1
00:00:00,000 --> 00:00:07,180

2
00:00:07,180 --> 00:00:09,970
In this video, we're
going to jump in and take

3
00:00:09,970 --> 00:00:13,210
a look at the IS-IS hierarchy.

4
00:00:13,210 --> 00:00:19,010
Much like OSPF, this is also
based on a two-layer hierarchy.

5
00:00:19,010 --> 00:00:22,210
Now in OSPF, that
would be area 0

6
00:00:22,210 --> 00:00:25,930
being the backbone and all of
the other areas being connected

7
00:00:25,930 --> 00:00:27,370
to area 0.

8
00:00:27,370 --> 00:00:29,920
That's your two-layer hierarchy.

9
00:00:29,920 --> 00:00:33,770
In IS-IS, it's done a
little bit differently.

10
00:00:33,770 --> 00:00:37,840
We have level 1 and
level 2 adjacencies.

11
00:00:37,840 --> 00:00:40,240
Now some adjacencies
can be both,

12
00:00:40,240 --> 00:00:43,700
and we'll get into all of that
as we go through this video.

13
00:00:43,700 --> 00:00:46,780
But just be aware that
there's essentially level 1

14
00:00:46,780 --> 00:00:47,950
and level 2.

15
00:00:47,950 --> 00:00:51,580
The level 1 adjacencies are
generally for within an area,

16
00:00:51,580 --> 00:00:55,850
and the level 2 adjacencies
are for between areas.

17
00:00:55,850 --> 00:00:58,000
Therefore, the
level 2 adjacencies

18
00:00:58,000 --> 00:01:00,820
are effectively acting
as the backbone.

19
00:01:00,820 --> 00:01:04,420
Now since this is just done
with a string of adjacencies,

20
00:01:04,420 --> 00:01:09,430
it is a bit more flexible
than OSPF's area 0.

21
00:01:09,430 --> 00:01:13,610
But all in all, it serves
the same basic function.

22
00:01:13,610 --> 00:01:19,210
Now, just like OSPF area
0 has to be contiguous,

23
00:01:19,210 --> 00:01:22,420
well, we have the same thing
with level 2 adjacencies

24
00:01:22,420 --> 00:01:24,280
in IS-IS.

25
00:01:24,280 --> 00:01:27,610
The level 2 adjacencies
have to be connected

26
00:01:27,610 --> 00:01:29,870
through our entire environment.

27
00:01:29,870 --> 00:01:32,840
So that's what we're going to
be focusing on in this video.

28
00:01:32,840 --> 00:01:35,930
Let's jump in and take a look
at some of the facts, figures,

29
00:01:35,930 --> 00:01:38,560
and specifications
and some of the rules

30
00:01:38,560 --> 00:01:40,190
that go along with all of this.

31
00:01:40,190 --> 00:01:44,760
So let's jump in and take a look
at this in some more detail.

32
00:01:44,760 --> 00:01:47,810
So it doesn't really make much
sense to talk about a hierarchy

33
00:01:47,810 --> 00:01:50,730
unless we understand
that, of course,

34
00:01:50,730 --> 00:01:56,340
we can partition our entire
environment into areas, again,

35
00:01:56,340 --> 00:01:58,040
just like OSPF.

36
00:01:58,040 --> 00:02:00,720
And we're doing it
for the same reason.

37
00:02:00,720 --> 00:02:05,240
So what happens is we use
something called LSPs or Link

38
00:02:05,240 --> 00:02:10,050
State Packets to exchange
the link state information.

39
00:02:10,050 --> 00:02:14,660
This is the equivalent
of LSAs in OSPF.

40
00:02:14,660 --> 00:02:19,670
This builds the link state
database or the LSDP,

41
00:02:19,670 --> 00:02:21,650
which we're pretty much
calling the same thing

42
00:02:21,650 --> 00:02:23,300
as we did in OSPF.

43
00:02:23,300 --> 00:02:27,800
And then of course, we run
Dijkstra's Shortest Path First

44
00:02:27,800 --> 00:02:32,970
algorithm against that database
to populate our routing table.

45
00:02:32,970 --> 00:02:36,530
That's just the general
idea behind how this works.

46
00:02:36,530 --> 00:02:40,190
And for the exact same
reason we do it in OSPF,

47
00:02:40,190 --> 00:02:44,030
we want to split up the
database here in IS-IS.

48
00:02:44,030 --> 00:02:47,540
If the database gets too
large, well, Dijkstra's

49
00:02:47,540 --> 00:02:49,790
going to take a really
long time to run.

50
00:02:49,790 --> 00:02:54,440
It could cause a total failure,
although on today's equipment

51
00:02:54,440 --> 00:02:56,760
that would take a
pretty large area.

52
00:02:56,760 --> 00:02:59,960
But even so, it's going to
slow down convergence times.

53
00:02:59,960 --> 00:03:03,140
So a bigger area
wouldn't be better

54
00:03:03,140 --> 00:03:06,050
just because the equipment
can necessarily handle it.

55
00:03:06,050 --> 00:03:07,610
We still have to
be careful that we

56
00:03:07,610 --> 00:03:10,380
don't make our convergence
times be too high.

57
00:03:10,380 --> 00:03:13,610
So again, we're breaking
this up into areas

58
00:03:13,610 --> 00:03:17,420
for the exact same reason
that we do in OSPF.

59
00:03:17,420 --> 00:03:19,970
The next thing to discuss
is just a little bit

60
00:03:19,970 --> 00:03:21,600
of terminology here.

61
00:03:21,600 --> 00:03:24,440
So we already mentioned
during the introduction

62
00:03:24,440 --> 00:03:26,750
that this is a
two-level hierarchy.

63
00:03:26,750 --> 00:03:29,700
And well, let's talk
about its components.

64
00:03:29,700 --> 00:03:32,690
First is the domain
itself the domain

65
00:03:32,690 --> 00:03:37,220
itself, of course, is just
referring to the entire IS-IS

66
00:03:37,220 --> 00:03:41,340
environment, just like we
refer to an OSPF domain.

67
00:03:41,340 --> 00:03:43,460
So it's the
environment as a whole.

68
00:03:43,460 --> 00:03:44,930
Then of course, as
we've mentioned,

69
00:03:44,930 --> 00:03:46,610
we split this up into areas.

70
00:03:46,610 --> 00:03:48,110
This is, of course,
just going to be

71
00:03:48,110 --> 00:03:50,990
a set or a subset
of routers that's

72
00:03:50,990 --> 00:03:52,400
going to be defined by you.

73
00:03:52,400 --> 00:03:54,720
This is going to be handled
by the administrator.

74
00:03:54,720 --> 00:03:57,530
You have to determine for
each environment based

75
00:03:57,530 --> 00:04:01,130
on the number of routes how
big each area should be.

76
00:04:01,130 --> 00:04:03,620
Then we have the
level 1 routing.

77
00:04:03,620 --> 00:04:06,870
Again, this is the
routing within an area

78
00:04:06,870 --> 00:04:09,440
or the intra-area routes.

79
00:04:09,440 --> 00:04:12,860
Then we have level 2
routing, which again this

80
00:04:12,860 --> 00:04:15,030
is between the areas.

81
00:04:15,030 --> 00:04:18,570
This is going to be
your inter-area routes.

82
00:04:18,570 --> 00:04:22,400
So again, just to compare
this a little bit to OSPF,

83
00:04:22,400 --> 00:04:27,650
your level 1s are like your
type 1 and type 2 LSAs.

84
00:04:27,650 --> 00:04:31,040
These would be the LSBLK A's
that stay within the area.

85
00:04:31,040 --> 00:04:34,340
In the case of OSPF,
we have type 3s

86
00:04:34,340 --> 00:04:37,070
that go between areas
that's essentially

87
00:04:37,070 --> 00:04:39,590
the equivalent of level 2s.

88
00:04:39,590 --> 00:04:43,430
They function very
much in the same way.

89
00:04:43,430 --> 00:04:47,590
So this is an idea of what
that hierarchy may look like.

90
00:04:47,590 --> 00:04:50,270
Now one thing to note
here, and of course,

91
00:04:50,270 --> 00:04:52,210
we could have used
any number we wanted,

92
00:04:52,210 --> 00:04:56,860
but I essentially made
the backbone here area 1.

93
00:04:56,860 --> 00:05:02,080
But a couple of things to note
here about the IS-IS topology.

94
00:05:02,080 --> 00:05:05,380
First off, you don't
have to have an area 0,

95
00:05:05,380 --> 00:05:07,460
just like we're picturing here.

96
00:05:07,460 --> 00:05:10,210
But the other thing
is the backbone

97
00:05:10,210 --> 00:05:12,250
doesn't have to be an area.

98
00:05:12,250 --> 00:05:15,760
This area 1 here that
we're using as a backbone

99
00:05:15,760 --> 00:05:20,380
could very easily have level 1
connections and routers in it

100
00:05:20,380 --> 00:05:21,620
as well.

101
00:05:21,620 --> 00:05:24,520
So there's not a dedicated
area that's going

102
00:05:24,520 --> 00:05:27,250
to be the backbone,
be it area 0 or not.

103
00:05:27,250 --> 00:05:31,480
This is all about how you
choose to design your backbone.

104
00:05:31,480 --> 00:05:32,920
But in this case,
what we've done

105
00:05:32,920 --> 00:05:36,670
is we've used area 1 here
sort of as a backbone area.

106
00:05:36,670 --> 00:05:43,540
First thing to note is that
IS-IS areas cross on links, not

107
00:05:43,540 --> 00:05:44,830
on routers.

108
00:05:44,830 --> 00:05:49,390
Remember in OSPF, we had
that special router, the ABR,

109
00:05:49,390 --> 00:05:51,230
the area border router.

110
00:05:51,230 --> 00:05:54,910
Well, as we can sort
of see here, in IS-IS,

111
00:05:54,910 --> 00:05:58,000
the area borders
are happening here

112
00:05:58,000 --> 00:06:00,860
on the links
between the routers.

113
00:06:00,860 --> 00:06:04,570
Now again, these links
are going to be level 2.

114
00:06:04,570 --> 00:06:07,480
That's what sort of makes
them the backbone links.

115
00:06:07,480 --> 00:06:10,930
And just like our
type 3 LSAs in OSPF,

116
00:06:10,930 --> 00:06:16,270
these level 2 routing exchanges
have less information in them

117
00:06:16,270 --> 00:06:19,430
about the links in other areas.

118
00:06:19,430 --> 00:06:23,200
So they're sort of like the
old summary LSAs from OSPF.

119
00:06:23,200 --> 00:06:26,840
And of course, the whole goal
of this, as we've mentioned,

120
00:06:26,840 --> 00:06:31,540
is it reduces the
load on the routers.

121
00:06:31,540 --> 00:06:33,280
Of course, this
is going to allow

122
00:06:33,280 --> 00:06:36,070
for scalability, and
that, of course, also

123
00:06:36,070 --> 00:06:38,620
leads to performance
and making sure

124
00:06:38,620 --> 00:06:42,550
that our adjacencies won't
take too long to come up.

125
00:06:42,550 --> 00:06:46,780
Now the IS-IS adjacencies
can be either level 1,

126
00:06:46,780 --> 00:06:49,270
like we're showing
here and here,

127
00:06:49,270 --> 00:06:53,710
or they can be level 2, like of
course, we're showing up here.

128
00:06:53,710 --> 00:06:56,350
Or they can be level 1, level 2.

129
00:06:56,350 --> 00:06:58,390
We don't have any of
those pictured here,

130
00:06:58,390 --> 00:07:00,310
but we could just as
easily have a level 1

131
00:07:00,310 --> 00:07:02,680
adjacency between
some of these routers

132
00:07:02,680 --> 00:07:06,470
and be connecting these
as level 1 and level 2.

133
00:07:06,470 --> 00:07:09,110
The reason we may want
them to be both Is we

134
00:07:09,110 --> 00:07:12,590
would use the level 1 to
pass routes within the area

135
00:07:12,590 --> 00:07:17,360
because again, maybe there are
N systems in this area 1 here,

136
00:07:17,360 --> 00:07:21,980
but we still need to have a
contiguous level 2 connection.

137
00:07:21,980 --> 00:07:26,220
So we need to carry both
level 1 and level 2 on these.

138
00:07:26,220 --> 00:07:28,610
And again, a lot of this
comes down to design

139
00:07:28,610 --> 00:07:30,950
and how we're going to build
this, which we'll touch

140
00:07:30,950 --> 00:07:33,060
more on in the next video.

141
00:07:33,060 --> 00:07:35,300
So just to add some
more detail to these,

142
00:07:35,300 --> 00:07:39,800
you can have any type of
adjacency, again, meaning

143
00:07:39,800 --> 00:07:44,330
either the level 1, the
level 2, or the level 1-2.

144
00:07:44,330 --> 00:07:49,110
You can have any of
these in the same area.

145
00:07:49,110 --> 00:07:51,530
But again, between
areas, we're going

146
00:07:51,530 --> 00:07:54,800
to be limited to
only the level 2.

147
00:07:54,800 --> 00:07:57,560
That's its entire
purpose in existing,

148
00:07:57,560 --> 00:08:02,170
is that it goes between
routers in different areas.

149
00:08:02,170 --> 00:08:04,680
Now another consideration
we have to think about

150
00:08:04,680 --> 00:08:07,740
is how the device
or the router itself

151
00:08:07,740 --> 00:08:09,330
is going to be configured.

152
00:08:09,330 --> 00:08:13,170
And it can also be configured
much like our links.

153
00:08:13,170 --> 00:08:15,480
It can either just be level 1.

154
00:08:15,480 --> 00:08:17,430
It can just be level 2.

155
00:08:17,430 --> 00:08:19,650
Or it could be level 1-2.

156
00:08:19,650 --> 00:08:23,400
In the case of our examples
here, this router, for example,

157
00:08:23,400 --> 00:08:27,240
could be level 1 only because
all of its adjacencies

158
00:08:27,240 --> 00:08:29,346
are in area 10.

159
00:08:29,346 --> 00:08:33,760
The level 2 devices could
again be any device that

160
00:08:33,760 --> 00:08:36,250
only has level 2 adjacencies.

161
00:08:36,250 --> 00:08:39,520
It doesn't have any
level 1 adjacencies.

162
00:08:39,520 --> 00:08:42,710
And then of course,
last but not least,

163
00:08:42,710 --> 00:08:45,580
the level 1-2
adjacencies would be

164
00:08:45,580 --> 00:08:50,320
any of the routers that have to
have both a level 1 and a level

165
00:08:50,320 --> 00:08:52,130
2 adjacency.

166
00:08:52,130 --> 00:08:55,540
So not only should we
configure our links

167
00:08:55,540 --> 00:08:59,710
to be what we want them to
be, level 1, level 2, or both,

168
00:08:59,710 --> 00:09:02,570
but we should also
configure our devices.

169
00:09:02,570 --> 00:09:07,330
It's not strictly necessary, but
it is generally best practice

170
00:09:07,330 --> 00:09:12,060
to configure the routers for
their role in the network.

171
00:09:12,060 --> 00:09:13,800
So again, we've sort
of mentioned this,

172
00:09:13,800 --> 00:09:15,950
but just to make sure
we're clear on all of it,

173
00:09:15,950 --> 00:09:22,550
the IS-IS backbone is simply
a contiguous string of level 2

174
00:09:22,550 --> 00:09:25,130
routers and adjacencies.

175
00:09:25,130 --> 00:09:29,720
And like we said, this can
traverse many other areas.

176
00:09:29,720 --> 00:09:33,470
You don't have to have
an area either dedicated

177
00:09:33,470 --> 00:09:37,490
to be the backbone or
that is only the backbone.

178
00:09:37,490 --> 00:09:41,630
Just to be clear, I could have
run my level 2 adjacencies

179
00:09:41,630 --> 00:09:44,990
between the routers like
this and maybe not even

180
00:09:44,990 --> 00:09:46,490
done all of this up here.

181
00:09:46,490 --> 00:09:50,640
I wouldn't even need area 1, and
this is still perfectly valid.

182
00:09:50,640 --> 00:09:54,530
This is simply an example
of the backbone traversing

183
00:09:54,530 --> 00:09:55,790
many other areas.

184
00:09:55,790 --> 00:09:59,010
It's traversing area
10, 20, 30, and 40 here.

185
00:09:59,010 --> 00:10:01,760
So this is something that
can absolutely be done.

186
00:10:01,760 --> 00:10:04,460
You see what I mean when I
say you have to actually sort

187
00:10:04,460 --> 00:10:06,140
of design your backbone here.

188
00:10:06,140 --> 00:10:10,160
Again, there is no
dedicated backbone area

189
00:10:10,160 --> 00:10:13,460
as we would refer
to it in, say, OSPF.

190
00:10:13,460 --> 00:10:18,920
And this does in fact add a lot
of flexibility to our design.

191
00:10:18,920 --> 00:10:22,970
And this is why I said you
do have to design the IS-IS

192
00:10:22,970 --> 00:10:28,820
hierarchy a bit more than
we do with OSPF in that OSPF

193
00:10:28,820 --> 00:10:32,450
is a little more strict, so
there's not as much to design.

194
00:10:32,450 --> 00:10:35,000
We may have to work
around those restrictions,

195
00:10:35,000 --> 00:10:38,000
but here we have flexibility,
and flexibility means

196
00:10:38,000 --> 00:10:40,580
that we have to design it.

197
00:10:40,580 --> 00:10:45,680
In this video, we talked a
bit about the IS-IS hierarchy.

198
00:10:45,680 --> 00:10:48,760
We talked about how it
is a two-layer hierarchy.

199
00:10:48,760 --> 00:10:51,880
It consists of
level 1 and level 2

200
00:10:51,880 --> 00:10:55,820
adjacencies, where the level
1 adjacencies are, of course,

201
00:10:55,820 --> 00:11:00,220
within the area, acting much
like OSPF type 1 and type

202
00:11:00,220 --> 00:11:04,450
2 LSAs, where the
level 2 adjacencies go

203
00:11:04,450 --> 00:11:09,340
between areas acting
sort of like LSA type 3s.

204
00:11:09,340 --> 00:11:13,300
Now again, some major
differences here number one.

205
00:11:13,300 --> 00:11:16,900
The area is not dedicated
to be a backbone.

206
00:11:16,900 --> 00:11:22,090
Again, it's just a contiguous
string of level 2 adjacencies.

207
00:11:22,090 --> 00:11:24,460
It can run all in one area.

208
00:11:24,460 --> 00:11:27,020
It can run between areas.

209
00:11:27,020 --> 00:11:29,440
So we have a lot of
flexibility when it comes

210
00:11:29,440 --> 00:11:32,050
to the IS-IS backbone design.

211
00:11:32,050 --> 00:11:34,480
And that we'll be
covering a little bit more

212
00:11:34,480 --> 00:11:36,080
in the next video.

213
00:11:36,080 --> 00:11:39,490
But also keep in mind here,
the second major difference

214
00:11:39,490 --> 00:11:43,990
is that areas in
IS-IS cross on links.

215
00:11:43,990 --> 00:11:48,190
They don't cross on routers,
like they do in OSPF,

216
00:11:48,190 --> 00:11:50,110
where we had the ABR.

217
00:11:50,110 --> 00:11:52,540
So keep these
differences in mind

218
00:11:52,540 --> 00:11:54,570
as we jump into our
next video, where

219
00:11:54,570 --> 00:11:57,130
we're going to talk a little
bit about some common designs

220
00:11:57,130 --> 00:12:00,970
that we use with
this IS-IS backbone.

221
00:12:00,970 --> 00:12:03,100
I hope this has been
informative for you,

222
00:12:03,100 --> 00:12:06,240
and I'd like to thank
you for viewing.

223
00:12:06,240 --> 00:12:10,000