-: Troubleshooting network issues, 2 2
in this section, 3 3
we're going to discuss how to troubleshoot general 4 4
networking issues. 5 5
As we go through this section of the course, 6 6
we're going to again focus on Domain 5 network troubleshooting 7 7
and our final objective, 8 8
objective 5.5. 9 9
Objective 5.5 states, 10 10
that given a scenario, 11 11
you must troubleshoot general networking issues. 12 12
Now, as we go through this section of the course, 13 13
we're going to talk about collisions, and broadcast storms, 14 14
and duplicate addresses, 15 15
and flooding and switching, and routing loops, 16 16
DHCP issues, IP misconfigurations, 17 17
VLM misconfigurations, firewall issues, DNS issues, 18 18
NTP issues, network performance issues, 19 19
low optical link budgets, 20 20
certificate issues, 21 21
licensed feature issues, BYOD challenges, 22 22
and some hardware failures. 23 23
But before we cover all those different issues 24 24
in this section of the course, 25 25
I want to use the rest of this lesson 26 26
to discuss a few things you need to consider 27 27
during your general network troubleshooting efforts, 28 28
including device configuration reviews, 29 29
routing tables, interface status, 30 30
VLAN assignments, 31 31
and your network performance baselines. 32 32
First, we have device configuration reviews. 33 33
Now previously 34 34
we've gone through a standard device configuration 35 35
and we reviewed it section by section. 36 36
But here, I just wanted to remind you 37 37
that whenever you're experiencing a networking issue, 38 38
it's always a good idea 39 39
to look at your configuration files once more. 40 40
Most network devices have two configurations, 41 41
a startup configuration and a running configuration. 42 42
The startup configuration file is stored in NVRAM, 43 43
and it contains the commands needed 44 44
to initially configure a router 45 45
when it's being booted up. 46 46
It also creates the running configuration file 47 47
that's going to to be stored in RAM 48 48
and used during normal operations. 49 49
The running configuration 50 50
is the configurations that are actively 51 51
being used by the router 52 52
at any given time. 53 53
Any changes made to the configuration 54 54
by a network technician 55 55
in the command line interface 56 56
is applied first to the running configuration. 57 57
But, 58 58
if it doesn't saved back to the startup configuration, 59 59
those changes will be lost 60 60
the next time you reboot the router. 61 61
So if everything was working and you rebooted the router 62 62
and things stop working, 63 63
it may be that you're missing a configuration. 64 64
Now if you want to see the startup configuration, 65 65
you can enter the command, Show startup-config 66 66
at the command line interface. 67 67
If you want to see the running configuration, 68 68
you're going to enter show running-config 69 69
at the command line interface. 70 70
Now if you need to save any changes 71 71
that you've made to the running config, 72 72
to the startup config, 73 73
you can do this by entering the command; 74 74
copy running-config, startup-config 75 75
into the command line interface. 76 76
Remember 77 77
these three commands work on Cisco devices, 78 78
but other network devices use similar commands 79 79
to conduct these same types of actions. 80 80
Now, next, you need to look at your routing tables. 81 81
Anytime data isn't flowing properly in your networks, 82 82
it could be a routing issue 83 83
to verify your routing tables. 84 84
You need to check your routers, 85 85
your layer three switches 86 86
and any workstations or servers you may have 87 87
to view the routes on a network device 88 88
like a router or a layer three switch. 89 89
You can enter the command Show IP route 90 90
from the command line interface. 91 91
Now, if you need a view, the routing table 92 92
on a Windows, workstation or server, 93 93
you can use the command Route print at the command prompt. 94 94
If you need to display the routing table on a Linux 95 95
Unix or OSX workstation or server, 96 96
you can enter the command Route-N at the shell. 97 97
Next, you should look at the interface status itself 98 98
when there's networking issues going on 99 99
to do this, you're going to enter the show interface 100 100
or show interfaces command 101 101
at the prompt within the command line environment 102 102
of your router or switch 103 103
again, this is a Cisco specific command, 104 104
but similar commands exist for other brands 105 105
of network devices. 106 106
From this interface status, 107 107
we're going to be able to determine all sorts of issues, 108 108
such as duplexing issues, DHCP issues, collisions, 109 109
and broadcast storms. 110 110
Next, we can check the VLAN assignments being used 111 111
for our network and our devices, 112 112
the VLAN assignments or VLAN tagging 113 113
is the practice of segmenting an organizational network 114 114
and separating users into respective networks Sections 115 115
based on their roles and responsibilities. 116 116
Most commonly 117 117
VLAN assignments are done using static assignments 118 118
based on the ports and the interfaces being used. 119 119
Conversely, 120 120
you can also set up dynamic VLAN assignments 121 121
based on a Mac address of a given client. 122 122
And that way, when that client connects, 123 123
it will be put into the right VLAN, 124 124
regardless of which physical port it's being connected to, 125 125
to use dynamic VLAN configurations though, 126 126
you need to use a VLAN Membership Policy Server or VMPS. 127 127
As well as a client server and database. 128 128
Now, remember, 129 129
If you're using VLANs, 130 130
these will be things that will break up 131 131
your broadcast domains 132 132
and reduce the chance of a broadcast on from occurring. 133 133
But because these VLANs break up broadcast domains, 134 134
you need to route traffic from one VLAN to another, 135 135
using a router or layer three switch. 136 136
If you forget to include a router in your network 137 137
architecture 138 138
and you're using VLANs, 139 139
your traffic is not going to be able to flow 140 140
between those VLANs. 141 141
So make sure you keep this in mind. 142 142
Finally, 143 143
you need to ensure that you have 144 144
a good network performance baseline 145 145
created for your network after you initially configure it 146 146
in the simplest terms, 147 147
a network performance baseline, 148 148
it's just a set of metrics used 149 149
in network performance monitoring 150 150
to define what the normal working conditions 151 151
of this enterprise network infrastructure 152 152
are going to look like. 153 153
Engineers are going to use network performance baselines 154 154
for comparison 155 155
to catch changes in traffic that could indicate 156 156
there's a problem with your network. 157 157
It also lets networks operations personnel 158 158
like network technicians, 159 159
understand the before and after effects. 160 160
Whenever a change is made, 161 161
this makes it easier to measure the benefit 162 162
and calculate a return on investment 163 163
when you're making network changes. 164 164
Basically, 165 165
if you think there's a problem 166 166
and something looks abnormal, 167 167
you should consult your network baseline 168 168
to see if there really is something that's wrong 169 169
or something that's been changed. 170 170
And how large of a change has caused 171 171
on your performance. 172 172
Because if you change one thing by adding a route 173 173
and your network performance went down by 50%, 174 174
you probably don't want to add that route anymore. 175 175
And so you need to look into that closer 176 176
again there's lots of places and lots of things 177 177
that can go wrong in our networks. 178 178
So with all these considerations in mind, 179 179
let's jump into this section 180 180
on troubleshooting network issues. 181 181
(upbeat music)