18 Troubleshooting IP, IPv6, and VLANs Flashcards
Server Roles
Most common kinds of servers found in a network:
Active Directory
Microsoft AD is Windows Server’s flagship role for User and Computer management and it’s used one way or another by almost every company in existence!
DNS
Using the Internet in any kind of efficient way depends on DNS because without it, we would all be surfing by memorizing IP addresses.
DHCP
Covered earlier, DHCP is how your endpoints dynamically learn their IP address to get on the network.
Hypervisor
I’ll talk about this role more when we get into virtualization, but for now, just know it’s what allows us to run virtual machines.
RADIUS
This role is largely used by wireless to authenticate connections into the network.
TACACS+
This role is used for device administration and can control what a user has access to when they log into a device.
Email
The type of server that manages sending and receiving email messages.
File
File servers store a large number of files for users to access.
Databases
These servers store data in mysterious tables ran by crazy wizards known as DBAs. Avoid DBAs at all cost!
Web
This type of server runs the webpages we browse on the Internet.
Using SPAN for Troubleshooting
A traffic sniffer can be a valuable tool for monitoring and troubleshooting your network.
However, since the inception of switches into our networks more than 20 years ago, troubleshooting
has become tougher because we can’t just plug an analyzer into a switch port
and be able to read all the network traffic. Before we had switches, we used hubs, and when
a hub received a digital signal on one port, the hub sent that digital signal out on all ports
except the port it was received on. This allows a traffic sniffer that’s connected to a hub
port to receive all traffic in the network.
Modern local networks are essentially switched networks. After a switch boots, it starts
to build up a layer 2 forwarding table based on the source MAC addresses of the different
packets that the switch receives. After the switch builds this forwarding table, it forwards
traffic destined for a known MAC address directly to the exit port associated with that
MAC address. By default, this prevents a traffic sniffer connected to another port from
receiving the unicast traffic. SPAN helps us analyze network traffic passing through the port by sending a copy of the
traffic to another port on the switch that’s been connected to a network analyzer or other monitoring device. SPAN copies the traffic that the device receives and/or sends on source
ports to a destination port for analysis.
For example, if you would like to analyze the traffic flowing from PC1 to PC2, shown
in Figure 18.17, you need to specify a source port where you want to capture the data. You
can either configure the interface Fa0/1 to capture the ingress traffic or configure the interface
Fa0/3 to capture the egress traffic—your choice. Next, specify the destination port
interface where the sniffer is connected and will capture the data, in this example, Fa0/2.
The traffic flowing from PC1 to PC2 will then be copied to that interface and you’ll be able
to analyze it with a traffic sniffer.
VLAN Troubleshooting
A couple of key times to troubleshoot VLANs are when and if you lose connectivity
between hosts and when you’re configuring new hosts into a VLAN but they’re not
working.
Here are the steps we’ll follow to troubleshoot VLANs:
1. Verify the VLAN database on all your switches.
2. Verify your content addressable memory (CAM) table.
3. Verify that your port VLAN assignments are configured correctly.
And here’s a list of the commands we’ll be using in the section coming up:
Show vlan
Show mac address-table
Show interfaces interface switchport
switchport access vlan vlan
Trunk Troubleshooting
You’ll need to troubleshoot trunk links when you lose connectivity between hosts that are
in the same VLAN but located on different switches. Cisco refers to this as “VLAN leaking.”
Seems to me we are leaking VLAN 10 between switches somehow.
These are the steps we’ll take to troubleshoot VLANs:
1. Verify that the interface configuration is set to the correct trunk parameters.
2. Verify that the ports are configured correctly.
3. Verify the native VLAN on each switch.
And here are the commands we’ll use to perform trunk troubleshooting:
Show interfaces trunk
Show vlan
Show interfaces interface trunk
Show interfaces interface switchport
Show dtp interface interface
switchport mode
switchport mode dynamic
switchport trunk native vlan vlan
