System information

Troubleshooting Ethernet
Book Title
4-60
When you’re troubleshooting Ethernet media in a Cisco router environment, the show interfaces
ethernet command provides several key fields of information that can assist with isolating problems.
The following section provides a detailed description of the show interfaces ethernet command and
the information it provides.
show interfaces ethernet
Use the show interfaces ethernet privileged exec command to display information about an
Ethernet interface on the router:
Media Problem Suggested Actions
Excessive noise
Step 1 Use the show interfaces ethernet exec command to determine the
status of the router’s Ethernet interfaces. The presence of many CRC
errors but not many collisions is an indication of excessive noise.
Step 2 Check cables to determine whether any are damaged.
Step 3 Look for badly spaced taps causing reflections.
Step 4 If you are using 100BaseTX, make sure you are using Category 5
cabling and not another type, such as Category 3.
Excessive collisions Step 1 Use the show interfaces ethernet command to check the rate of
collisions. The total number of collisions with respect to the total
number of output packets should be around 0.1 percent or less.
Step 2 Use a TDR to find any unterminated Ethernet cables.
Step 3 Look for a jabbering transceiver attached to a host. (This might
require host-by-host inspection or the use of a protocol analyzer.)
Excessive runt frames Step 1 In a shared Ethernet environment, runt frames are almost always
caused by collisions. If the collision rate is high, refer to the problem
“Excessive collisions” earlier in this table.
Step 2 If runt frames occur when collisions are not high or in a switched
Ethernet environment, then they are the result of underruns or bad
software on a network interface card.
Step 3 Use a protocol analyzer to try to determine the source address of the
runt frames.
Late collisions
1
1 A late collision is a collision that occurs beyond the first 64 bytes of an Ethernet frame.
Step 1
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.
Step 2 Check the diameter of the network and make sure it is within
specification.
No link integrity on 10BaseT,
100BaseT4, or 100BaseTX
Step 1 Make sure you are not using 100BaseT4 when only two pairs of wire
are available. 100BaseT4 requires four pairs.
Step 2 Check for 10BaseT, 100BaseT4, or 100BaseTX mismatch (for
example, a card different from the port on a hub).
Step 3 Determine whether there is cross-connect (for example, be sure
straight-through cables are not being used between a station and the
hub).
Step 4 Check for excessive noise (see the problem “Excessive noise” earlier
in this table).