Analyzing Errors on Switched Ethernet Networks

Switches are replacing hubs in many campus networks. A switch port that is in half-duplex mode follows the normal rules of CSMA/CD. The port senses carrier, defers if necessary, detects collisions, backs off, and retransmits. Whether a collision can occur depends on what is connected to the switched port. If a shared medium is connected to the switch, collisions may occur. A good rule of thumb is that less than 0.1 percent of frames should encounter collisions. There should be no late collisions. Late collisions are collisions that happen after a port or interface has sent the first 64 bytes of a frame.

If the switch port connects a single device, such as another switch, a server, or a single workstation, then both ends of this point-to-point link should be configured for full duplex. In this case, collisions should never occur. Full-duplex Ethernet isn't CSMA/CD. There are only two stations that can send, because full duplex requires a point-to-point link, and each station has its own private transmit channel. So full duplex isn't multiple access (MA). There's no need for a station to sense the carrier to see if someone else is sending on its transmit channel. There isn't anyone else. So full duplex doesn't use carrier sense (CS). There are no collisions. Both stations sending at the same time is normal. Receiving while sending is normal. So, there is no collision detection (CD) either.

Unfortunately, the autonegotiation of half versus full duplex has been fraught with problems over the years, resulting in one end of a point-to-point link being set to half duplex and the other being set to full duplex. This is a misconfiguration and must be fixed. Autonegotiation problems can result from hardware incompatibilities and old or defective Ethernet software drivers. Some vendors' NICs or switches do not conform exactly to the IEEE 802.3u specification, which results in incompatibilities. Hardware incompatibility may also occur when vendors add advanced features, such as autopolarity, that are not in the IEEE 802.3u specification. (Autopolarity corrects reversed polarity on the transmit and receive twisted pairs.)

The autonegotiation of speed isn't usually a problem. If the speed doesn't negotiate correctly, the interface doesn't work and the administrator hopefully notices and corrects the problem immediately. Manually configuring the speed for 10, 100, or 1000 Mbps usually isn't necessary, (except for cases where the user interface requires this before it will allow manual configuration of duplex mode). If a LAN still has Category 3 cabling, then manually configuring the speed to 10 Mbps is recommended, however. Errors can increase on a LAN that has autonegotiated for 100 or 1000 Mbps if there is Category 3 cabling that does not support the high-frequency signal used on 100- or 1000-Mbps Ethernet.

Duplex negotiation happens after the speed is negotiated. Problems with duplex negotiation are harder to detect because any performance impact is dependent on the link partners transmitting at the same time. A workstation user who doesn't send much traffic may not notice a problem, whereas a server could be severely impacted by a duplex mismatch. As part of analyzing the performance of the existing network, be sure to check for duplex mismatch problems. A surprisingly high number of networks have been hobbling along for years with performance problems related to a duplex mismatch.

To detect a duplex mismatch, look at the number and type of errors on either end of the link. You can view errors with the show interface or show port command on Cisco routers and switches. Look for CRC and runt errors on one side and collisions on the other side of the link. The side that is set for full duplex can send whenever it wants. It doesn't need to sense carrier. The side that is set for half duplex does sense carrier and will stop transmitting if it detects a simultaneous transmission from the other side. It will back off and retransmit, and report a collision. The result of the half-duplex station's stopping transmission is usually a runt frame (shorter than 64 bytes) and is always a CRC-errored frame.

The full-duplex side receives runts and CRC errored-frames and reports these errors. The half-duplex station reports collisions. Most of these will be legal collisions; some may be illegal late collisions. When checking the health of Ethernet LANs, check for these errors. Notice the asymmetry of the errors when there is a duplex mismatch. If you see collisions and CRC errors on both sides of the link, the problem is probably something other than a duplex mismatch, perhaps a wiring problem or bad NIC.

Until recently, most engineers recommended avoiding autonegotiation, but that is changing. Improvements in the interoperability of autonegotiation and the maturity of the technology mean that it is generally safer to rely on autonegotiation than to not rely on it.

There are numerous problems with not using autonegotiation. The most obvious one is human error. The network engineer sets one end of the link and forgets to set the other end. Another problem is that some NICs and switch ports don't participate in autonegotiation if manually set. This means they don't send the link pulses to report their setting.

How should the partner react to such a situation? The answer is undefined. Some NICs and switch ports assume the other side is too old to understand full duplex and must be using half. This causes the NIC or switch port to set itself to half. This is a serious problem if the other side is manually configured to full. On the other hand, there are cases where autonegotiation simply does not work and you may need to carefully configure the mode manually.

Continue reading here: Analyzing Network Efficiency

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