Troubleshooting Switched Networks

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As the number of switch features grows, so does the possibility that things will go wrong.
This section presents recommendations for implementing a functional network. It also
addresses some of the common reasons that port connectivity, VLAN configuration, VTP,
and STP can fail, as well as what information to look for to identify the source of a problem.
Troubleshooting Switches
There are many ways to troubleshoot a switch. Developing a troubleshooting approach or
test plan works much better than using a hit-or-miss approach. Here are some general
suggestions to make troubleshooting more effective:
■ Take the time to become familiar with normal switch operation: The Cisco website
(Cisco.com) has a lot of technical information that describes how its switches work.
The configuration guides in particular are helpful.
■ For more large multiswitch environments, have an accurate physical and logical
map of the network on hand: A physical map shows how the devices and cables are
connected. A logical map shows what segments (VLANs) exist in the network and
which routers provide routing services to these segments. A spanning-tree map is also
useful for troubleshooting complex issues. Because a switch can create different
segments by implementing VLANs, the physical connections alone do not tell the
whole story. You must know how the switches are configured to determine which
segments (VLANs) exist and how they are logically connected.
Troubleshooting Switched Networks 77
■ Have a plan: Some problems and solutions are obvious; others are not. The symptoms
that you see in the network can be the result of problems in another area or layer.
Before jumping to conclusions, try to verify in a structured way what is working and
what is not. Because networks can be complex, it is helpful to isolate possible problem
domains. One way to do this is to use the OSI seven-layer model. For example: Check
the physical connections involved (Layer 1), check connectivity issues within the
VLAN (Layer 2), check connectivity issues across different VLANs (Layer 3), and so
on. Assuming that the switch is configured correctly, many of the problems you
encounter will be related to physical layer issues (physical ports and cabling).
■ Do not assume a component is working without first verifying that it is: If a PC is
not able to log into a server across the network, it could be due to any number of things.
Do not assume basic components are working correctly without testing them first;
someone else might have altered their configurations and not informed you of the
change. It usually takes only a minute to verify the basics (for example, that the ports
are correctly connected and active), and it can save you valuable time.
Figure 2-37 outlines a basic flow for troubleshooting switch problems that will be used in
this section.
Figure 2-37 Troubleshooting Flow
Troubleshooting Port Connectivity
If you are experiencing connectivity problems, the first thing to check is the port. Ports are
the foundation of the switched network. If they do not work, nothing works! Some ports
have special significance because of their location in the network and the amount of traffic
they carry. These include ports that have connections to other switches, routers, and servers.
They can be more complicated to troubleshoot because they often take advantage of special
Troubleshoot
Physical Layer
Port Connectivity
Issues
Troubleshoot
VLAN/Trunking
Issues
Troubleshoot
VTP
Issues
Troubleshoot
Spanning-Tree
Issues
78 Chapter 2: Medium-Sized Switched Network Construction
features, such as trunking and EtherChannel. However, do not overlook the other ports;
they, too, are significant because they connect users in the network. Figure 2-38 shows the
flow for troubleshooting port connectivity.
Figure 2-38 Troubleshooting Port Connectivity
Hardware Issues
Hardware issues can be one of the reasons a switch has connectivity issues. To rule out
hardware issues, verify the following:
■ The port status for both ports involved in the link: Ensure that neither is shut down.
The administrator may have manually shut down one or both ports, or the switch
software may have shut down one of the ports because of a configuration error. If one
side is shut down and the other is not, the status on the enabled side will be
“notconnected” (because it does not sense a neighbor on the other side of the wire).
The status on the shutdown side will say something like “disable” or “errDisable”
(depending on what actually shuts down the port). The link will not be active unless
both ports are enabled.
■ The type of cable used for the connection: You should use at least Category 5 cable
for 100 Mbps connections, and Category 5e for 1 Gbps or faster. You use a straightthrough
RJ-45 cable for end stations, routers, or servers to connect to a switch or hub.
You use an Ethernet crossover cable for switch-to-switch connections or hub-to-switch
connections. The maximum distance for Ethernet, FastEthernet, or Gigabit Ethernet
copper wires is 100 meters.
Troubleshoot
Physical Layer/
Port Connectivity
Issues
Troubleshoot
VLAN/Trunking
Issues
Troubleshoot
VTP
Issues
Troubleshoot
Spanning-Tree
Issues
Correct
cable type,
category,
length?
Is port VLAN
membership
correct?
Are
local and peer
ports up and not
admin down or
err-disabled?
Duplex
mismatch?
Troubleshooting Switched Networks 79
■ A software process disables a port: A solid orange light on the port indicates that the
switch software has shut down the port, either by way of the user interface or by
internal processes such as spanning tree BPDU guard; Root Guard, which prevents a
port from becoming the root port; or port security violations.
Configuration Issues
Configuration of the port is another possible reason the port may be experiencing
connectivity issues. Some of the common configuration issues are as follows:
■ The VLAN to which the port belongs has disappeared: Each port in a switch
belongs to a VLAN. If the VLAN is deleted, then the port becomes inactive.
The following set of code illustrates that the command show interface interface will
not reveal a problem when a port is configured to be part of a VLAN that does not exist.
SwitchX# sh int fa0/2
FastEthernet0/2 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0017.596d.2a02 (bia 0017.596d.2a02)
Description: Interface to RouterA F0/0
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is 10/100BaseTX
However, the command show interface interface switchport does show that the port
is inactive and will not be functional until the missing VLAN is replaced.
SwitchX# sh int fa0/2 switchport
Name: Fa0/2
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 5 (Inactive)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Some switches show a steady orange light on each port for which the associated VLAN
has disappeared. If many orange lights appear on the switch, do not panic. It may be
that all the ports belong to the same VLAN, which was accidentally deleted. When you
add the VLAN back into the VLAN table, the ports become active again. A port
remembers its assigned VLAN.
80 Chapter 2: Medium-Sized Switched Network Construction
■ Autonegotiation is enabled: Autonegotiation is an optional function of the
FastEthernet (IEEE 802.3u) standard that enables devices to automatically exchange
information about speed and duplex abilities over a link. You should not use
autonegotiation for ports that support network infrastructure devices, such as switches,
routers, or other nontransient end systems, such as servers and printers.
Autonegotiating speed and duplex settings is the typical default behavior on switch
ports that have this capability. However, you should always configure ports that
connect to fixed devices for the correct speed and duplex setting, rather than allow
them to autonegotiate these settings. This configuration eliminates any potential
negotiation issues and ensures that you always know exactly how the ports should be
operating.