Step 1 Use the show interfaces ethernet EXEC command to determine the status of the router Ethernet interfaces. The presence of many cyclicredundancy- check (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 100BASE-TX, make sure you are using Category 5 cabling and not another type, such as Category 3. | Troubleshooting Procedures for Common Ethernet Media Problems Media Problem Suggested Actions Excessive Noise Step 1 Use the show interfaces ethernet EXEC command to determine the status of the router Ethernet interfaces. The presence of many cyclic-redundancy-check 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 100BASE-TX 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 approximately percent or less. Step 2 Use a time domain reflectometer 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 NIC . Step 3 Use a protocol analyzer to try to determine the source address of the runt frames. Late Collisions1 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 10BASE-T 100BASE-T or 100BASE-TX Step 1 Make sure you are not using 100BASE-T4 when only two pairs of wire are available .
