CCNA Icnd110 s02l08

© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—2-1
Ethernet LANs
Troubleshooting
Switch Issues

The Layered Approach
 Switches operate at Layer 2 of the OSI model.
 Switches provide an interface with the physical media.
 Problems generally are seen at Layer 1 and Layer 2.
 Layer 3 issues could arise related to access to the management
functions of the switch.

Switched Media Issues
Media issues have several possible sources:
 Wiring gets damaged.
 New EMI sources are introduced.
 Traffic patterns change.
 New equipment is installed.

show interface
SwitchX#show interface fastethernet 0/0
Ethernet 0 is up, line protocol is up
Hardware is MCI Ethernet, address is aa00.0400.0134 (via 0000.0c00.4369
Internet address is 131.108.1.1, subnet mask is 255.255.255.0
.
.
Output Omitted
.
.
2295197 packets input, 305539992 bytes, 0 no buffer
Received 1925500 broadcasts, 0 runts, 0 giants
3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 input packets with dribble condition detected
3594664 packets output, 436549843 bytes, 0 underruns
8 output errors,
1790 collisions,
10 interface resets,
0 restarts
Fastethernet 0/0 is up, line protocol is up [1]
3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort [2]
8 output errors, [3]
1790 collisions, [4]
0 restarts [5]

Excessive Noise
Suggested steps:
 Use the show interface ethernet EXEC command to determine
the status of the device Ethernet interfaces. The presence of
many CRC errors but not many collisions is an indication of
excessive noise.
 Inspect the cables for damage.
 If you are using 100Base-TX, make sure you are using Category
5 cabling.

Excessive Collisions
Suggested steps:
 Use the show interface ethernet command to check the rate of
collisions. The total number of collisions with respect to the total
number of output packets should be 0.1 percent or less.
 Use a time-domain reflectometer (TDR) to find any unterminated
Ethernet cables. A TDR is a device that sends signals through a
network medium to check cable continuity and other attributes.
 Look for a jabbering transceiver attached to a host. This might
require host-by-host inspection or the use of a protocol analyzer.
Jabber occurs when a device that is experiencing circuitry or logic
failure continuously sends random (garbage) data.

Late Collisions
Suggested steps:
 Use a protocol analyzer to check for late collisions. Late collisions
should never occur in a properly designed Ethernet network. They
usually occur when Ethernet cables are too long or when there
are too many repeaters in the network.
 Verify that the distance between the first and last host on the
segment is within specification.

Port Access Issues
 Media-related issues
 Duplex-related issues
 Speed-related issues

Duplex modes:
 One end set to full and the other set to half results in a mismatch.
 One end set to full and autonegotiation set on the other end:
– Autonegotiation fails, and that end reverts to half.
– Results in a mismatch.
 One end set to half and autonegotiation set on the other:
– Autonegotiation fails, and that end reverts to half.
– Both ends at half; no mismatch.
 Autonegotiation on both ends:
– One end fails to full, and the other end fails to half.
– Example: A Gigabit Ethernet interface defaults to full, while a
10/100 defaults to half.
– Autonegotiation fails on both ends, and they revert to half.
– Both end at half; no mismatch.
Duplex-Related Issues

Duplex modes:
 One end set to one speed and the other set to another, resulting
in a mismatch.
 One end set to a higher speed and autonegotiation enabled on
the other end.
– If autonegotiation fails, the autonegotiation end reverts to its
lowest speed.
– Results in a mismatch.
– Autonegotiation fails on both ends, and they revert to their
lowest speed.
– Both end at half; no mismatch.
Speed-Related Issues

Configuration Issues
 Know what you have before you start.
– Hard copy
– Text file
– TFTP server
 Verify changes before you save.
– Confirm that the issue was corrected and no new issues were
created.
 Save the current configuration.
– copy running-config start-config
 Secure the configuration.
– Password-protect the console.
– Password-protect the vty.
– Password-protect EXEC mode.

Summary
 Use a layered approach to troubleshooting.
 Use the show interface command to troubleshoot these issues:
– Media issues
– Duplex issues
– Speed issues
 Keep a copy of configurations and protect the running
configuration.

CCNA Icnd110 s02l08

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CCNA Icnd110 s02l08

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