1
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2
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Now that we have our
routing protocol configured

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to support segment routing, and
we've configured our devices

4
00:00:14,460 --> 00:00:18,870
to inject a prefix SID for
their loopback interfaces,

5
00:00:18,870 --> 00:00:22,710
it's probably about time that
we configure our environment

6
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to actually use the segment
routing labels or the prefix

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00:00:27,090 --> 00:00:27,930
SIDs.

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00:00:27,930 --> 00:00:31,890
Now, as we saw in the
previous video, the reason

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00:00:31,890 --> 00:00:36,420
it's not doing it right now
is that LDP is still enabled

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00:00:36,420 --> 00:00:40,030
and being preferred
in our environment.

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Now, we could just jump
right in and turn off LDP

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00:00:44,400 --> 00:00:46,270
and hope everything works.

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00:00:46,270 --> 00:00:49,770
But probably a better choice,
particularly if you're doing

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00:00:49,770 --> 00:00:54,270
a migration would be to actually
just change the preference

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to prefer segment routing,
then check and verify

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00:00:58,410 --> 00:01:00,040
that everything's working.

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00:01:00,040 --> 00:01:04,750
And then finally, turn off
LDP if that's our end goal.

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So in this video,
we're going to be

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00:01:06,960 --> 00:01:09,100
doing the first half of that.

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00:01:09,100 --> 00:01:12,280
We're going to configure our
devices to prefer segment

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00:01:12,280 --> 00:01:15,040
routing over the LDP labels.

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00:01:15,040 --> 00:01:17,320
Then of course, we'll
verify that it's still

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00:01:17,320 --> 00:01:19,390
working, we still
have reachability,

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00:01:19,390 --> 00:01:22,250
and that it's in fact
using those labels.

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00:01:22,250 --> 00:01:24,830
So let's go ahead and
jump in and get started.

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00:01:24,830 --> 00:01:28,030
We'll bring up our topology
real quickly just as a reminder

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00:01:28,030 --> 00:01:29,480
of what we're working with.

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00:01:29,480 --> 00:01:31,690
And then we're going to
jump into the command line

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00:01:31,690 --> 00:01:34,090
and configure our
devices to prefer

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00:01:34,090 --> 00:01:36,145
segment routing over LDP.

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00:01:36,145 --> 00:01:38,860

32
00:01:38,860 --> 00:01:42,510
So we are still on
MPLS Topology 1.

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00:01:42,510 --> 00:01:45,090
And again, all we're
going to be doing now

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00:01:45,090 --> 00:01:48,970
is preferring this
segment routing labels.

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00:01:48,970 --> 00:01:52,030
So there's not going to be a lot
to see as far as the topology.

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But we are going to be
focusing on the traffic going

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00:01:54,750 --> 00:01:58,710
from router 6 to
router 5 loopback.

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00:01:58,710 --> 00:02:00,750
And what we're
really looking to see

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00:02:00,750 --> 00:02:07,140
is which label router 1 is
imposing on to this traffic.

40
00:02:07,140 --> 00:02:10,210
We'll verify this when we
get to the command line.

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00:02:10,210 --> 00:02:13,620
But right now, it's
using the LDP label.

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00:02:13,620 --> 00:02:16,080
We want to stop
using that and we

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00:02:16,080 --> 00:02:19,330
want to start using the
segment routing label.

44
00:02:19,330 --> 00:02:22,440
So let's jump into the
command line there on router 6

45
00:02:22,440 --> 00:02:24,720
and verify what the
environment is currently

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00:02:24,720 --> 00:02:29,000
doing before we switch it over

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00:02:29,000 --> 00:02:32,600
So we'll start off tracing
the router 5 loopback sourced

48
00:02:32,600 --> 00:02:34,400
from our loopback.

49
00:02:34,400 --> 00:02:38,240
And what we see focusing
particularly on hop 2

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00:02:38,240 --> 00:02:41,810
is when router 1 is
putting this label on,

51
00:02:41,810 --> 00:02:47,900
it's putting on label
24,006 or 24,009

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00:02:47,900 --> 00:02:50,750
depending on which
equal cost path it's

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00:02:50,750 --> 00:02:54,020
choosing towards router
three or router 4.

54
00:02:54,020 --> 00:02:58,770
But the point is router 1 is
clearly using the LDP label.

55
00:02:58,770 --> 00:03:01,670
So now let's go ahead and
change that preference.

56
00:03:01,670 --> 00:03:03,980
We'll start here on router 1.

57
00:03:03,980 --> 00:03:08,180
So all we have to do is go into
configuration, into the ISIS

58
00:03:08,180 --> 00:03:11,970
process, into the
address family.

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00:03:11,970 --> 00:03:16,790
And then on the command where
we did segment routing MPLS,

60
00:03:16,790 --> 00:03:20,930
we simply have to add sr-prefer.

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00:03:20,930 --> 00:03:22,880
We can see here that
it flat out says

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00:03:22,880 --> 00:03:25,100
that it's going to prefer
segment routing labels

63
00:03:25,100 --> 00:03:29,220
over LDP labels and that's,
of course, exactly what we're

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00:03:29,220 --> 00:03:30,640
trying to do.

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00:03:30,640 --> 00:03:34,650
So we'll Enter that
command, commit our changes,

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00:03:34,650 --> 00:03:38,460
and end configuration.

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00:03:38,460 --> 00:03:42,150
We can now do a show
cef for 10.5.5.5.

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00:03:42,150 --> 00:03:45,630
And notice here,
regardless of which path

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00:03:45,630 --> 00:03:49,990
it's choosing, either the path
through router 4 or router 3,

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00:03:49,990 --> 00:03:54,750
notice that both of these paths
are now imposing the same label

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00:03:54,750 --> 00:04:00,300
and it's the segment
routing label 16,005.

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00:04:00,300 --> 00:04:03,330
Don't forget that's
the prefix SID

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00:04:03,330 --> 00:04:07,870
that we assigned to router 5's
loopback in the previous video.

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00:04:07,870 --> 00:04:10,890
So now we see that it's clearly
using the segment routing

75
00:04:10,890 --> 00:04:13,380
labels to do the
forwarding, at least

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00:04:13,380 --> 00:04:14,740
according to the cef table.

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00:04:14,740 --> 00:04:17,570
Let's do some more verification.

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00:04:17,570 --> 00:04:20,510
We could also do a
trace right from here.

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00:04:20,510 --> 00:04:23,280
Now, since this is
router 1 doing the trace,

80
00:04:23,280 --> 00:04:28,640
we see that it's locally
imposing this label of 16,005.

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00:04:28,640 --> 00:04:31,100
So now that we see that
this is working the way

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00:04:31,100 --> 00:04:33,380
we want it to, let's
go ahead and turn this

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00:04:33,380 --> 00:04:35,460
on on the rest of our routers.

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00:04:35,460 --> 00:04:37,580
So let's hop over to router 2.

85
00:04:37,580 --> 00:04:40,490
Once again, we go into
configuration mode,

86
00:04:40,490 --> 00:04:45,480
into router ISIS 1,
into our address family.

87
00:04:45,480 --> 00:04:48,110
And we turn on sr-prefer.

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00:04:48,110 --> 00:04:50,520
Routers 3 and 4
will be identical

89
00:04:50,520 --> 00:04:53,490
so we'll simply paste
that configuration in.

90
00:04:53,490 --> 00:04:57,350
So there's router
3 and router 4.

91
00:04:57,350 --> 00:05:00,960
Again, iOS XE, slightly
different configuration.

92
00:05:00,960 --> 00:05:03,600
So let's go over to
router 5 and take a look.

93
00:05:03,600 --> 00:05:07,010
So on iOS XE we want to go
into configuration mode.

94
00:05:07,010 --> 00:05:09,830
Then we go into segment
routing MPLS right

95
00:05:09,830 --> 00:05:12,020
from global configuration.

96
00:05:12,020 --> 00:05:16,400
And there's a choice here that
we can use to set attributes.

97
00:05:16,400 --> 00:05:19,570
And this will Enter the
sr. set attributes mode.

98
00:05:19,570 --> 00:05:21,880
So it's a sub
configuration mode.

99
00:05:21,880 --> 00:05:24,090
So we'll say set attributes.

100
00:05:24,090 --> 00:05:27,610
And then under here we go
into our address family.

101
00:05:27,610 --> 00:05:30,180
And again, the command is
slightly different here.

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00:05:30,180 --> 00:05:35,280
It's sr-label preferred
so we'll configure that.

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00:05:35,280 --> 00:05:37,300
Exit configuration.

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00:05:37,300 --> 00:05:40,200
And now let's do some
final verification.

105
00:05:40,200 --> 00:05:42,360
Let's go back over to router 6.

106
00:05:42,360 --> 00:05:44,560
We'll do our trace again.

107
00:05:44,560 --> 00:05:48,840
So based on this output, let's
take a look at two things,

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00:05:48,840 --> 00:05:51,795
again focusing on hop 2.

109
00:05:51,795 --> 00:05:54,900
First, we see it's
still doing equal cost

110
00:05:54,900 --> 00:05:59,190
multipathing between routers
3 and 4 as it should.

111
00:05:59,190 --> 00:06:03,510
However, notice that it's now
using the exact same label

112
00:06:03,510 --> 00:06:06,690
for both routers 3 and 4.

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00:06:06,690 --> 00:06:09,330
If you remember, we had
mentioned previously

114
00:06:09,330 --> 00:06:13,110
in a different skill that one
of the things about segment

115
00:06:13,110 --> 00:06:19,200
routing is the labels or the
prefix SIDs become global.

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00:06:19,200 --> 00:06:22,020
Since router 5 is
the one announcing

117
00:06:22,020 --> 00:06:25,230
what label to use
for its loopback,

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00:06:25,230 --> 00:06:27,790
everybody's using
the same label.

119
00:06:27,790 --> 00:06:31,110
So previously with
LDP implementation,

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00:06:31,110 --> 00:06:34,500
each device was making
up its own label

121
00:06:34,500 --> 00:06:36,430
to get to a destination.

122
00:06:36,430 --> 00:06:39,030
Now, theoretically,
if all the routers

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00:06:39,030 --> 00:06:41,430
had the exact same routes
with the exact same time when

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00:06:41,430 --> 00:06:45,090
they all reloaded and everything
was hyperconverged identically,

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00:06:45,090 --> 00:06:47,490
well, all the routers might
have magically come up

126
00:06:47,490 --> 00:06:50,700
with the exact same labels
for the exact same prefix

127
00:06:50,700 --> 00:06:52,980
but it would have
just been random luck.

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00:06:52,980 --> 00:06:58,200
Here, the label becomes global
because router 5 is telling

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00:06:58,200 --> 00:07:00,370
everybody which label to use.

130
00:07:00,370 --> 00:07:03,510
And we can see that here
represented in the traces.

131
00:07:03,510 --> 00:07:07,180
When we were using LDP, we
were using different labels

132
00:07:07,180 --> 00:07:09,550
for routers 3 and
4 because that's

133
00:07:09,550 --> 00:07:11,110
what they told us to use.

134
00:07:11,110 --> 00:07:14,230
But now since we're using
the label from router 5,

135
00:07:14,230 --> 00:07:17,290
we can see that
because it's 16,005

136
00:07:17,290 --> 00:07:19,660
and that's what we had
configured on router 5.

137
00:07:19,660 --> 00:07:22,420
So routers 3 and 4 aren't
making their own labels

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00:07:22,420 --> 00:07:24,370
up for this destination.

139
00:07:24,370 --> 00:07:27,190
And of course, the other
thing to pick up off the trace

140
00:07:27,190 --> 00:07:29,050
since we said there
were two things

141
00:07:29,050 --> 00:07:32,950
is that it is in fact now
using the segment routing label

142
00:07:32,950 --> 00:07:34,970
which we've already discussed.

143
00:07:34,970 --> 00:07:36,550
So that's obvious.

144
00:07:36,550 --> 00:07:38,560
But those are the
two big things here.

145
00:07:38,560 --> 00:07:42,100
The label is global
now and we're actually

146
00:07:42,100 --> 00:07:44,070
using that label.

147
00:07:44,070 --> 00:07:48,160
Now, just to verify our
iOS XE configuration,

148
00:07:48,160 --> 00:07:51,400
we should probably do a
trace the other direction.

149
00:07:51,400 --> 00:07:56,790
So now let's go from
router 8 over to router 1.

150
00:07:56,790 --> 00:07:58,980
So what we see here
out of our trace,

151
00:07:58,980 --> 00:08:02,790
particularly here on hop
2 is that we are in fact

152
00:08:02,790 --> 00:08:07,350
using that segment routing
label, the 16,001 which

153
00:08:07,350 --> 00:08:11,370
of course, is the prefix SID
that we configured on router 1.

154
00:08:11,370 --> 00:08:14,280
So now our environment
is doing segment

155
00:08:14,280 --> 00:08:20,000
routing all the way through
end to end in both directions.

156
00:08:20,000 --> 00:08:23,990
In this video, we saw how
to configure our devices.

157
00:08:23,990 --> 00:08:29,090
Again, iOS XR and iOS XE
being slightly different,

158
00:08:29,090 --> 00:08:32,690
but we saw how to configure them
to prefer the segment routing

159
00:08:32,690 --> 00:08:35,750
labels over the
LDP learned labels.

160
00:08:35,750 --> 00:08:38,330
And we saw that it
switched over and started

161
00:08:38,330 --> 00:08:42,270
using those segment routing
labels as we desired.

162
00:08:42,270 --> 00:08:48,080
Now, since we're not using
the LDP labels anymore,

163
00:08:48,080 --> 00:08:51,410
we probably don't
need LDP anymore.

164
00:08:51,410 --> 00:08:54,220
So why don't we go
ahead and turn that off.

165
00:08:54,220 --> 00:08:58,040
That's exactly what we're
going to do in our next video.

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00:08:58,040 --> 00:09:01,630
Until then, I hope this has
been informative for you

167
00:09:01,630 --> 00:09:04,650
and I'd like to thank
you for viewing.

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