Cisco Catalyst 3750-X 12S-S

Cisco · 1U switch, 12 SFP 1 Gb/s ports, StackWise Plus stacking · Cisco Catalyst WS-C3750X-12S-S

what it is

A 1U switch with twelve SFP 1 Gb/s ports — no copper ports for end devices. Next to the data ports there is a separate 10/100 management port (Fa0), which does not take part in switching. It has two StackWise stack ports and two power supply bays; in the units we checked, one C3KX-PWR-350WAC (350 W) power supply was running and the second bay was empty.

The model does not deliver PoE: the power supply provides 350 W for the system and 0 W for PoE (show env all). It is a switch for a rack with fiber links — core or distribution, into which other switches connect.

License in the units we checked: IP Base, permanent. Alongside it there is an unused eight-week IP Services evaluation license. The factory resource template ("desktop default”) provides for 8 routed interfaces, 1024 VLANs, 6 thousand MAC addresses and 8 thousand IPv4 routes (show sdm prefer).

StackWise stack

Two units connected with two stack cables operate as one switch: one configuration, one management address, ports numbered by member number (Gi1/0/1 … Gi2/0/12).

• The role is chosen by priority. show switch detail shows the role (active, member), priority and state of each unit, as well as the state of both stack ports with the neighbor number.
• The switch itself reports the stack cable length — show switch stack-ports summary showed 50 cm on each of the four ports, together with a link state change counter.
• All members must run the same firmware version.
• The member number and priority are stored outside the configuration — erasing the configuration does not change them. A unit bought used may arrive with a number other than 1.

Without stack power cables (StackPower) each unit is a separate power group: show stack-power showed two groups of 350 W each, each with one switch and one power supply.

what we read from it

We read this model over SNMP and from the CLI (show commands only). Over SNMP, the following branches respond: interfaces, error counters, bridge, component inventory, Cisco sensors, environment, CPU, memory, flash, image, license, configuration, spanning tree, err-disabled ports, VLANs, port security and stack.

The switch reports:

• The state of each port with speed and duplex, traffic and error counters.
• The temperature of each stack member with thresholds — factory yellow 46°C, red 60°C.
• Two fans, the power supply fan and the power supplies with the state of each.
• Stack state: members, their roles and stack ports (CISCO-STACKWISE-MIB branch).
• SFP transceiver diagnostics — described below.
• Ports disabled due to an error (err-disable) together with the cause.

SFP transceiver diagnostics

For a transceiver with digital optical diagnostics (DOM), the switch shows temperature, supply voltage, transmit power and receive power, each with four thresholds — alarm and warning, high and low. The thresholds come from the transceiver's own memory, so each transceiver model has its own.

• In the CLI, diagnostics are provided only by show interfaces transceiver detail. show idprom interface on this model does not print the diagnostic page of the transceiver: the EEPROM PAGE A2 Bytes section actually contains the continuation of the identification page (with the Cisco part number string). It looks like a transceiver without diagnostics, even though it has them.
• Laser bias current is not available in the CLI. The table legend lists milliamperes, but firmware 15.2(4)E10 on this hardware does not print the current block. Over SNMP, laser bias current is available: each transceiver with diagnostics has five sensors — temperature, voltage, laser bias current, transmit and receive power — together with thresholds.
• Threshold exceeded without a flag. Measured: receive power −37.0 dBm with an alarm threshold of −21 dBm, and the show interfaces transceiver detail table did not place a -- flag next to it. Alarm assessment has to be computed from the values and thresholds, not from the absence of a flag.

show inventory reports the part number (PID) only for Cisco-coded transceivers. The others have an empty PID and only a description, e.g. 1000BaseSX SFP.

third-party transceivers

A transceiver without Catalyst coding — including Cisco ones from the Small Business line, for example the MGBSX1 — is marked as unsupported, and the port goes into the err-disable state with the cause gbic-invalid. After two configuration commands, such a transceiver works (measured: link 1000 Mb/s, full duplex):

• service unsupported-transceiver,
• no errdisable detect cause gbic-invalid.

Some transceivers are rejected by the switch entirely: the log shows %PHY-4-SFP_NOT_SUPPORTED, show idprom responds with %No module is present, and the port type is unknown. Such a transceiver cannot even be identified.

The switch also recognizes 100BASE-FX transceivers (e.g. GLC-GE-100FX) — show interfaces status shows them as 100BaseFX SFP.

firmware

15.2(4)E10 (April 2020) is the last release for the 3750-X line. A single universalk9 image supports the whole line — features are selected by the license.

• The first boot of a new version takes longer: the switch programs the port microcode for several minutes. The bootloader does not change during this upgrade.
• The image is 25.5 MB. Over the console (XMODEM at 115 200 b/s) it takes about 40 minutes to load; over the network from a TFTP server — about 3.5 minutes.

console and access recovery

Console: a mini-USB port on the front panel and an RJ-45 port on the rear (next to the management port), 9600 b/s, 8N1. Plugging in a USB cable disables the RJ-45 console (Console media-type is USB). The USB port is visible to the computer even when the switch is powered off — it then returns a single repeated character, which is not a baud rate error. The terminal program must set the DTR and RTS signals; without them the screen shows only garbage.

Recovering access to a unit with an unknown password requires physical access and the console. Condition: the boot messages contain The password-recovery mechanism is enabled.

• First, all power cables must be disconnected — a second power supply or a StackPower cable keeps the switch running, and unplugging one cable does not restart it.
• The MODE button is pressed after power is applied, in the first seconds of boot, and held until the console shows the boot interruption and the switch: prompt. MODE pressed before connecting power did not work; two units of the same model required a slightly different moment.
• From this mode the configuration files are deleted and the switch boots in its factory state.

what to watch out for

• The number of interfaces in show version is not the number of ports. A stack of two units reported 32 gigabit interfaces and 4 ten-gigabit ones, although the enclosures have 24 SFP ports — the rest belong to the network module slot, which was not present in the units we checked. Monitoring that counts ports from this list will overstate their number.

what we cannot read

• Sensors from the standard RFC 3433 branch — it is empty. All sensors, including the transceivers, are only in the Cisco branch.
• Host resources (HOST-RESOURCES-MIB) and component states (ENTITY-STATE-MIB) — the branches are empty.
• VLANs from the standard Q-BRIDGE table — it is empty; VLANs are reported by the Cisco VTP branch.
• StackPower state over SNMP — the CISCO-STACKPOWER-MIB branch is empty; the power group budget is shown only by show stack-power.
• Laser bias current from the CLI and diagnostics from show idprom — see above.

transceivers tested in this device

Transceivers we checked in this device, and whether the device showed their optical diagnostics (temperature, power, alarm thresholds):

• Avago SFBR-5716PZ (1000BASE-SX), firmware 15.2(4)E10: diagnostics read (2 units).
• Finisar FTLF8519P3BNL-CS (1000BASE-SX), firmware 15.2(4)E10: diagnostics read (3 units).
• Avago SFBR-5766PZ-CS2 (1000BASE-SX), firmware 15.2(4)E10: diagnostics not shown (1 units).
• E2O Communications ES212-LP3TA-A-F1 (1000BASE-LX; Fibre Channel 1/2G LW), firmware 15.2(4)E10: diagnostics not shown (6 units).
• Finisar FTLF8519P2BCL (1000BASE-SX), firmware 15.2(4)E10: diagnostics not shown (3 units).
• Opnext TRF5325ANLB202 (100BASE-FX), firmware 15.2(4)E10: diagnostics not shown (3 units).
• Ubiquiti CM-RJ45-1G (1000BASE-T), firmware 15.2(4)E10: diagnostics not shown (2 units).

All transceivers and switches in one place, with filters: what each device shows.

full list of parameters read via CLI and SNMP

The list shows WHAT the device exposes — including parameters we do not use ourselves. There are no values here at all: it is a catalog of capabilities, not a reading from anyone's network. It is not a closed list either — explanation below the table.

368 parameters

List source: reading of 1 working unit (as of 2026-10-18). The list is the sum of what we observed — more units reveal more parameters, so numbers for different models are not directly comparable.