What does some of the physical network errors means e.g. CRC errors?

  • Article ID:100004082
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  • Product(s):NetBackup

Problem

When troubleshooting networking issues it can help to understand the meaning on some of the errors seen in the interface statistics.

Error Message

 

1. Alignment Errors
 
An Alignment Error indicates a received frame in which both are true:

a. The number of bits received is an uneven byte count (that is, not an integral multiple of 8)

b. The frame has a Frame Check Sequence (FCS) error.

Alignment Errors often result from MAC layer packet formation problems, cabling problems that cause corrupted or lost data, and packets that pass through more than two cascaded multiport transceivers.

2. Collisions

Collisions indicate that two devices detect that the network is idle and try to send packets at exactly the same time (within one round-trip delay). Because only one device can transmit at a time, both devices must stop sending and attempt to retransmit. Collisions are detected by the transmitting stations.

The retransmission algorithm helps to ensure that the packets do not retransmit at the same time. However, if the two devices retry at nearly the same time, packets can collide again; the process repeats until either the packets finally pass onto the network without collisions, or 16 consecutive collisions occur and the packets are discarded.

3. Excessive Collisions

Excessive Collisions indicate that 16 consecutive collisions have occurred, usually a sign that the network is becoming congested. For each excessive collision count (or after 16 consecutive collisions), a packet is dropped. If you know the normal rate of excessive collisions, then you can determine when the rate of packet loss is affecting your network's performance.

4. FCS Errors

Frame Check Sequence (FCS) Errors, a type of CRC, indicate that frames received by an interface are an integral number of octets long but do not pass the FCS check. The FCS is a mathematical way to ensure that all the frame's bits are correct without having the system examine each bit and compare it to the original. Packets with Alignment Errors also generate FCS Errors.

Both Alignment Errors and FCS Errors can be caused by equipment powering up or down or by interference (noise) on unshielded twisted-pair (10BASE-T) segments. In a network that complies with the Ethernet standard, FCS or Alignment Errors indicate bit errors during a transmission or reception. A very low rate is acceptable. Although Ethernet allows a 1 in 108 bit error rate, typical Ethernet performance is 1 in 1012 or better

5. CRC Errors

A Cyclic Redundancy Check (CRC) Error is an RMON statistic that combines "FCS Errors" and "Alignment Errors". These errors indicate that packets were received with:

a. A bad FCS and an integral number of octets (FCS Errors)

b. A bad FCS and a non-integral number of octets (Alignment Errors)

CRC Errors can cause an end station to freeze. If a large number of CRC Errors are attributed to a single station on the network, replace the station's network interface board. Typically, a CRC Error rate of more than 1 percent of network traffic is considered excessive.

6. Late Collisions

Late Collisions indicate that two devices have transmitted at the same time, but cabling errors (most commonly, excessive network segment length or repeaters between devices) prevent either transmitting device from detecting a collision. Neither device detects a collision because the time to propagate the signal from one end of the network to the other is longer than the time to put the entire packet on the network. As a result, neither of the devices that cause the late collision senses the other's transmission until the entire packet is on the network.

Although late collisions occur for small packets, the transmitter cannot detect them. As a result, a network suffering measurable Late Collisions for large packets is losing small packets as well. Late collisions are also an indication of an potential duplex miss match between the server NIC and the switch port.


7. Receive Discards

Receive Discards indicate that received packets could not be delivered to a high-layer protocol because of congestion or packet errors.


8. Too Long Errors (Giants/Jumbo)

A Too Long Error indicates that a packet is longer than 1518 octets (including FCS octets) but otherwise well formed. Too Long Errors are often caused by a bad transceiver, a malfunction of the jabber protection mechanism on a transceiver, or excessive noise on the cable.

9. Too Short Errors (Runt)

A Too Short Error, also called a runt, indicates that a packet is fewer than 64 octets long (including FCS octets) but otherwise well formed.


10. Transmit Discards

Transmit Discards indicate that packets were not transmitted because of network congestion.

Solution

If these errors are seen on the network then the network administrator should be contacted to investigate the cause of the errors and a remedy provided.


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