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 greater than two or three percent, performance
degradation can 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, caused when the frame is not completely copied to the wire, which results in fragmented frames.
When operating at full−duplex, FCS, cyclic redundancy checks (CRC), alignment errors, and runt counters
are probably minimal. If the link operates at full−duplex, the collision counter is not active. If the FCS, CRC,
alignment, or runt counters increment, check for a duplex mismatch. Duplex mismatch is a situation in which
the switch operates at full−duplex and the connected device operates at half−duplex, or the other way around.
The result of a duplex mismatch is extremely slow performance, intermittent connectivity, and loss of
connection. Other possible causes of data link errors at full−duplex are bad cables, a faulty switch port, or
NIC software or hardware issues.
When you troubleshoot NIC performance issues, view the output of the show port mod/port command and
the show mac mod/port command, and note the counter information.
Table 2Explanation of CatOS show port Command Counters
Counter
Description
Alignment
Errors
Alignment errors are a count of the number of
frames received that do not end with an even
number of octets and have a bad CRC.
FCS
FCS error count is the number of frames that were
transmitted or received with a bad checksum (CRC
value) in the Ethernet frame. These frames are
dropped and not propagated onto other ports.
Xmit−Err This is an indication that the internal transmit
buffer is full.
Rcv−Err
This is an indication that the receive buffer is full.
UnderSize These are frames that are smaller than 64 bytes,
which includes FCS, and have a good FCS value.
Single
Collisions
Single collisions are the number of times the
transmitting port had one collision before
successfully transmitting the frame to the media.
Multiple
Collisions
Multiple collisions are the number of times the
transmitting port had more than one collision before
successfully transmitting the frame to the media.
Late
Collisions
A late collision occurs when two devices transmit
at the same time and neither side of the connection
detects a collision. The reason for this occurrence is
that the time to propagate the signal from one end
of the network to another is longer than the time to
put the entire packet on the network. The two
devices that cause the late collision never see that
the other sends until after it puts the entire packet
on the network. Late collisions are detected by the
transmitter after the first time slot of the
