LRS-24 Installation and Operation Manual Appendix C SNMP Management
SNMP Traps C-7

Routing of IP Management Traffic

The routing of SNMP messages is made in accordance with the IP addresses
assigned to the various modules (the CM-2 module also has its own IP address).
IP addresses are assigned during the preliminary configuration activities. The CM-2
module automatically learns the IP addresses of all the existing I/O modules,
without any user intervention. For this purpose, updates are automatically
performed upon power-up, whenever a module is inserted or moved to a new
position in the enclosure, and also when a new CM-2 module is installed.
Using this information, the CM-2 module can operate as a bridge/router and ARP
(address resolution protocol) server with respect to the modules with SNMP agents
installed in the LRS-24:
• The first time that a management station which is authorized to manage the
LRS-24 needs to send a message to one of the modules installed in it, it does
not know the MAC address to be used; therefore it issues an ARP request with
the IP address of the desired module. The CM-2 module of LRS-24 containing
the module with the desired IP address answers the message, and includes the
MAC address of the Ethernet interface located on its interface module
(LRSI-F1-CM2 or LRSI-F2-CM2).
The next time, the CM-2 module routes the management message to the
appropriate module in accordance with the IP address included in the
message.
• A similar procedure is used when an I/O module needs to send a message to a
management station. In this case, the module sends a SLIP message with the
desired IP address, and the CM-2 module converts the SLIP message to a
regular message, by adding the MAC information for the addressed
management station.
If the CM-2 module does not yet know the MAC address, it temporarily
queues the message, and then sends an ARP request to learn that address.
After receiving the answer, the original message is sent.

C.4 SNMP Traps

The SNMP agent of LRS-24 supports the MIB-II authentication trap, and in
addition generates enterprise-specific traps for each alarm message, and for errors
detected as a result of the sanity check performed by modules after changes in
configuration.
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