Cisco Systems 17053 appendix Explanation of CatOS show port Command Counters, Fcs

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 2￿Explanation of CatOS show port Command Counters

Counter	Description
Alignment	Alignment errors are a count of the number of
Errors
	frames received that do not end with an even
	number of octets and have a bad CRC.
	FCS error count is the number of frames that were
FCS	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	Single collisions are the number of times the
Collisions
	transmitting port had one collision before
	successfully transmitting the frame to the media.
Multiple	Multiple collisions are the number of times the
Collisions
	transmitting port had more than one collision before
	successfully transmitting the frame to the media.
Late	A late collision occurs when two devices transmit
Collisions	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