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Certain counters rapidly increase in the show controllers ethernet controller command output on the Catalyst fixed configuration switches, including alignment, Undersized frames, FCS, transmit and receive errors or various collisions counters

Core issue

Many performance issues with switches are related to data link errors. Excessive errors usually indicate a problem. When operating at half-duplex setting, some data link errors like Frame Check Sequence (FCS), alignment, runts and collisions are normal. Generally, a one percent ratio of errors to total traffic is acceptable for half-duplex connections. If the ratio of errors to input packets is more than two or three percent, performance degradation may be noticed as a result of this configuration.

In half-duplex environments, it is possible for both the switch and the connected device to sense that the wire is not busy and transmit at exactly the same time, which results in a collision. Collisions can cause runts, FCS and alignment errors due to the frame not being completely copied to the wire. This results in fragmented frames being on the wire.

When operating at full-duplex, FCS, Cyclic Redundancy Checks (CRCs), alignment errors and runt counters should be minimal. If the link is operating at full-duplex, the collision counter is not active. If the FCS, CRC, alignment or runt counters are incrementing, this is usually the result of a duplex mismatch. A duplex mismatch occurs when the switch is operating at full-duplex and the connected device is operating at half-duplex or the other way around. Duplex mismatches can cause extremely slow performance, intermittent connectivity and loss of connection.

Other possible causes of data-link errors when configured for full-duplex are bad cables, faulty switch ports or Network Interface Card (NIC) software or hardware issues.

Undersized Frames: When using dot1q on the trunk interface on the Catalyst 3750, runts may be seen on show interfaces output because valid dot1q encapsulated packets which are 61 to 64 bytes including the q-tag are counted by the Catalyst 3750 as undersized frames, even though these packets are forwarded correctly. In addition, these packets are not reported in the appropriate category (unicast, multicast, broadcast) in receive statistics. For more information, refer to Cisco bug ID CSCec14238.


In order to resolve switch performance issues, view the output of the show controllers ethernet-controller <mod#>/<port#> command. Issue the command several times to determine if the counter is incrementing.

In case of undersized frames, it is only a cosmetic error and does not affect the operation of your switch or network.

For an explanation of the show controllers ethernet-controller command output in the Catalyst Fixed Configuration Switches 3750, 3550, 2970, 2950, 2955, 2940 and 2900 and 3500XL, refer to the table in the Show Controller Ethernet-Controller for Cisco IOS section of Troubleshooting Switch Port and Interface Problems.