In case the trap sending equipment does not allow to change the destination port (e.g. Mikrotik routers), it is necessary to forward the received packets to the new port.
This can be done either by software like _snmptrapd_ or by adding a firewall rule to your system, e.g. by executing
It can be extended with channels of type `number`, `string`, `switch`.
All channel-types have one mandatory parameter: `oid`.
It defines the OID that should be linked to this channel in dotted format (e.g. .1.2.3.4.5.6.8).
Channels can be configured in four different modes via the `mode` parameter.
Available options are `READ`, `WRITE`, `READ_WRITE` and `TRAP`.
`READ` creates a read-only channel, i.e. data is requested from the target but cannot be written.
`WRITE` creates a write-only channel, i.e. the status is never read from the target but changes to the item are written to the target.
`READ_WRITE` allows reading the status and writing it for controlling remote equipment.
`TRAP` creates a channel that ONLY reacts to traps.
It is never actively read and local changes to the item's state are not written to the target.
Using`TRAP` channels requires configuring the receiving port (see "Binding configuration").
The `datatype` parameter is needed in some special cases where data is written to the target.
The default `datatype` for `number` channels is `UINT32`, representing an unsigned integer with 32 bit length.
Alternatively `INT32` (signed integer with 32 bit length), `COUNTER64` (unsigned integer with 64 bit length) or `FLOAT` (floating point number) can be set.
For `string` channels the default `datatype` is `STRING` (i.e. the item's will be sent as a string).
If it is set to `IPADDRESS`, an SNMP IP address object is constructed from the item's value.
The `HEXSTRING` datatype converts a hexadecimal string (e.g. `aa bb 11`) to the respective octet string before sending data to the target (and vice versa for receiving data).
`number`-type channels can have a parameter `unit` if their `mode` is set to `READ`. This will result in a state update applying [UoM](https://www.openhab.org/docs/concepts/units-of-measurement.html) to the received data if the UoM symbol is recognised.
The standard behaviour if an SNMP exception occurs this is to log at `INFO` level and set the channel value to `UNDEF`.
This can be adjusted at channel level with advanced options.
The logging can be suppressed with the `doNotLogException` parameter.
If this is set to `true` any SNMP exception is not considered as faulty.
The default value is `false`.
By setting `exceptionValue` the default `UNDEF` value can be changed.
Valid values are all valid values for that channel (i.e. `ON`/`OFF` for a switch channel, a string for a string channel and a number for a number channel).
