By the SNMP Monitoring team · Reviewed July 2026
A fibre link doesn't usually fail all at once — it fades. An optic ages, a connector gets dirty, a bend tightens, and the received light level slowly drops until errors creep in and eventually the link flaps. DOM (Digital Optical Monitoring, also called DDM) is how a transceiver reports that light level, and SNMP can read it — most portably as a dBm-typed sensor in the ENTITY-SENSOR-MIB, and more richly through vendor MIBs. This page is the paste-ready reference for reading optical power over SNMP so you catch a fading link before it drops.
Related how-to context lives in network monitoring; the interface objects are on the interface reference. Hubs: sensors, all OIDs.
What DOM/DDM exposes
A DOM-capable SFP/SFP+/QSFP reports a handful of live diagnostics per transceiver:
- Rx optical power — received light level, in dBm. The headline number for link health.
- Tx optical power — transmitted light level, in dBm.
- Module temperature — the optic's own temperature, in °C.
- Laser bias current — the drive current to the laser, in mA; drift signals an aging optic.
- Supply voltage — the transceiver's supply rail.
Rx power is the one you watch hardest — it's the direct measure of how much light is arriving, and a slow decline is the classic signature of a link heading for trouble.
ENTITY-SENSOR dBm objects
Where a platform maps DOM into the standard sensor table, optical power is an entry at 1.3.6.1.2.1.99.1.1.1 whose entPhySensorType is dBm:
| Object | OID | Meaning / type |
|---|---|---|
| entPhySensorType | 1.3.6.1.2.1.99.1.1.1.1 | Sensor type — dBm for optical power |
| entPhySensorScale | 1.3.6.1.2.1.99.1.1.1.2 | SI multiplier |
| entPhySensorPrecision | 1.3.6.1.2.1.99.1.1.1.3 | Decimal places |
| entPhySensorValue | 1.3.6.1.2.1.99.1.1.1.4 | The dBm reading (integer, apply precision) |
dBm is a logarithmic unit and often negative for received power (e.g. −5.2 dBm), so precision matters: a value of -52 at precision 1 is −5.2 dBm. Read the precision and expect signed values — clamping to positive integers will mangle Rx readings.
Vendor approaches
DOM predates a universal standard mapping, so in practice a lot of optical monitoring is vendor-specific. Cisco, for instance, exposes transceiver diagnostics through the CISCO-ENTITY-SENSOR-MIB under 1.3.6.1.4.1.9, with per-sensor Rx/Tx power, temperature and bias. Other vendors have their own trees. The exact DOM leaf OIDs are platform-specific, so this reference describes the ENTITY-SENSOR pattern rather than inventing leaves — pull the precise objects from your vendor's MIB. On Cisco gear, start at Cisco SNMP. The portable takeaway: whether standard or vendor, DOM optical power reads as a value plus a scale, and Rx power is what you trend.
Worked snmpwalk example
Walk the sensor table and pick out the dBm entries:
snmpwalk -v2c -c <RO_string> <host> 1.3.6.1.2.1.99.1.1.1
ENTITY-SENSOR-MIB::entPhySensorType.1001 = INTEGER: dBm(14)
ENTITY-SENSOR-MIB::entPhySensorPrecision.1001 = INTEGER: 1
ENTITY-SENSOR-MIB::entPhySensorValue.1001 = INTEGER: -52
ENTITY-SENSOR-MIB::entPhySensorType.1002 = INTEGER: dBm(14)
ENTITY-SENSOR-MIB::entPhySensorValue.1002 = INTEGER: -18
Sensor .1001 is an Rx power of -52 × 10^(−1) = −5.2 dBm (healthy); .1002 reading −1.8 dBm is a Tx power. Cross-reference the index against the ENTITY-MIB inventory (1.3.6.1.2.1.47) to learn which port and which direction. <RO_string> is a placeholder; keep live communities out of committed scripts.
Interpreting dBm & link budget
Every optic has a receiver sensitivity — a minimum Rx power below which the link degrades and eventually fails. The gap between the light you're receiving and that floor is your link budget margin. The read:
- Note the transceiver's rated Rx sensitivity and Rx range from its datasheet.
- Trend the live Rx power (dBm) over time.
- Watch the margin: a healthy link sits comfortably above the sensitivity floor.
- Alert when Rx power falls toward the floor — a steady decline means a dirtying connector, a bending fibre, or an aging laser.
A falling Rx power is an early, quiet warning: the link still works, but its margin is eroding. Catching that trend lets you clean a connector or swap an optic during a window instead of during an outage. Because a fading optical link often shows up first as rising interface errors, watch dBm alongside the interface error counters.
Related
DOM optical power pairs naturally with the interface layer. For the traffic/error/status side of the same ports, see the interface OIDs reference and the network monitoring how-tos. Module temperature ties into temperature sensors. Vendor specifics: Cisco SNMP. The full index: all sensors; definitions: glossary.
Frequently asked questions
What OID is SFP optical power?
Where a platform maps DOM into the standard table, optical power is an ENTITY-SENSOR-MIB sensor typed dBm, read from entPhySensorValue (1.3.6.1.2.1.99.1.1.1.4) with its precision applied. Many vendors instead expose it through an enterprise MIB — on Cisco, the CISCO-ENTITY-SENSOR-MIB under 1.3.6.1.4.1.9.
What is DOM/DDM?
DOM (Digital Optical Monitoring), also called DDM (Digital Diagnostics Monitoring), is a transceiver feature that reports live diagnostics — Rx and Tx optical power, module temperature, laser bias current, and supply voltage. It lets you read an optic's health over SNMP instead of physically metering the fibre.
How do I read Rx power in dBm?
Read the dBm-typed entPhySensorValue for the receive sensor and apply its precision — values are logarithmic and usually negative, so a raw -52 at precision 1 is −5.2 dBm. On vendor platforms, read the equivalent object from the vendor MIB (e.g. Cisco's CISCO-ENTITY-SENSOR-MIB) and apply the same value-plus-scale logic.
Why is my Rx power too low?
A low or falling Rx power means less light is arriving than the optic needs — commonly a dirty or loose connector, a tight fibre bend, a damaged cable, or an aging laser at the far end. As Rx power approaches the transceiver's receiver-sensitivity floor, the link budget margin shrinks and errors rise, so a declining trend is worth acting on before the link flaps.
Key takeaways
- Optical power (DOM/DDM) reads as a
dBm-typed ENTITY-SENSOR value atentPhySensorValue(1.3.6.1.2.1.99.1.1.1.4). - Rx power is the headline — dBm is logarithmic and usually negative; apply precision and expect signed values.
- Vendors expose richer DOM via enterprise MIBs — e.g. CISCO-ENTITY-SENSOR-MIB under
1.3.6.1.4.1.9; exact leaves are platform-specific. - Trend Rx power against the optic's receiver sensitivity — a decline erodes link-budget margin.
- Watch dBm alongside interface errors — a fading optic shows up as rising errors.
- Related: interface OIDs, network monitoring; catalog: sensors/all.