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FS-8704-09 SNMP Driver Manual Page 13 of 39
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Appendix A. Advanced Topics
Appendix A.1. Receiving SNMP TRAPS
A trap can reveal the causal event in two ways:
• The cause can be indicated by the OID number inside the trap with the event state reported by
the associated data value.
• The cause can be indicated by the value of one OID field and the event state by the value of
another OID field.
Appendix A.1.1. Trap Source IP Address
When a trap is received it contains its source IP address information within the UDP packet
header. It is possible for a trap to reveal its source IP address by coding an IP_ADDRESS field
inside the trap.
To catch a trap it is necessary to define Map Descriptors containing the OID’s of the trap. Map
Descriptors are associated with a Node. A Node is defined by its IP address amongst other
parameters. When a trap is received the source of the IP address must match the IP address of
the Node associated with the Map Descriptor.
A trap may originate from one IP address, and contain an alternative source IP Address within the
message. In this case, the FieldServer will use the IP_Address contained within the message as
the source rather than the IP address of the sending device. When this situation occurs the driver
prints a message to the error log so that you know why, perhaps, the trap was ignored/processed
differently from your expectations."