MIB Table

The snmp_mib table is defined as an array. The components of this array are of type MIB_ENTRY.

const MIB_ENTRY snmp_mib[] = {
  /* ---------- System MIB ----------- */
  /* SysDescr Entry */
  { MIB_OCTET_STR | MIB_ATR_RO,
    8, {OID0(1,3), 6, 1, 2, 1, 1, 1, 0},
    MIB_STR("Embedded System SNMP V1.0"),
    NULL },
  /* SysObjectID Entry */
  { MIB_OBJECT_ID | MIB_ATR_RO,
    8, {OID0(1,3), 6, 1, 2, 1, 1, 2, 0},
    MIB_STR("\x2b\x06\x01\x02\x01\x01\x02\x00"),
    NULL },
  /* SysUpTime Entry */
  { MIB_TIME_TICKS | MIB_ATR_RO,
    8, {OID0(1,3), 6, 1, 2, 1, 1, 3, 0},
    4, &snmp_SysUpTime,
    NULL },
    ..
}

In the following example, we will construct a MIB variable entry LedOut. It will allow SNMP Manager to control LED diodes on an evaluation board.

• The MIB variable type is Integer. An U8 variable is sufficient, because the LED port is 8-bit:

    /* LedOut Entry */
    { MIB_INTEGER,
  

• The OID reference is 1.3.6.1.3.1.0. It is defined in the Experimental MIB branch of the MIB tree:

      6, {OID0(1,3), 6, 1, 3, 1, 0},
  

  - the first byte defines the length of the OID name,
  - macro OID0 calculates the first byte of OID value from 1st and 2nd address byte,
  - the value of an OID address byte must be less than 128, othervise an extended encoding must be used.
• The variable size and location is described with the help of MIB_INT macro:

      MIB_INT(LedOut),
  

  The following macros are defined:

  Macro	Variable Definition
  MIB_STR	Octet String size and location.
  MIB_INT	Signed or Unsigned Integer size and location.
  MIB_IP	IP Address size and location.

• The write_leds is specified as callback function. It gets called when the LedOut is written:

      write_leds },
  

• Finally we need the actual variable definition:

  static U8 LedOut;
  

For the LedOut control we actually need the following parts of code to be defined in SNMP_MIB.c module:

static U8 LedOut;
static void write_leds (int mode);

const MIB_ENTRY snmp_mib[] = {
    ..
  /* LedOut Entry */
  { MIB_INTEGER,
    6, {OID0(1,3), 6, 1, 3, 1, 0},
    MIB_INT(LedOut),
    write_leds },
    ..
}

static void write_leds (int mode) {
  /* No action on read access. */
  if (mode == MIB_WRITE) {
    LED_out (LedOut);
  }
}

Extended OID encoding

The value of OID address byte must be less than 128. If it is not, an OID address must be encoded in extended format. This is because the high bit of an address byte is an address extension bit.

For example, the OID address 1.3.6.1.4.1.311.0 is encoded as:

  8, {OID0(1,3), 6, 1, 4, 1, 130, 55, 0},

The address value for the highlighted numbers is calculated as:

  (130-128) * 128 + 55 = 311

The OID address 1.3.6.1.4.1.31036.50.1.1.0 is encoded as:

  12, {OID0(1,3), 6, 1, 4, 1, 129, 242, 60, 50, 1, 1, 0},

where the value 31036 is calculated as:

  (129-128) * 128 * 128 + (242-128) * 128 + 60 = 31036