Catalyst 3850-24XS 10G port down although DOM shows normal RX power from carrier DWDM
A Cisco Catalyst 3850-24XS with an SFP-10G-LR (1310 nm) connects over a single-mode cross-connect to a Tier-1 carrier's layer-2 DWDM equipment in the same data center. When the carrier shuts the circuit down, TenGigabitEthernet2/0/23 goes 'down, line protocol is down (notconnect)' yet 'show interface transceiver' still reports RX power of about -5.1/-5.2 dBm, identical to the value seen while the port is up. 'show interfaces' shows 'link type is force-up, media type is SFP-10GBase-LR' with zero input packets. A loopback plug brings the port up, and the same SFP+ model in another device reports DOM correctly, includign -40 dBm with no signal. The poster asked whether the SFP+ module was at fault.
- Hardware: Cisco Catalyst 3850-24XS (WS-C3850-24XS)
- Transceivers: SFP-10G-LR (SFP-10GBase-LR)
- Form factor: SFP+
Symptoms / messages:
TenGigabitEthernet2/0/23 is down, line protocol is down (notconnect)
RX power -5.1 dBm reported while port is down
link type is force-up
0 packets input
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Comments 4
Turns out there's no switch fault - the confusion came from my 'monitor capture' test. Looped-back frames carrying the sending interface's own MAC address get discarded by the switch before the CPU-level capture ever sees them.
What's the module model, far-end equipment, distance, and fiber type? Also post the log messages you see when the cable is connected.
How the carrier 'shuts down' matters: if only higher-layer services are disabled, layer-1 light can remain present on the fiber, which explains a stable RX reading on a dead circuit.
Test with a loopback: the link comes up when looped, and an identical optic elsewhere reads -40 dBm without signal, so the optic is fine; move the loop further along the cable path to ioslate the fault.