SCALANCE X101-1 Fiber Optic Link Troubleshooting: P2 LED Green But No Communication

The SCALANCE X101-1 is a Siemens unmanaged Industrial Ethernet media converter that maps 10/100 Mbps twisted-pair (RJ45) to 100 Mbps fiber optic. When port P2 shows a permanent green LED but the connected device cannot be reached (no ICMP ping, no PROFINET IO, no S7 communication), the physical link layer is intact and the fault sits in layer-2 configuration of the remote managed switch. The most common case field engineers encounter is a SCALANCE X101-1 patched into a Dell PowerConnect 2824 (or a Cisco Catalyst) where the fiber port LEDs are healthy, the RJ45 link LED on the SCALANCE is healthy, and yet no packet crosses the boundary. This article walks through the hardware, the LED semantics, the candidate root causes, and the exact configuration corrections on the managed-switch side that restore the link.

Problem Statement

Symptoms reported on the field:

• SCALANCE X101-1 port P2 (fiber) LED = permanent green, no flashing even during heavy traffic attempts.
• SCALANCE X101-1 port P1 (RJ45) LED = green, but no ICMP echo reply to the connected CPU/PG.
• Remote managed switch (Dell PowerConnect 2824 or Cisco Catalyst) shows the corresponding fiber port as "up, line protocol up" with no input or output counters incrementing.
• ARP resolution fails. ping from the SCALANCE side hangs with "Destination host unreachable" or 100% packet loss.

Hardware configuration under test:

• SCALANCE X101-1 article number 6GK5101-1BB00-2AA3 with one 100Base-FX multi-mode SC fiber port and one 10/100Base-TX RJ45 port.
• Dell PowerConnect 2824 (24-port Gigabit managed switch, later firmware revisions) on the remote end.
• Multi-mode 50/125 µm (OM2) or 62.5/125 µm (OM1) duplex SC-SC patch cable, length well under the optical budget.

SCALANCE X101-1 Hardware Overview

The SCALANCE X101-1 (Siemens article number 6GK5101-1BB00-2AA3) is documented on the Siemens SiePortal product page as an "IE media converter, unmanaged, 1x 10/100 Mbps RJ45 port, LED diagnostics, error signaling contact with set pushbutton." A summary of the relevant technical data is given in the table below; always cross-check against the current operating instructions on the SiePortal because Siemens periodically revises the catalog text.

Reference: SCALANCE X101-1 — Siemens SiePortal product page.

Two facts from the data sheet drive every conclusion in this article:

1. The device is transparent at layer 2. It forwards any Ethernet frame whose destination MAC is reachable, with no VLAN tag filtering. The unit will pass 802.1Q-tagged frames unchanged and will also pass untagged frames unchanged.
2. Port P2 is hard-locked to 100 Mbps, full duplex, 100Base-FX. There is no autonegotiation on the fiber side, no 10 Mbps option, and no half-duplex option. The remote fiber port must therefore be manually set to 100 Mbps full duplex if the managed switch does not autonegotiate to the same value.

P2 LED Status Decoding

The SCALANCE X LED legend (per the operating instructions) is small but unambiguous. The two relevant LEDs for this fault are P1 (RJ45) and P2 (fiber).

When the field engineer reports "P2 is permanent green, no flashing", the layer-1 optical link is up. Frames that arrive at the SCALANCE P2 from the fiber are forwarded to P1 unchanged, and vice versa. If the LED never flashes, the higher layers on the connected device (the S7-CPU, the PG, the managed switch) are not generating any frame that reaches the SCALANCE at all, or the remote port is silently dropping the frames after they have left the SCALANCE. P2 green is therefore a useful negative signal: it rules out the cable, the optical power, and the SC connector pair as the cause.

Root Cause Analysis: Why Link Is Up But Traffic Fails

Once the physical layer is confirmed, the candidate root causes shrink to a small set of managed-switch behaviors. The five causes listed below account for the great majority of field reports of this exact symptom.

Cause 1 — VLAN membership or untagged membership mismatch

The Dell PowerConnect 2824 ships with all 24 copper ports in VLAN 1, untagged, with a PVID of 1. The fiber port (when an SFP is inserted) is in the same default VLAN 1. If the host CPU and the SCALANCE-attached PC are placed in different VLANs — for example, the CPU is in VLAN 10 and the SCALANCE P1 host is in VLAN 1 — frames cannot traverse the boundary. The SCALANCE itself does not tag or untag frames, so the VLAN assignment of the device connected to P1 is the VLAN the managed switch sees on its fiber port.

Verify with: show vlan on the Dell CLI, or the Web UI under Switching → VLAN → VLAN Membership.

Cause 2 — Hard-coded speed/duplex mismatch on the managed-switch fiber port

SCALANCE X101-1 P2 is fixed at 100 Mbps / full duplex. If the managed switch fiber port is forced to 100/half, autonegotiated to 1000, or left in a default of 10/half, the SCALANCE will still see link up (100Base-FX only checks 100/full on the line coding) but the link will not pass traffic. Autonegotiation on 100Base-FX is defined in IEEE 802.3u but is rarely implemented consistently on managed-switch SFP/GBIC ports, so the safe practice is to force both sides explicitly.

Verify with: show interface status on the Dell CLI. The port must read 100-full or 100-Full. The SCALANCE will not negotiate to 1000Base-SX even if the SFP on the switch supports it.

Cause 3 — Spanning Tree Protocol (STP / RSTP / MSTP) blocking the port

By default, the Dell PowerConnect 2824 runs RPVST+ (Rapid Per-VLAN Spanning Tree) with all ports enabled. If the SCALANCE fiber port comes up after the switch has finished its initial STP convergence, the port may sit in "blocking" state for the first 30–50 seconds. After that, if a loop is detected anywhere else in the network, the port can go back to blocking permanently. The LED is still green because the link is up at layer 1, but no data frame is forwarded.

Verify with: show spanning-tree on the Dell CLI. The port state should read Forwarding, not Blocking or Disabled.

Cause 4 — Port security / 802.1X / DHCP snooping table full

Dell PowerConnect 2824 enables port security and/or 802.1X on ports by default in higher firmware loads. A single MAC address limit of 1 (the default for "secure" ports) will lock the port the moment a second device sends a frame. The SCALANCE's MAC is learned on the P1 side and reappears on the P2 side, which can be interpreted as two distinct devices by the security module. Symptom is identical: link up, no traffic.

Verify with: show port-security and show dot1x. Disable both as a test, or raise the MAC limit to 10 and clear the violation table.

Cause 5 — Fiber polarity, connector, or SFP compatibility

If the SCALANCE P2 LED is solid green, the optical link is healthy and this cause is already ruled out. If P2 is yellow or off, the cause reappears and the fault sits in:

• Mismatched transmit/receive strands (TX/RX crossed). The SC connector is keyed, so a cross-over is unusual but does occur with patch panels.
• Single-mode fiber used on a multi-mode SCALANCE (or vice versa). The wavelength is 1310 nm on the multi-mode variant; using a 1550 nm SFP on the other end kills the link.
• Wrong SFP form factor. The SCALANCE X101-1 uses an integrated SC interface; a 100Base-FX GBIC on the Dell is the right partner, not a 1000Base-SX SFP that has been forced to 100.

Managed Switch Default Behavior (Dell PowerConnect 2824 and Cisco Catalyst)

The SCALANCE X101-1 is most often connected to a managed switch that the engineer did not configure for the link. The table below summarizes the factory defaults that are most likely to break the path.

The single most common cause on a Dell PowerConnect 2824 is "fiber SFP defaulting to 1000/full". The SFP inserted in the GBIC slot is a 1000Base-SX, but the SCALANCE X101-1 P2 is hard-locked to 100/full. The link comes up only when 1000Base-SX autonegotiates down to 100/full, which is not guaranteed on every SFP. When it does not, the symptom is exactly what the field engineer reported: P2 LED green, no communication.

A second common cause on Cisco Catalyst switches is "switchport port-security" with a limit of 1, which is enabled on some access ports. The Cisco community reference How to connect the Cisco Catalyst switch to Siemens SCALANCE confirms that direct gigabit-to-fastethernet bridging between the two vendors is a known configuration scenario that requires explicit speed and duplex settings on the Cisco side.

Step-by-Step Diagnostic Procedure

Use the sequence below on a live system. Each step rules out one root cause; do not skip ahead.

1. Confirm physical layer. Look at SCALANCE P2 LED. If green, proceed. If yellow or off, replace the patch cable, swap the TX/RX strands, and re-check the SFP type. Continue only after P2 is solid green.
2. Confirm the SCALANCE has power and P1 link is up. P1 LED must be green or flashing. If P1 is off, the SCALANCE has no power or the RJ45 host is not transmitting. The fiber P2 LED can still light in that case because it depends only on the optical input.
3. Capture the MAC table on the managed switch. From the Dell CLI: show mac-address-table interface tengigabitethernet1/1 (substitute the actual fiber port). The SCALANCE P2 MAC address should appear within 1 second of any traffic. If the MAC never appears, the SCALANCE is not transmitting on the fiber side and the issue is in the SCALANCE's P1 host or the SCALANCE itself.
4. Read the port status. From the Dell CLI: show interfaces status. Look for the fiber port. Confirm the admin state is Up, the speed is 100, and the duplex is Full. If the column reads auto or 1000, that is the fault.
5. Read the STP state. From the Dell CLI: show spanning-tree detail. Confirm the port is Forwarding. If Blocking, either wait 30–50 s for convergence or disable STP on the port for test purposes only.
6. Read counters. From the Dell CLI: show interfaces counters. The Input packets counter on the fiber port should be incrementing if the SCALANCE is sending. The Output packets counter should be incrementing if the rest of the network is reaching the SCALANCE. Zero on both with a green link is the signature of a VLAN / STP / port-security problem.
7. Ping from the SCALANCE side. Use a PG/PC connected to P1, set a static IP in the same subnet as the managed switch management IP, and ping <switch IP>. If the ping fails, the SCALANCE is forwarding the ICMP frame to the managed switch and the switch is dropping it. If the ping succeeds, the SCALANCE is fine and the original problem is somewhere further upstream (CPU side).
8. Ping from the switch side. From the Dell CLI: ping <IP of host on P1> source-interface vlan 1. If this succeeds but a ping from the host to the switch does not, the switch is replying on a different VLAN than the host is using.

Configuration Corrections on Dell PowerConnect 2824

The exact CLI commands to recover a SCALANCE X101-1 fiber link on a Dell PowerConnect 2824 are listed below. Use them in order; the order is significant because CLI sessions are not transactional on the Dell platform.

1. Disable STP on the fiber port (test mode). console# configure → console(config)# interface tengigabitethernet1/1 → console(config-if)# spanning-tree portfast → console(config-if)# spanning-tree guard root → console(config-if)# exit. PortFast puts the port into Forwarding immediately, bypassing the 30–50 s listening/learning delay. Root guard prevents the SCALANCE-attached segment from ever becoming the STP root.
2. Force speed and duplex. console(config)# interface tengigabitethernet1/1 → console(config-if)# speed 100 → console(config-if)# duplex full → console(config-if)# no negotiation. The no negotiation command disables autonegotiation on the SFP port, which is required because the SCALANCE X101-1 P2 does not autonegotiate on the fiber side.
3. Disable port security and 802.1X on the port. console(config-if)# no port-security → console(config-if)# no dot1x port-control → console(config-if)# exit. Either feature can lock the port silently; both are off by default but can be enabled by templates or by another engineer on the same switch.
4. Confirm VLAN membership. console# show vlan. The fiber port must appear in VLAN 1 (or whichever VLAN the SCALANCE P1 host belongs to) as untagged. If the port is in VLAN 1 but the P1 host is in VLAN 10, move the host or reconfigure the P1 port. Remember: the SCALANCE does not tag, so the VLAN on P1 = the VLAN on P2.
5. Save the configuration. console# write memory or console# copy running-config startup-config. The Dell PowerConnect 2824 does not auto-save; the changes are lost on the next reload if this step is missed.

Fiber Optic Cable and Connector Verification

Although the LED evidence in the field report points away from the cable, the verification is cheap and worth doing once, because the most common field fault at this point is "wrong patch cable in the wrong cabinet".

• Fiber type. SCALANCE X101-1 P2 is multi-mode, 1310 nm, 100Base-FX. The fiber in the cabinet must be 50/125 µm (OM2, OM3) or 62.5/125 µm (OM1). Single-mode 9/125 µm (OS1, OS2) will not light the SCALANCE even if the SFP on the other end is single-mode, because the SCALANCE has an integrated multi-mode transceiver.
• Connector. The SCALANCE P2 uses an SC duplex connector. The patch cable must be SC duplex on the SCALANCE end. The other end depends on the SFP/GBIC in the managed switch: SC, LC, or ST are all possible. Use a hybrid patch cable or a media converter if the connector types differ.
• Length. The optical budget on the SCALANCE X101-1 multi-mode variant is sized for up to 5 km of OM2. Stay under 4 km for margin; over 5 km the link is not guaranteed.
• Polarity. The SC duplex connector is keyed. If a single-strand patch panel is in the path, the TX of the SCALANCE must reach the RX of the managed-switch SFP, and vice versa. The simplest test is to reverse the pair at one end and re-check the LED; if the link goes from green to off, the strands are crossed.

Optical power measurement is not required when the P2 LED is solid green, but if the LED is yellow or off, measure the receive power on the SCALANCE side with an optical power meter. Anything below -30 dBm on a multi-mode 1310 nm link indicates either a broken strand, a contaminated connector, or a fiber length beyond budget.

PROFINET and S7 Communication Port Requirements

The PROFINET and S7 protocols use the following UDP/TCP port set, all of which must be allowed by any ACL or firewall in the path between the CPU and the SCALANCE-attached device:

The Dell PowerConnect 2824 does not filter by UDP/TCP port at layer 3 or 4 by default — it is a layer-2 switch. The concern is therefore not "is the port open" but "is the ACL on the layer-3 device further upstream blocking the port". A direct SCALANCE-to-managed-switch link with no router or firewall in between carries all of the above ports transparently. If a layer-3 device is in the path, the engineer must explicitly allow 102, 34962, 34963, 34964, and 67/68 in the ACL.

The S7 port 102 is also the port used by the default Siemens S7 driver in TIA Portal and STEP 7 for online operations. Some managed switches have a "filtering" or "smart" mode that drops ISO packets (EtherType 0x88A4 used by PROFINET and 0xFEFE used by S7). The Dell PowerConnect 2824 does not have such a filter by default, but the engineer should verify by looking for any traffic-shaping or protocol-filter feature on the Web UI under Security → ACL or QoS → Packet Classification.

Verification Procedure

After applying the configuration corrections, verify the link in this order. Each step is mandatory; do not move on until the previous step passes.

1. LED check. SCALANCE P2 green, P1 green, managed-switch fiber port LED green. All three must be green at the same instant. If any is yellow or off, return to the fiber section.
2. MAC table check. From the Dell CLI: show mac-address-table. The MAC of the SCALANCE-attached device must appear in the table within 1 second. If the MAC does not appear, the SCALANCE is not forwarding, which is a hardware fault and requires replacement.
3. ICMP ping. From the host on P1: ping 192.168.0.1 (substitute the managed-switch management IP). Five echoes, all replies. If replies are intermittent, suspect duplex mismatch — re-verify speed 100 and duplex full on both sides.
4. ARP table check. From the Dell CLI: show arp. The IP of the host on P1 must appear with a MAC and the fiber port as the interface. If the MAC is on a different port, the SCALANCE is not on the port you think it is.
5. PROFINET discovery. From TIA Portal, open "Online → Accessible devices". The CPU on the SCALANCE side must appear in the list within 5 seconds. If the CPU does not appear, check that the CPU's PROFINET device name and IP are configured and that the CPU is reachable on the management VLAN.
6. S7 online. From TIA Portal, open "Online → Go online" against the CPU. The connection must establish within 3 seconds. If it does not, verify TCP 102 is not blocked upstream and that the CPU is in RUN or STOP (not in a fault state that disables communication).
7. Long-duration soak. Leave the link running for at least 30 minutes. Watch the input/output counters on the fiber port. If the counters continue to increment without errors, the link is stable. If the counters show input errors, CRC errors, or runts, suspect a marginal fiber or a duplex mismatch that the ping did not reveal.

Record the result of each step. If step 7 produces CRC errors, return to step 4 of the diagnostic procedure and force the speed/duplex again — a half-duplex mismatch can pass a ping but fail under load.

Field Notes and Edge Cases

A handful of additional conditions have been observed in the field and are worth documenting for the next engineer who sees this symptom.

• SCALANCE X101-1 in a ring. If the SCALANCE is part of a PROFINET MRP ring, the management switch must support MRP or be set to MRP-disabled for that port. The SCALANCE itself does not run MRP — it is transparent — but the MRP manager (typically a SCALANCE XC or XF) can still mark the link as "not MRP capable" if the managed switch does not transmit the correct MRP frames. The symptom is identical: link up, no PROFINET IO. The fix is to either enable MRP on the managed switch or assign the SCALANCE-attached port to a non-MRP VLAN.
• Power-saving on managed-switch SFP. Some Dell and Cisco SFPs enter a low-power state when the link is idle for 5+ minutes. The SCALANCE X101-1 does not generate keep-alive frames, so the SFP can go to sleep and wake only when the SCALANCE sends data. The first frame after wake-up is dropped. Disable "SFP power saving" or "Energy Efficient Ethernet" on the switch port.
• Replacing the SCALANCE with a managed switch. If the link cannot be made stable with the SCALANCE in the path, replace the SCALANCE with a SCALANCE XC-208 (managed, 8 ports) or SCALANCE XF-204 (managed, 4 ports) and run PROFINET on the SCALANCE directly. This is more expensive but removes the unmanaged-vs-managed layer-2 asymmetry and exposes the diagnostic counters that the X101-1 hides.
• EMC and ground. The SCALANCE X101-1 ships with a ground screw on the housing. If the SCALANCE is mounted on a DIN rail without an EMC grounding strap, the fiber can pick up common-mode noise that the LED logic interprets as a valid link. Symptom: P2 green but CRC errors on the switch side. Always use a short, fat ground strap from the SCALANCE ground screw to the cabinet backplane.

Frequently Asked Questions