Information Technology Reference
In-Depth Information
Troubleshooting Ethernet
Table 4-5 provides troubleshooting procedures for common Ethernet media problems.
Table4-5
Troubleshooting Procedures for Common Ethernet Media Problems
Media Problem
Suggested Actions
Excessive noise
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.
1.
Check cables to determine whether any are damaged.
2.
Look for badly spaced taps causing reflections.
3.
If you are using 100BaseTX, make sure you are using Category
5 cabling and not another type, such as Category 3.
4.
Excessive
collisions
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.
2.
Use a TDR to find any unterminated Ethernet cables.
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
In a shared Ethernet environment, runt frames are almost always
caused by collisions. If the collision rate is high, refer to the
problem of excessive collisions, earlier in this table.
If runt frames occur when collisions are not high or when in a
switched Ethernet environment, then they are the result of
underruns or bad software on a network interface card.
Use a protocol analyzer to try to determine the source address of the
runt frames.
Late collisions
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.
2.
Check the diameter of the network, and make sure that it is
within specification.
No link
integrity on
10BaseT,
100BaseT4, or
100BaseTX
Make sure that you are not using 100BaseT4 when only two
pairs of wire are available. 100BaseT4 requires four pairs.
1.
Check for a 10BaseT, 100BaseT4, or 100BaseTX mismatch
(for example, a card different from the port on a hub).
2.
Determine whether there is cross-connect. (For example, be
sure that straight-through cables are not being used between a
station and the hub.)
3.
Check for excessive noise (see the problem of excessive noise,
earlier in this table).
4.
