1
00:00:00,000 --> 00:00:02,934
[MUSIC PLAYING]

2
00:00:02,934 --> 00:00:07,830

3
00:00:07,830 --> 00:00:10,950
So let's get started
here with an introduction

4
00:00:10,950 --> 00:00:12,640
to segment routing.

5
00:00:12,640 --> 00:00:15,090
Now, what we're going
to really be focusing on

6
00:00:15,090 --> 00:00:17,910
is, what does this
give us that we

7
00:00:17,910 --> 00:00:21,540
don't get from traditional
MPLS forwarding.

8
00:00:21,540 --> 00:00:24,220
The fact of the matter
is at the end of the day,

9
00:00:24,220 --> 00:00:27,300
this really is MPLS forwarding.

10
00:00:27,300 --> 00:00:30,840
What changes a bit is how
we're using the labels

11
00:00:30,840 --> 00:00:33,720
and how the labels get
advertised to neighbors

12
00:00:33,720 --> 00:00:35,200
and things like that.

13
00:00:35,200 --> 00:00:37,530
So that's the
purpose of this video

14
00:00:37,530 --> 00:00:40,560
just to get some of the concepts
and a little bit of terminology

15
00:00:40,560 --> 00:00:41,620
out of the way.

16
00:00:41,620 --> 00:00:44,410
And then, of course,
in the later videos,

17
00:00:44,410 --> 00:00:47,220
we'll be jumping into
most of these topics

18
00:00:47,220 --> 00:00:48,790
in a bit more detail.

19
00:00:48,790 --> 00:00:50,580
So let's go ahead
and get started

20
00:00:50,580 --> 00:00:55,200
with a good introduction
to segment routing.

21
00:00:55,200 --> 00:00:59,660
So again, keep in mind that
segment routing is really

22
00:00:59,660 --> 00:01:04,430
just building on top of the
basic or traditional MPLS

23
00:01:04,430 --> 00:01:06,110
forwarding paradigm.

24
00:01:06,110 --> 00:01:07,970
We're still using labels.

25
00:01:07,970 --> 00:01:09,740
We're still
forwarding our traffic

26
00:01:09,740 --> 00:01:13,070
and doing label switching
based on those labels.

27
00:01:13,070 --> 00:01:15,710
However, the way we
use them and the way

28
00:01:15,710 --> 00:01:18,960
that they're going to be
learned is slightly different.

29
00:01:18,960 --> 00:01:22,850
But again, the data plane,
at the end of the day,

30
00:01:22,850 --> 00:01:25,620
still using MPLS.

31
00:01:25,620 --> 00:01:27,860
So that part's not changing.

32
00:01:27,860 --> 00:01:29,360
The headers don't change.

33
00:01:29,360 --> 00:01:31,760
The label structure
itself doesn't change.

34
00:01:31,760 --> 00:01:34,880
Everything here as
far as the MPLS side

35
00:01:34,880 --> 00:01:40,070
is exactly the same as
traditional or legacy MPLS.

36
00:01:40,070 --> 00:01:43,490
The major difference
between the segment routing

37
00:01:43,490 --> 00:01:46,460
and the legacy or
traditional, or you

38
00:01:46,460 --> 00:01:52,460
could say LDP-based MPLS,
lies in the control plane.

39
00:01:52,460 --> 00:01:56,620
And that main difference is
we're going to see as we go.

40
00:01:56,620 --> 00:01:59,820
We're going to be
getting rid of the LDP.

41
00:01:59,820 --> 00:02:02,010
We're not going to be
using label distribution

42
00:02:02,010 --> 00:02:04,150
protocol any longer.

43
00:02:04,150 --> 00:02:07,150
So what does that really
mean at the end of the day?

44
00:02:07,150 --> 00:02:11,400
Well, it means that we're going
to be changing things in sort

45
00:02:11,400 --> 00:02:13,630
of two significant ways.

46
00:02:13,630 --> 00:02:16,470
The first is the labels
are usually going

47
00:02:16,470 --> 00:02:19,200
to be having identical values.

48
00:02:19,200 --> 00:02:22,800
And because of this,
in the SR domain,

49
00:02:22,800 --> 00:02:26,340
these labels are going
to have global value

50
00:02:26,340 --> 00:02:32,110
because it'll be the same label
for the prefix on every device.

51
00:02:32,110 --> 00:02:34,050
And as we sort of
already mentioned,

52
00:02:34,050 --> 00:02:37,290
our second big change
here is that the labels

53
00:02:37,290 --> 00:02:43,140
are going to be advertised not
by LDP but by our IGP, which

54
00:02:43,140 --> 00:02:47,040
would be either OSPF or IS-IS.

55
00:02:47,040 --> 00:02:49,500
Of course, these
two protocols will

56
00:02:49,500 --> 00:02:51,970
need to be modified
to support this,

57
00:02:51,970 --> 00:02:54,630
and we'll be talking
about how they're modified

58
00:02:54,630 --> 00:02:56,910
as we move through this skill.

59
00:02:56,910 --> 00:02:59,460
So those are some
changes to the operation,

60
00:02:59,460 --> 00:03:01,680
but what is this
really all about?

61
00:03:01,680 --> 00:03:03,720
What are we trying
to achieve here?

62
00:03:03,720 --> 00:03:06,630
Well, there's really
two major concepts

63
00:03:06,630 --> 00:03:09,550
that surround the idea
of segment routing.

64
00:03:09,550 --> 00:03:14,310
The first is this is a
form of source routing.

65
00:03:14,310 --> 00:03:16,740
In case you're not familiar
with source routing, what

66
00:03:16,740 --> 00:03:21,330
this means is that the
source, or the ingress router,

67
00:03:21,330 --> 00:03:26,280
is the one that actually
chooses the path that is then

68
00:03:26,280 --> 00:03:28,830
encoded into the actual packet.

69
00:03:28,830 --> 00:03:32,940
This is going to be done in
an ordered list of segments,

70
00:03:32,940 --> 00:03:35,880
or we can also
call these labels.

71
00:03:35,880 --> 00:03:40,080
And then the rest of the
network simply executes

72
00:03:40,080 --> 00:03:42,270
those encoded instructions.

73
00:03:42,270 --> 00:03:44,670
In other words, we're going
to forward the traffic

74
00:03:44,670 --> 00:03:48,630
along the path
specified by the source.

75
00:03:48,630 --> 00:03:52,270
Then of course, we have
the segments themselves.

76
00:03:52,270 --> 00:03:55,080
Now, like we said, the
segment is a label,

77
00:03:55,080 --> 00:03:58,950
but that segment or label
can mean different things

78
00:03:58,950 --> 00:04:01,450
or be used for different things.

79
00:04:01,450 --> 00:04:05,910
The segment can really identify
any type of instruction.

80
00:04:05,910 --> 00:04:07,830
It can be a service.

81
00:04:07,830 --> 00:04:13,980
It can be a context, a locator,
an IGP-based forwarding

82
00:04:13,980 --> 00:04:17,970
construct, a BGP-based
forwarding construct.

83
00:04:17,970 --> 00:04:21,899
It can be a local value
or a global index.

84
00:04:21,899 --> 00:04:25,380
Now, of course, we're going to
talk about more of these things

85
00:04:25,380 --> 00:04:28,020
as we go through the rest
of this skill as well.

86
00:04:28,020 --> 00:04:30,150
We're not going to
cover all of these,

87
00:04:30,150 --> 00:04:32,340
but we'll cover the
major ones that are

88
00:04:32,340 --> 00:04:34,420
involved in segment routing.

89
00:04:34,420 --> 00:04:37,890
So each of these segments
is actually identified

90
00:04:37,890 --> 00:04:43,080
by a segment ID, or what
we generally call the SID.

91
00:04:43,080 --> 00:04:47,590
Those SIDs also come
in two different types.

92
00:04:47,590 --> 00:04:51,930
So the two different types
of segments, or SIDs,

93
00:04:51,930 --> 00:04:55,380
are first we have
the global segment.

94
00:04:55,380 --> 00:04:59,220
The global segment
represents a node,

95
00:04:59,220 --> 00:05:02,430
also known as a prefix-SID.

96
00:05:02,430 --> 00:05:06,990
This is really any
node in the SR domain

97
00:05:06,990 --> 00:05:11,310
that can interpret an
associated instruction.

98
00:05:11,310 --> 00:05:15,840
Each node in the SR
domain then installs

99
00:05:15,840 --> 00:05:19,530
that associated instruction
into the forwarding table.

100
00:05:19,530 --> 00:05:21,030
This, of course,
is what we're going

101
00:05:21,030 --> 00:05:25,800
to be using in MPLS to actually
forward the traffic then.

102
00:05:25,800 --> 00:05:28,980
Now, one interesting thing
here about segment routing

103
00:05:28,980 --> 00:05:36,240
is these prefix-SIDs have a
label value that is manually

104
00:05:36,240 --> 00:05:41,250
assigned from what's called the
Segment Routing Global Block,

105
00:05:41,250 --> 00:05:43,710
or the SRGB.

106
00:05:43,710 --> 00:05:48,060
Now, we'll be talking about this
manual assignment and the SRGB

107
00:05:48,060 --> 00:05:50,590
in more detail here coming up.

108
00:05:50,590 --> 00:05:54,120
Now, since this is a manual
assignment essentially

109
00:05:54,120 --> 00:05:58,200
identifying a prefix,
much like IP addresses,

110
00:05:58,200 --> 00:06:03,150
this cannot be duplicated
in the SR domain.

111
00:06:03,150 --> 00:06:05,590
This must be a unique value.

112
00:06:05,590 --> 00:06:09,990
So keep that in mind when
we're assigning prefix-SIDs.

113
00:06:09,990 --> 00:06:12,520
What we're actually
talking about here

114
00:06:12,520 --> 00:06:16,540
are the labels that we're going
to be using for our destination

115
00:06:16,540 --> 00:06:17,480
networks.

116
00:06:17,480 --> 00:06:21,580
So typically, client-facing
networks, loopback interfaces

117
00:06:21,580 --> 00:06:25,150
on PE routers, things
like this, those

118
00:06:25,150 --> 00:06:28,450
are the things that are going
to get prefix-SIDs identified

119
00:06:28,450 --> 00:06:30,070
or assigned to them.

120
00:06:30,070 --> 00:06:31,840
We're not going to
be assigning these

121
00:06:31,840 --> 00:06:35,890
to every prefix in our routing
table, just the prefixes

122
00:06:35,890 --> 00:06:38,710
that we're going to be
using as destinations.

123
00:06:38,710 --> 00:06:43,450
The next type of SID we're going
to have is a local segment.

124
00:06:43,450 --> 00:06:46,340
This represents a link.

125
00:06:46,340 --> 00:06:50,230
This is known as
the adjacency SID.

126
00:06:50,230 --> 00:06:55,670
And here, this is only going to
be for devices on the segment.

127
00:06:55,670 --> 00:06:58,330
So only the segment
originating nodes

128
00:06:58,330 --> 00:07:01,750
can interpret the associated
instruction on this type

129
00:07:01,750 --> 00:07:04,270
because it's for
connected neighbors.

130
00:07:04,270 --> 00:07:08,920
This label, much like
traditional MPLS labels,

131
00:07:08,920 --> 00:07:11,680
are still going to be
dynamically allocated

132
00:07:11,680 --> 00:07:14,660
from the dynamic block,
so in other words,

133
00:07:14,660 --> 00:07:17,020
the same block that
MPLS would normally

134
00:07:17,020 --> 00:07:18,670
be pulling its labels from.

135
00:07:18,670 --> 00:07:23,800
Now, this does vary a little
bit on iOS XE versus iOS XR,

136
00:07:23,800 --> 00:07:27,070
but we'll cover that more
when we get to implementation.

137
00:07:27,070 --> 00:07:29,500
And again, since
this is an adjacency,

138
00:07:29,500 --> 00:07:33,130
this is going to be
allocated to physical links

139
00:07:33,130 --> 00:07:35,030
in our environment.

140
00:07:35,030 --> 00:07:36,700
So now that we have
sort of a rough idea

141
00:07:36,700 --> 00:07:38,650
here of these two
different segment types,

142
00:07:38,650 --> 00:07:43,640
let's focus in a little
bit on that SRGB.

143
00:07:43,640 --> 00:07:46,250
So the Segment
Routing Global Block

144
00:07:46,250 --> 00:07:49,790
defines the actual
label range that we're

145
00:07:49,790 --> 00:07:52,970
going to be using
for segment routing.

146
00:07:52,970 --> 00:07:54,830
This has to, of
course, be defined

147
00:07:54,830 --> 00:07:57,410
on all of the devices
in the environment,

148
00:07:57,410 --> 00:08:02,480
and then it gets
distributed as a combination

149
00:08:02,480 --> 00:08:07,490
of the label range itself,
as well as an index number

150
00:08:07,490 --> 00:08:09,410
for things like the prefix-SID.

151
00:08:09,410 --> 00:08:12,450

152
00:08:12,450 --> 00:08:22,020
Now, on iOS XR, this defaults
to 16,000 through 23,999.

153
00:08:22,020 --> 00:08:26,220
Now, this range is going
to be reserved on iOS XR

154
00:08:26,220 --> 00:08:29,910
even if segment
routing is not enabled.

155
00:08:29,910 --> 00:08:35,010
The IGP will register
this range when

156
00:08:35,010 --> 00:08:37,169
segment routing is enabled.

157
00:08:37,169 --> 00:08:39,690
We'll, of course,
see this in action

158
00:08:39,690 --> 00:08:42,780
when we get to our
implementation skill.

159
00:08:42,780 --> 00:08:45,630
So ultimately, this
is all put together.

160
00:08:45,630 --> 00:08:50,100
The assigned label to
a particular prefix

161
00:08:50,100 --> 00:08:55,810
will be the prefix index
number plus the SRGB base.

162
00:08:55,810 --> 00:08:59,080
So let's discuss a little
bit about what that means.

163
00:08:59,080 --> 00:09:03,000
Let's say that we have a
prefix that we have, we'll say,

164
00:09:03,000 --> 00:09:07,320
10.1.1.1-32.

165
00:09:07,320 --> 00:09:11,670
We want to give this a
label in segment routing.

166
00:09:11,670 --> 00:09:15,090
Well, there's actually
two ways to do that.

167
00:09:15,090 --> 00:09:17,730
We could say that the
absolute value is say,

168
00:09:17,730 --> 00:09:22,070
for example, 16,010.

169
00:09:22,070 --> 00:09:28,800
Or as another choice, we could
say that it's an index of 10.

170
00:09:28,800 --> 00:09:31,670
So what it'll do is
take this index number

171
00:09:31,670 --> 00:09:37,160
and add to that the base of
the label range, which again,

172
00:09:37,160 --> 00:09:43,760
remember is 16,000 by default.
So it would add 16,000 to this.

173
00:09:43,760 --> 00:09:46,820
And ultimately, of course,
we're going to end up

174
00:09:46,820 --> 00:09:51,410
with the same label 16,010.

175
00:09:51,410 --> 00:09:54,650
So as we're configuring
this, we can actually

176
00:09:54,650 --> 00:09:56,060
configure either one.

177
00:09:56,060 --> 00:09:59,150
We can configure this
as an absolute value,

178
00:09:59,150 --> 00:10:02,420
or we can configure
it as an index.

179
00:10:02,420 --> 00:10:05,840
It really doesn't matter as
long as we end up with the label

180
00:10:05,840 --> 00:10:09,450
that we want to use
for each given prefix.

181
00:10:09,450 --> 00:10:12,410
Now, as far as the rest
of the label range,

182
00:10:12,410 --> 00:10:16,650
the range above
the 24,000 is still

183
00:10:16,650 --> 00:10:19,710
going to be used
for dynamic labels,

184
00:10:19,710 --> 00:10:23,490
such as the adjacency
set and any other MPLS

185
00:10:23,490 --> 00:10:27,130
labels that might be used
on this particular device.

186
00:10:27,130 --> 00:10:34,530
So again, just keep in mind that
16,000 to 23,999 is on iOS XR,

187
00:10:34,530 --> 00:10:36,850
that default label range.

188
00:10:36,850 --> 00:10:39,330
Now again, we'll see
as we jump into this,

189
00:10:39,330 --> 00:10:43,620
iOS XE acts just a little
bit differently here as far

190
00:10:43,620 --> 00:10:45,360
as the SRGB goes.

191
00:10:45,360 --> 00:10:46,920
But we'll see how
that works when

192
00:10:46,920 --> 00:10:48,690
we get into implementation.

193
00:10:48,690 --> 00:10:52,950
Just to point it out, it simply
doesn't reserve the range

194
00:10:52,950 --> 00:10:57,660
automatically and by default
and start the dynamic range

195
00:10:57,660 --> 00:10:59,970
above 24,000.

196
00:10:59,970 --> 00:11:02,640
It actually starts
the dynamic range

197
00:11:02,640 --> 00:11:08,070
at 16, which is above the
reserved MPLS labels of 0

198
00:11:08,070 --> 00:11:09,160
through 15.

199
00:11:09,160 --> 00:11:15,210
So it just doesn't pre-reserve
the SRGB range on iOS XE.

200
00:11:15,210 --> 00:11:18,840
But again, we'll see that
when we get to implementation.

201
00:11:18,840 --> 00:11:21,120
Now, something else
pretty cool here

202
00:11:21,120 --> 00:11:25,620
is since segment routing
is still just using

203
00:11:25,620 --> 00:11:29,340
MPLS for the data
plane, that means

204
00:11:29,340 --> 00:11:34,740
that segment routing and
traditional or legacy MPLS

205
00:11:34,740 --> 00:11:38,310
can be mixed with
no problems, which

206
00:11:38,310 --> 00:11:40,770
means these can easily coexist.

207
00:11:40,770 --> 00:11:47,160
Now, if they do coexist,
and we have both LDP and SR

208
00:11:47,160 --> 00:11:54,090
labels, by default LDP is
still going to be preferred.

209
00:11:54,090 --> 00:11:56,580
Now, we're going to see when
we get to implementation

210
00:11:56,580 --> 00:11:58,660
we can, of course, change that.

211
00:11:58,660 --> 00:12:02,160
But if they do both exist,
the default behavior

212
00:12:02,160 --> 00:12:04,830
is still to prefer LDP.

213
00:12:04,830 --> 00:12:07,930
That behavior may
change in the future.

214
00:12:07,930 --> 00:12:10,350
However, the way it
is right now, this

215
00:12:10,350 --> 00:12:12,510
is actually very,
very convenient

216
00:12:12,510 --> 00:12:16,030
when it comes to doing
things like migration.

217
00:12:16,030 --> 00:12:20,070
This means that you can have
your LDP functional environment

218
00:12:20,070 --> 00:12:24,700
continue operating while you
implement segment routing,

219
00:12:24,700 --> 00:12:27,930
and then at some point
in the future, simply

220
00:12:27,930 --> 00:12:29,850
change this
preference right here

221
00:12:29,850 --> 00:12:34,350
and switch over and start
using segment routing.

222
00:12:34,350 --> 00:12:37,910
So as we've mentioned,
a segment is simply

223
00:12:37,910 --> 00:12:41,840
encoded as an MPLS label.

224
00:12:41,840 --> 00:12:44,990
An ordered list of segments
that we want to traverse

225
00:12:44,990 --> 00:12:49,590
is going to be encoded
as an MPLS label stack.

226
00:12:49,590 --> 00:12:52,790
We know that MPLS can
support more than one label.

227
00:12:52,790 --> 00:12:56,570
Each label is simply put in
the order of the segments

228
00:12:56,570 --> 00:12:58,070
that we wish to cross.

229
00:12:58,070 --> 00:13:01,190
That's how the source-based
routing operates.

230
00:13:01,190 --> 00:13:04,410
Once that segment is actually
used, in other words,

231
00:13:04,410 --> 00:13:07,520
once we're past that
segment in the forwarding,

232
00:13:07,520 --> 00:13:10,590
that segment is then
removed from the stack

233
00:13:10,590 --> 00:13:13,400
so that the next label
for the next segment

234
00:13:13,400 --> 00:13:15,500
is then the top
label, and that's

235
00:13:15,500 --> 00:13:17,250
how we do our forwarding.

236
00:13:17,250 --> 00:13:19,190
So our final thing
to cover here,

237
00:13:19,190 --> 00:13:22,820
how does a node decide
whether it should impose

238
00:13:22,820 --> 00:13:26,150
a prefix-SID label on a packet?

239
00:13:26,150 --> 00:13:28,950
And the answer has
a couple of things.

240
00:13:28,950 --> 00:13:33,080
First, the destination
itself or the next hop

241
00:13:33,080 --> 00:13:36,290
that the destination
resolves on matches

242
00:13:36,290 --> 00:13:39,260
a forwarding equivalency
class that has

243
00:13:39,260 --> 00:13:41,820
a prefix-SID assigned to it.

244
00:13:41,820 --> 00:13:46,900
Also, the downstream neighbor
has to be SR-enabled.

245
00:13:46,900 --> 00:13:50,640
It will, of course,
know this from the IGP.

246
00:13:50,640 --> 00:13:55,890
Also, the node has to be
configured to prefer the SR

247
00:13:55,890 --> 00:13:59,160
label over the LDP label.

248
00:13:59,160 --> 00:14:02,190
Now, of course, we
only have to prefer one

249
00:14:02,190 --> 00:14:04,290
if LDP is configured.

250
00:14:04,290 --> 00:14:07,770
The other option would
be LDP is not configured.

251
00:14:07,770 --> 00:14:09,190
So guess what.

252
00:14:09,190 --> 00:14:13,380
It doesn't even
have an LDP label.

253
00:14:13,380 --> 00:14:15,600
If it doesn't have an
LDP label, then there's

254
00:14:15,600 --> 00:14:16,840
nothing to fight with.

255
00:14:16,840 --> 00:14:20,550
And if we have an SR label,
then that's clearly the one

256
00:14:20,550 --> 00:14:22,330
that we would use.

257
00:14:22,330 --> 00:14:25,980
So in this video, we jumped
in and got an introduction

258
00:14:25,980 --> 00:14:27,630
to segment routing.

259
00:14:27,630 --> 00:14:30,780
We talked about how it
uses source-based routing

260
00:14:30,780 --> 00:14:34,860
and a label stack with
prefix-SIDs to get traffic

261
00:14:34,860 --> 00:14:36,250
to the destination.

262
00:14:36,250 --> 00:14:38,970
We also talked a bit
about how the control

263
00:14:38,970 --> 00:14:41,340
plane is going to be replaced.

264
00:14:41,340 --> 00:14:44,040
We're no longer
going to use LDP.

265
00:14:44,040 --> 00:14:47,130
And instead, we're
going to use the IGP.

266
00:14:47,130 --> 00:14:49,470
Now, that change in
the control plane

267
00:14:49,470 --> 00:14:52,890
is, of course, going to
take a more detailed look

268
00:14:52,890 --> 00:14:54,660
to understand how it works.

269
00:14:54,660 --> 00:14:57,010
And that's exactly what
we're going to do next.

270
00:14:57,010 --> 00:15:00,330
Until then, I hope this has
been informative for you,

271
00:15:00,330 --> 00:15:03,410
and I'd like to thank
you for viewing.

272
00:15:03,410 --> 00:15:07,000