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Runts on Catalyst 6513 when interface in half-duplex.

I had a high runt count (<64 byte frames) on a 100Mb ethernet interface. The problem appeared to be with the FastHub 400 attached to it, but it turned out to be a problem of using a half-duplex connection on the 6513 interface instead. The FastHub will only do half-duplex. I attached one of the machines, that was connected to the hub, directly to the 6513 and forced the 6513 into half-duplex and the problem with runts on the 6513 interface remained. The problem went away when I allowed the interface to do full-duplex. There appears to be a problem with the way that the 6513 handles any half-duplex traffic. IOS ver. 12.1(19)E1.

New Member

Re: Runts on Catalyst 6513 when interface in half-duplex.

Well.... Not exactly. It looks like I am seeing runts with either full or half duplex. The problem may be better associated with IBM machines or ... Anyone seen this?


Re: Runts on Catalyst 6513 when interface in half-duplex.

Runts are normal in a half duplex environment . The more traffic you pass over a half duplex environment the more runts,crc's,collisions you will see , this is not a problem unless they become a high percentage of traffic passed . You should not see any runts in a full duplex environment that is working correctly .

New Member

Re: Runts on Catalyst 6513 when interface in half-duplex.

Half duplex is problematic.


Understanding Specific Port and Interface Counter Output for CatOS and Cisco IOS

Excessive errors for certain counters usually indicate a problem. When operating at half-duplex setting, some data link errors incrementing in Frame Check Sequence (FCS), alignment, runts, and collision counters 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 greater than two or three percent, performance degradation may be noticed.

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

When operating at full-duplex, errors in FCS, Cyclic Redundancy Checks (CRC), alignment, 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, check for a duplex mismatch. Duplex mismatch is a situation where the switch is operating at full-duplex and the connected device is operating at half-duplex, or vice versa. The results of a duplex mismatch will be extremely slow performance, intermittent connectivity, and loss of connection. Other possible causes of data link errors at full-duplex are bad cables, faulty switch ports, or NIC software/hardware issues. Refer to the Common Port and Interface Problems section of this document for more information.

New Member

Re: Runts on Catalyst 6513 when interface in half-duplex.


Runts are normal in half-duplex devices such as Fasthubs. They can also show up in full-duplex devices if you set either end to 100Mb/full-duplex and the other end to auto. You will end up with half-duplex on the auto end and excessive runts, which it turns out are really only collisions in the first 64 bytes; they are not really runts as seen when cut-through was a good idea.

Good link:

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