Overview
PTP (Precision Time Protocol, IEEE 1588-2008) is a network time synchronisation protocol that achieves sub-microsecond accuracy across Ethernet networks. In industrial OT environments — including power grid protection relaying, SCADA event correlation, and motion control — devices must share a precise common time reference. ORing 9000 Series managed switches ([PRODUCT_MODELS]) support IEEE 1588-2008 PTP as an Ordinary Boundary Clock, distributing time from a grandmaster clock to downstream PTP clients across multiple network hops. This guide covers configuration in P2P (Peer-to-Peer) delay measurement mode with Two-Step operation, using an OMICRON OTMC 100p as the grandmaster clock reference.
Prerequisites
- Applicable switch models: ORing 9000 Series ([PRODUCT_MODELS])
- Minimum firmware: [FIRMWARE_VERSION]
- Login level required: Administrator
- PTP grandmaster clock: OMICRON OTMC 100p or any IEEE 1588-2008 compliant grandmaster
- All devices in the PTP domain must use the same delay mechanism (P2P or E2E) and operation mode (Two-Step)
Key concepts before you start
| Concept | This guide's setting | Why it matters |
|---|---|---|
| Delay mechanism | P2P (Peer-to-Peer) | Must match on every device in the PTP domain. Mixing P2P and E2E causes synchronisation failure. |
| Operation mode | Two-Step | ORing 9000 Series supports Two-Step only. PTP clock must also be set to Two-Step. |
| Device type | Ordinary Boundary Clock | Switch acts as Slave toward upstream (grandmaster) and Master toward downstream clients. |
| DIM field | P2P | Per-port setting on the switch. Must match the domain delay mechanism (P2P in this guide). |
Test topology
| Device | Role | Port | Connected to | Expected PTP state |
|---|---|---|---|---|
| OMICRON OTMC 100p | PTP Grandmaster | — | Switch A Port 5 | Master |
| Switch A | Ordinary Boundary Clock | Port 5 | PTP Grandmaster | Slave |
| Switch A | Ordinary Boundary Clock | Port 7 | Switch B Port 7 | Master |
| Switch B | Ordinary Boundary Clock | Port 7 | Switch A Port 7 | Slave |
| Switch B | Ordinary Boundary Clock | Port 8 | PTP Client | Master |
| PTP Client | Ordinary Clock (Slave) | — | Switch B Port 8 | Slave |
Part 1 — Verify PTP grandmaster clock settings
Before configuring the switches, verify the PTP grandmaster clock settings. Every device in a PTP domain must use the same delay mechanism and operation mode. A single mismatch prevents synchronisation across the entire chain.
Step 1 — Confirm PTP delay mechanism: P2P
On the OMICRON OTMC 100p (or your grandmaster clock), navigate to the PTP settings page and confirm the delay mechanism is set to P2P (Peer-to-Peer). If it is set to E2E (End-to-End), change it to P2P before proceeding. All switches in this guide will be configured for P2P to match.
P2P measures link delay between each adjacent pair of PTP-capable devices using Pdelay_Req messages, providing more accurate per-hop delay compensation than E2E in multi-hop topologies.
Step 2 — Set PTP operation mode to Two-Step
On the PTP grandmaster clock, confirm or change the operation mode to Two-Step. ORing 9000 Series switches support Two-Step mode only and do not support One-Step mode.
In Two-Step mode, the Sync message is transmitted first, followed immediately by a Follow_Up message containing the precise hardware transmit timestamp. In One-Step mode, the timestamp is embedded directly in the Sync message — a feature that requires specialised hardware not present in all switches. If the grandmaster is set to One-Step, the ORing switch will receive Sync messages but cannot extract the correct timestamp, and synchronisation will fail silently.
Part 2 — Configure Switch A (Ordinary Boundary Clock)
Switch A acts as a Boundary Clock: its Port 5 synchronises to the grandmaster (Slave role) and its Port 7 distributes time to Switch B (Master role). The Ordinary Boundary Clock device type handles this dual role automatically once configured.
Step 3 — Create a PTP clock instance on Switch A
Navigate to [MENU_PATH_PTP] on Switch A. Create a new PTP clock instance with the following settings:
- Device type: Ordinary Boundary Clock
- PTP working ports: Port 5 (toward PTP grandmaster) and Port 7 (toward Switch B)
- All other parameters: leave at default values
The Ordinary Boundary Clock device type configures the switch to terminate the incoming PTP session on its upstream port and regenerate a new PTP session on its downstream port(s), absorbing upstream jitter at each hop.
Step 4 — Configure PTP port settings on Switch A: set DIM to P2P
Click the Clock Instance number to enter the PTP Port Configuration page. For each working port (Port 5 and Port 7), change the DIM (Delay Interval Mechanism) field from the default E2E to P2P. Click Apply to save.
The DIM setting is per-port and must match the delay mechanism set on the PTP grandmaster clock (Step 1). A DIM mismatch between the switch port and the grandmaster is the most common cause of PTP ports remaining stuck in Listening state.
Step 4 — Verify Switch A time synchronisation
Wait 30–60 seconds after applying the DIM setting. Then verify the following on Switch A's PTP status page:
- PTP Time field: should display the current time received from the grandmaster clock (not zero and not the switch's local clock time)
- Offset From Master: should show a non-zero value — a value less than 1,000 ns (1 µs) indicates good synchronisation; a value greater than 1 ms (1,000,000 ns) indicates a synchronisation problem
- Port 5 role: Slave (toward grandmaster)
- Port 7 role: Master (toward Switch B)
Part 3 — Configure Switch B (Ordinary Boundary Clock)
Switch B has the same device type as Switch A (Ordinary Boundary Clock) but uses different ports: Port 7 connects to Switch A (Slave role) and Port 8 connects to downstream PTP clients (Master role). All DIM and operation mode settings are identical to Switch A.
| Setting | Switch A | Switch B |
|---|---|---|
| Device type | Ordinary Boundary Clock | Ordinary Boundary Clock |
| Operation mode | Two-Step | Two-Step |
| Delay mechanism (DIM) | P2P | P2P |
| Upstream port (Slave) | Port 5 → PTP Grandmaster | Port 7 → Switch A |
| Downstream port (Master) | Port 7 → Switch B | Port 8 → PTP Client |
Step 5 — Create PTP clock instance on Switch B and set DIM to P2P
Navigate to [MENU_PATH_PTP] on Switch B. Create a new PTP clock instance with:
- Device type: Ordinary Boundary Clock
- PTP working ports: Port 7 (toward Switch A) and Port 8 (toward PTP clients)
Then click the Clock Instance number to enter PTP Port Configuration. Change the DIM field to P2P on both Port 7 and Port 8. Click Apply.
Part 4 — Verify the complete PTP chain
After configuring both switches, verify the full PTP synchronisation chain from grandmaster to PTP clients. Check four items: PTP time, Offset From Master, port roles, and meanPathDelay values.
Expected port roles
| Device | Port | Expected role | Connected to |
|---|---|---|---|
| PTP Grandmaster | — | Master | Switch A Port 5 |
| Switch A | Port 5 | Slave | PTP Grandmaster |
| Switch A | Port 7 | Master | Switch B Port 7 |
| Switch B | Port 7 | Slave | Switch A Port 7 |
| Switch B | Port 8 | Master | PTP Client |
Verification checklist
- ✅ PTP Time field on both switches: displays grandmaster time (not zero, not local clock)
- ✅ Offset From Master on both switches: non-zero value, ideally < 1,000 ns (1 µs) for good synchronisation
- ✅ Port roles: Switch A Port 5 = Slave, Port 7 = Master; Switch B Port 7 = Slave, Port 8 = Master
- ✅ peer meanPathDelay: non-zero value on P2P ports, confirming Pdelay_Req exchange is active
Troubleshooting common PTP configuration issues
PTP port stuck in Listening state — does not reach Slave or Master
The most common cause is a DIM mismatch. Check in this order: (1) confirm the DIM field on the switch port is set to P2P — not left at the default E2E; (2) verify the PTP grandmaster clock delay mechanism is also P2P; (3) confirm the grandmaster is set to Two-Step operation mode; (4) check the PTP domain number on all devices matches (default is domain 0); (5) verify the physical link between the grandmaster and Switch A Port 5 is active (link LED on). If all settings match and the port still shows Listening, try deleting and recreating the PTP clock instance.
Offset From Master is zero or not displayed
A zero or blank Offset From Master means the switch has not received a valid Sync/Follow_Up message pair from the grandmaster. Check: (1) the PTP clock instance on the switch has the correct working ports assigned — missing the upstream port means no Sync messages are processed; (2) the grandmaster is transmitting PTP messages (verify on the grandmaster's status page); (3) there is no firewall or VLAN configuration blocking PTP multicast frames (destination MAC 01:1B:19:00:00:00 for general PTP, or 01:80:C2:00:00:0E for peer delay). Wait at least 60 seconds after applying configuration changes before judging Offset From Master.
Offset From Master is non-zero but larger than 1 ms
A large Offset From Master (greater than 1 ms) indicates synchronisation is occurring but with poor accuracy. This typically means: (1) the network path between grandmaster and switch has high asymmetric delay (e.g., long fibre runs, asymmetric queuing); (2) a switch in the path is not PTP-aware (transparent clock or boundary clock) and is adding variable delay; (3) the grandmaster clock itself has a configuration issue. For industrial OT applications requiring < 1 µs accuracy, all switches in the PTP path should be configured as Ordinary Boundary Clocks (as in this guide) rather than passing PTP traffic through non-PTP-aware switches.
Port role is neither Slave nor Master (shows Passive or Faulty)
A Passive port role indicates the port lost the Best Master Clock Algorithm (BMCA) election to another port — this is expected if two ports could both be Slave. A Faulty state indicates a protocol error. Check: (1) verify there is no loop in the PTP topology (PTP is not loop-tolerant like STP); (2) confirm only one PTP grandmaster clock is present in the domain; (3) re-check DIM and domain number settings on the affected switch.
Frequently asked questions
What is PTP (IEEE 1588) and why is it used in industrial networks?
PTP (Precision Time Protocol, IEEE 1588-2008) synchronises clocks across Ethernet networks to sub-microsecond accuracy. In industrial OT environments — power grid protection, SCADA event correlation, motion control — devices must share a common time reference. ORing 9000 Series L3 managed switches support IEEE 1588 as Ordinary Boundary Clocks, distributing time from a grandmaster to downstream clients.
What is the difference between P2P and E2E delay measurement?
E2E (End-to-End) measures delay between Slave and grandmaster using Delay_Req messages. P2P (Peer-to-Peer) measures delay between each adjacent pair of PTP devices using Pdelay_Req messages, giving more accurate per-hop compensation in multi-hop networks. All devices in a PTP domain must use the same mechanism — mixing P2P and E2E causes synchronisation failure.
What is the difference between One-Step and Two-Step PTP operation?
In One-Step mode, the transmit timestamp is embedded in the Sync message at hardware level. In Two-Step mode, the timestamp is sent in a separate Follow_Up message. ORing 9000 Series switches support Two-Step only. If the PTP grandmaster is set to One-Step, change it to Two-Step before connecting to ORing switches.
What is an Ordinary Boundary Clock in PTP?
A Boundary Clock has multiple PTP ports. It terminates the PTP session on its upstream (Slave) port and regenerates a new session on its downstream (Master) port(s), absorbing network jitter at each hop. ORing 9000 Series switches configured as Ordinary Boundary Clocks terminate the grandmaster's PTP session and re-originate it toward downstream clients — improving synchronisation accuracy across multi-hop industrial networks.
PTP port is stuck in Listening — what should I check first?
Start with the DIM field — a mismatch between the switch port DIM setting and the grandmaster delay mechanism (P2P vs E2E) is the most common root cause. Verify DIM = P2P on all switch ports, confirm the grandmaster is also P2P and Two-Step, and check that the PTP domain number matches on all devices. See the Troubleshooting section above for a full checklist.
