ModSnmp Manual v3.09 - 4.1.2. Response Handling
Always responds. When using Modbus RTU or ASCII over a serial interface, a device should not respond to a request for a different slave ID (this is necessary for RS485 multi-drop to work). When using Modbus TCP over a TCP socket interface, the device/bridge should respond to requests to unknown/unreachable devices with an exception 10 or 11 response. When using non-standard protocol variants (such as RTU encapsulated in TCP), the behaviour in these situations is not defined. You should select Always responds if the device behaves in the Modbus TCP manner, or deselect it if it behaves in the serial manner.
Number of Retries. This field specifies how many times ModSnmp should retry a request that it has not received a response to.