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26 Advanced System Diagnostics and Troubleshooting Guide
Packet Errors and Packet Error Detection
Definition of Terms
To establish a basis for the descriptions in this chapter, Table 3 lists and defines terms that are used
repeatedly throughout this chapter and those that follow. When any of these terms are used for their
precise meaning, they are shown emphasized in
bold type .
Table 3: Data Error Terms
Term Description
Packet error event When the contents of a network data or control packet are modified by the
transmission medium or a network device in a way that is not indicated by the rules
of standard network behavior such that the contents of the packet will be considered
invalid by upper layer protocols or applications, we say that a packet error event
has occurred.
Note that the term applies only to packet changes initiated by layer 1 interaction; that
is, if an error in the electrical or optical processing of the bit-level data in the packet
results in a change to the packet, we consider this a packet error event.
The term does not extend to systematic software or hardware errors that result in
valid but incorrect changes to the packet at higher OSI layers, such as inserting the
wrong next-hop MAC destination address into the packet header because of an
erroneous entry in the hardware forwarding database.
Checksum A value computed by running actual packet data through a polynomial formula.
Checksums are one of the tools used by Extreme Networks in attempts to detect
and manage packet error events.
Packet checksum A checksum value that is computed by the MAC chip when the packet is transferred
from the MAC chip to the switch fabric. This checksum value precedes the packet
as it transits the switch fabric.
Verification checksum A checksum value that is computed by the MAC chip when the packet is transferred
from the switch fabric to the MAC chip for transmission.
Checksum error When a packet exits the switch fabric, the packet checksum that follows the packet
must match the verification checksum computed as the packet leaves the switch
fabric. If the checksums do not match, then a checksum error results.
System health check A series of system tests and associated reporting mechanisms that are used to notify
network operators of potential system problems and to isolate and diagnose faulty
components when problems occur. The checksum error reporting mechanism is a
part of the system health check system.
System health check error This term refers to error messages in the system log that are generated by the
system health check system. Error messages generated by the system health
check system are prefaced by the text string “Sys-health-check.” Checksum error
messages are a subset of the system health check error messages.
Transient errors Errors that occur as one-time events during normal system processing. These types
of errors will occur as single events, or might recur for short durations, but do not
have a noticeable impact on network functionality and require no user intervention to
correct.
Soft-state errors These types of error events are characterized by a prolonged period of reported
error messages and might, or might not, be accompanied by noticeable degradation
of network service. These events require user intervention to correct, but are
resolved without replacing hardware.
Error messages of this type are the result of software or hardware systems entering
an abnormal operating state in which normal switch operation might, or might not, be
impaired.
Permanent errors These types of errors result from permanent hardware defects that might, or might
not, affect normal switch operation. They cannot be resolved by user intervention
and will not resolve themselves. You must replace hardware to resolve permanent
errors.