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[AUDIO LOGO]

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So now that we've
taken a deeper look

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at things like IS-IS
neighborships and packet types,

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let's jump in and take a deeper
look at the IS-IS database.

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So we'll start off by
looking at some attributes

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and things we need to
know about the database.

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And then we'll jump
into the command line

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and take a look at
the database itself.

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So let's go ahead
and jump in and get

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started with looking at
some of those attributes.

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So as we've
mentioned previously,

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we do exchange LSPs to
build the link state

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database or the LSDB.

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Our focus in this video is
going to be on the link state

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database itself as we've already
sort of looked at those LSPs.

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Now what we use the link state
database for is to, of course,

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build the topology which
is, of course, then

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used to build the
shortest path first tree

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which is ultimately going
to build our routing table.

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Now we've mentioned this
a few times as well,

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but remember that the level 1
LSPs are for within an area.

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So this is the topology
for a single area.

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And if a router is in
just a single level,

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well, then it's just going
to have that one link state

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database and therefore only
run SPF once against that link

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state database.

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So there'll only be just
that one calculation.

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If it's a level 1 and
a level 2 router, well,

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then it's going to have
to link state databases.

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And of course, that
means it's going

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to also have to run two SPF
calculations against the two

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link state databases.

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If there's two, we
have to run SFP twice.

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Now something else
fundamental to keep in mind

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is these link state
packets are flooded to all

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the neighbors in an area.

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Now, of course, it's not
going to send it back

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to the one that originated it.

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There would be no
point in doing so.

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And we have mentioned
this a few times, but just

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as a final reminder
here, level 1 LSPs

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are only flooded within an
area, and the level 2 LSPs

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are, of course, flooded
into the backbone

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because level 2 adjacencies
are in fact the backbone.

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Now, each router is going
to originate its own LSPs.

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And this is going to be for
both level 1 and/or level 2.

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Of course, it does depend if the
router is level 1 or level 2.

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And it identifies itself
with the system ID.

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Now we do implement
error checking here

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in the form of a checksum.

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And of course, if
it's invalid, meaning

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it doesn't match up
with the checksum,

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it's of course, going
to be discarded.

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We're not going
to keep bad LSPs.

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Let's also discuss
the update process.

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So in other words, what happens
if we get an LSP, that LSP

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is valid meaning it did
in fact pass the checksum,

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and it's newer
than the one that's

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currently in our database?

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Well, it's a fairly
straightforward process.

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So the first thing
we do, of course,

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is update our own database
if it's newer information,

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we're going to want
that newer information.

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We then acknowledge the receipt.

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That way, the sender knows that
we got the updated information.

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And then we set the lifetime
to 1,200 and seconds.

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The entry would time out
after that for the purposes

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of database maintenance.

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Now if that LSP does
expire, well, we

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keep it for an
additional 60 seconds

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just to make sure that
there's no latency

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in the forwarding of the LSP.

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And then we go ahead and
flood it as well, expired.

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And that will remove it from
everybody else's database.

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Again, this is part of database
cleanup and maintenance.

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Now we did talk a
little bit before also

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about these complete versus
partial sequence number

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PDUs or SNPs.

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We actually saw how Wireshark
listed these as CSNP or PSNP.

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And as we mentioned at
the time, the complete

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is going to include
all of the LSPs,

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and the partial will be
a subset of the LSPs.

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Again, it just depends if we're
getting the whole database

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or just part of it.

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Now, again, both of
these carry a summary

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of the link state database.

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It's just a matter
of whether it's

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a complete or a partial summary.

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And again, remember the
whole purpose of this

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is to make sure that I have all
of the entries that you, you

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have all the
entries that I have.

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We're using these to keep
our databases in sync.

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Now, since it's
synchronizing the database,

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hopefully it makes sense
that there's also like,

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the database itself,
SNPs also are

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separate for level 1 and
level 2 because again, they're

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maintaining two
different databases.

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Now while these are exchanged
between neighbors, again,

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for the purpose of making sure
the databases are synchronized,

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this is handled a
little bit differently

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on our different network types.

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So on a broadcast
network type, it's

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the DIS or the DIS that will
send this multicast out.

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And it defaults to doing
this every 10 seconds.

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On point to point, it's going
to be a little different.

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It's really only sent once
when the link comes up.

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So since the job of
the SNP is to keep

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the databases synchronized,
if there are any differences,

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we're going to
send a partial SNP

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to indicate which entries
need to be updated

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or are needed to be sent again.

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Just as another
point of fact here,

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the PSNPs are also used
for the acknowledgments

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when we actually get an update.

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Now with integrated IS-IS,
a couple of other things

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to keep in mind here.

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The best path is calculated
to other routers,

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and then we simply add those
destinations from the database.

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The IP or IPv6
routes are treated

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as end system information.

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And since they're end
system information,

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if all that changes
is IP reachability,

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then all we need is that partial
route calculation or a PRC.

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We don't have to calculate
the whole topology.

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The main point of
these last few points

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we just talked about here
is we calculate routes

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between the routers themselves.

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So the best path is all
about getting to the router.

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The prefixes that are
available on that router

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are really just a subset of
getting to the router itself.

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So if that router loses
a particular prefix,

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an interface goes
down, something

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like that, all it has to do
is remove that IP reachability

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from the database.

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That doesn't cause a
complete recalculation of SPF

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because the router
itself is still

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reachable with the same cost
and everything like that.

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All it's doing is
removing the availability

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of a particular address.

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So let's jump in
to the command line

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and take a look
at this database,

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then we'll sort make a change
and see how it propagates

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to the other routers.

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So let's start by
looking at our topology

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just to see where we're
going to be working.

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And then we'll
jump into the CLI.

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So once again, we'll be
using IS-IS basic topology.

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And for this example, we're
just going to focus on router 1

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and router 2.

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So basically what
we're going to do

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is we're going to look at
the topology on router 1.

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We'll look at some
information from router 2.

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Then we'll go change that
information on router 2

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and just see how it
propagates to router 1.

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So let's jump into the
command line on router 1.

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So let's start off with
just to show IS-IS database.

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And basically what
this is showing us

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is a summary of the information
that we have in our database

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right now, including all of the
devices that we're aware of.

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So let's look at some
more detailed information

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on router 2 specifically.

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So we can just hit up to go
back to the show IS-IS database.

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And then we'll get more
specific by saying XR2.00-00,

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and we'll ask for the detail.

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Now the first thing to note
is under the area addresses,

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we have metrics to
get to its neighbors.

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So we can see that
it says it has

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a metric of 10 to get to router
5 as well as a metric of 10

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to get to router 1.

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By the way, we have not changed
any of the metrics on router 1.

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So it's also 10 for router
1 to get to router 2,

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although we're not
looking at that database

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right this moment.

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Also notice that router
2 is listing its attached

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interfaces, including,
for example,

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it's 10..2.2.2/32 loopback
also with a metric of 10.

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Remember that in Cisco iOS, the
default for all links is 10.

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But what happens if we go
change that on router 2?

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Let's take a look.

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So we jump over to router 2.

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We'll simply go in
and configuration.

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We'll go into our IS-IS instance
for our interface loopback 0,

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specifically for the
address family IPv4.

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And we'll simply set the metric.

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We're going to set it to 5, but
just so we see the range here

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depending on whether we're
doing narrow or wide metrics.

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We're, of course,
doing wide metrics.

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So we could go all the way
up to, well, that big number.

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And what we're going
to do is, like I said,

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we're just going to set it low.

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We're going to set it to 5.

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But we can see that there
is a wide range of numbers

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that we can use, particularly
if we're on wide metrics.

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Let's commit those changes.

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We'll exit out of configuration.

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And let's go back to router
1 and see what's changed.

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So on router 1, if we simply
look at the detail for router 2

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again, what we see
specifically is

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that the metric for
that loopback is now 5.

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And if we were to say, show
IP route for IS-IS and we look

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specifically at that 10.2.2.2,
we see it now has a metric

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of 15--

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10 to get to router 2 itself
and then 5 for the loopback.

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So we can see how this
information in the database

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can, of course, be changed and
propagated to other devices

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as well as, of
course, being used

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to propagate the routing table.

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In this video, we jumped in
and we took a closer look

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at the IS-IS database.

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Then, of course,
we made a change.

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We saw how that got propagated
to the neighbors, which

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of course, it should--

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wouldn't be much of a routing
protocol if it didn't--

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but then how that information
is also, of course,

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reflected right into the routing
table exactly as it should be.

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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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