again, let's just jump in and cover
some of those IS-IS basics.
So the first thing to mention about IS-IS
is this is a routing protocol designed by the OSI,
or Open Systems Interconnection.
And this is designed to be an interior routing protocol.
It's not meant to scale to, say, the size of the internet,
like BGP version 4.
It's really in direct competition, if you will,
with OSPF.
In fact, we'll be comparing this a lot to OSPF as we go.
They're very, very similar.
Now, a little bit of terminology to get out of the way--
we basically have two different types of nodes in IS-IS.
The first is the end system.
The end system is really just any host that's not a router.
So this could be a printer or a workstation
or a server, really just anything other than a router.
Then, of course, there's the Intermediate System, or the IS,
which, of course, is where the routing protocol gets
its name--
intermediate system to intermediate system.
And that is simply referring to a router.
So basically, this is the router-to-router protocol.
That's essentially what IS-IS is.
Now, a lot of people aren't really all that familiar
with IS-IS, just because it's not
as popular as its counterpart, again, OSPF.
However, it is very popular in large-scale service provider
networks, which of course, is why we need to talk about it.
And we'll see coming up why that's the case.
IS-IS supports a lot of features that service providers find
quite attractive and useful.
Now, we mentioned that this is very similar to OSPF.
Technically, IS-IS was actually started before OSPF.
But we're not really too concerned
about which came first.
The most important thing here is that there are many,
many similarities.
They both use the same algorithm.
They're both link-state routing protocols, and so on.
We'll be talking more about this as we go through IS-IS.
But the point is, yes, there are many similarities.
In fact, the way I usually say it-- they
actually have more in common than they have different.
But that said, there are fundamental differences,
of course, that we need to discuss.
So as we just mentioned, this is a link-state routing protocol,
just like OSPF here.
And as such, it has a lot of those same attributes.
Again, it exchanges databases, not routes.
Exchanging routes is distance vector behavior.
This is link state, again, like OSPF.
Routes are then calculated on each
IS based on that exchange database.
Once again, we run Dijkstra's algorithm,
or the shortest path first algorithm against the database
and come up with our own routes.
This, just like other modern routing protocols,
fully supports VLSM, or Variable-Length Subnet Masking.
So in other words, it is a classless protocol.
It doesn't rely on the traditional class A, class B,
class C subnet structure.
It can support any class network with any mask.
Now, one of the big features making
this very attractive to service providers
is this has a very, very proven track record.
It's very reliable and very scalable,
along with fast convergence.
All of these things make this very attractive to a service
provider.
We want reliability.
We want scalability.
And it has to be fast.
Remember, we're going to be using this in the service
provider as the underlay routing protocol to whatever
other structure we're running--
so for example, maybe MPLS.
Or maybe we're just running native BGP routing.
But either way, we need to know how
to get to the next hops of the other devices.
So this is usually going to be used--
again, or OSPF-- to be the underlay routing protocol.
This is why we need things like reliability, scalability,
and of course, fast convergence.
Now, when IS-IS was originally designed,
it was actually designed to be the routing protocol
for something called Connectionless Network
Service, or CLNS.
CLNS and, of course, therefore IS-IS
use something called Connectionless Network
Protocol, or CLNP, as the transport.
So essentially, what we're saying here
is that unlike most other routing protocols
that we use for IP, either IPv4 or IPv6,
IS-IS doesn't itself actually run on top of IP.
Now, we have a small environment set up right now.
We don't really need to look at the topology.
We're just going to look at the Wireshark capture
off of one router that's running IS-IS so we can take
a look at how this does not run over IP
but rather runs over CLNS.
So let's hop into Wireshark and take a look
at a capture off of our router 1,
which is in fact running IS-IS.
So here we have our Wireshark capture.
We're filtering based on IS-IS.
And we can see various IS-IS packet
types going back and forth.
Here we see some hellos.
We see some CNSP packets.
We'll get into these different types of protocol packets
in a future video.
But for now, what we want to see is here in the main capture
window, we see that we have an Ethernet header, an LLC header.
And then notice it goes right to ISO, ISIS InTRA Domain Routeing
Information Exchange Protocol.
If we open this up, we can see that the protocol identifier
is in fact ISIS.
And if we scroll down a little bit more here,
we see that this, in fact, is an ISIS HELLO that we captured.
And we can see some of this information.
And we'll be covering this in some more detail, again,
as we go through.
But the main point we want to see right now
is if we shrink these back down, what
we do not have is an IP header.
This is IS-IS running directly on top
of Ethernet in this case.
So now let's hop back over to our discussion.
And we'll pick back up here with another big advantage to IS-IS.
And that is that it is very extensible
because it uses something called a TLV, or Type Length Value
in order to exchange its routing information.
This is actually a very, very important feature
we have with IS-IS.
This extensibility means that we can support a variety
of different protocols.
Not only can it of course support CLNS,
which is what it was designed for, although we don't use it
for that very often these days.
Of course, it supports IPv4 and IPv6,
or we probably wouldn't really be talking about it at all.
But it can also do things like exchange MAC address
information and things like this.
So we can use this for other technologies such as--
well, Cisco used it for their FabricPath technology.
Cisco ACI can use it.
We can use it for VXLAN.
So there's a lot of different things
that we can do with IS-IS because it
is so extensible and easily updated
to support additional information.
About the only other protocol that's this flexible--
and also why it gets used so much--
is BGP.
So as far as extensibility and flexibility,
we can do a lot of the same things
both with IS-IS and BGP because of this ability
to just exchange information without it
being in such a strict format, such as something like OSPF.
Just to contrast, think of it this way.
When OSPF wanted to go from IPv4 to IPv6,
we needed a whole new version.
We had to go from OSPF version 2 to OSPF version 3.
Protocols such as IS-IS and, again, as an example, BGP, all
we had to do is add new address families.
So again, this extensibility, very important feature.
Particularly moving forward as new technologies come out
and things like this, it makes it a very good candidate
if not for the end engineer to use at least for the underlying
technologies to use it, such as controllers in something
like software-defined networking.
Some other information we want to get at here
is IS-IS actually supports two different metric types.
Specifically, it supports what's called the narrow-style metric.
And this is really the legacy style.
We really don't use this too much anymore.
You'll see why in just a moment.
It's still the default on most platforms.
So since we don't really want to use it,
we'll see that when we get into our configuration
later that we're going to most likely want to change this.
This uses a 6-bit link metric and a 10-bit path metric,
which in and of itself makes it fairly limiting.
But the more important thing that makes it limiting
is compared to the newer wide-style metric, which,
like we just said, is the new style, of course,
this one is required for IPv6 support.
And since even if today we're not doing IPv6,
we probably will be shortly.
This is almost always something we want.
This is a 24-bit link metric, allowing
us to be a lot more granular.
And it's a 32-bit path metric.
In this video, we jumped in and looked
at some of the basics of IS-IS.
We started off by looking at some of its advantages.
And then we saw how it doesn't actually
run on IP, because it was designed for CLNS,
but has been extended to support not only CLNS but also IP
versions, of course, 4 and 6.
Then we wrapped up here by talking about the two
different metric styles we have and how, generally speaking,
we want to prefer the wide metrics.
I hope this has been informative for you,
and I'd like to thank you for viewing.