System Management Configuration Guide, Cisco Catalyst IE3x00 Rugged, IE3400 Heavy Duty, and ESS3300 Series Switches

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System Management Configuration Guide, Cisco Catalyst IE3x00 Rugged, IE3400 Heavy Duty, and ESS3300 Series Switches

Precision time protocol

Updated: June 26, 2026

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Describes a time synchronization protocol designed for packet-based networks with distributed device clocks.

Precision Time Protocol (PTP) is a time synchronization protocol that

  • synchronizes clocks in packet-based networks that include distributed device clocks of varying precision and stability

  • requires minimal bandwidth and little processing overhead, and

  • is defined in IEEE 1588 as Precision Clock Synchronization for Networked Measurements and Control Systems.

Protocol characteristics

PTP is designed specifically for industrial, networked measurement and control systems, and is optimal for use in distributed systems.

Cisco Catalyst IE3x00 Rugged, IE3400 Heavy Duty, and ESS3300 Series Switches (referred to as the switch in the rest of the document) support PTP.

Why PTP?

Smart grid power automation applications such as peak-hour billing, virtual power generators, and outage monitoring and management, require extremely precise time accuracy and stability. Timing precision improves network monitoring accuracy and troubleshooting ability.

PTP benefits in ethernet networks

The benefits of using PTP in an Ethernet network include:

  • Low cost and easy setup in existing Ethernet networks

  • Limited bandwidth is required for PTP data packets

Ethernet switches and delays

Ethernet switches and delays are network timing challenges that

  • occur when switches buffer packets during simultaneous transmission attempts, causing timing delays,

  • result from packet processing variations during MAC address table searches and CRC verification, and

  • can cause device clocks on the network to lose synchronization with one another.

Switch delay mechanisms and PTP compensation

In an Ethernet network, switches provide a full-duplex communication path between network devices. Switches use the address information in data packets to determine their destinations. When a switch tries to send multiple packets at once, it buffers some packets to prevent loss before transmission. If the buffer becomes full, the switch delays packet transmission. This delay can cause device clocks on the network to lose synchronization with one another.

Additional delays may occur when entering packets are stored in local memory while the switch searches the MAC address table and verifies packet Cyclic Redundancy Check (CRC) fields. This process causes variations in packet-forwarding time latency, and these variations might result in asymmetrical packet-delay times.

Adding PTP to a network compensates for these latency and delay problems by correctly adjusting device clocks so that they stay synchronized with one another. PTP enables network switches to function as PTP devices, including boundary clocks and transparent clocks.

Note

To learn more about PTP clock devices and their role in a PTP network, see PTP clocks.

Message-based synchronization

Message-based synchronization is a PTP method that requires accurate measurement of communication path delay between the time source (grandmaster clock) and the time recipient. It sends messages between the time source and the time recipient to determine path delay. The method measures the exact transmit and receive times of messages to calculate communication path delay. It then adjusts current time information by the calculated delay to provide more accurate time information.

Path delay calculation and transparent clocks

This delay measurement principle determines the path delay between devices on a network. The local clocks are adjusted for this delay using a series of messages sent between the time source and the time recipient devices. The system calculates the one-way delay time by averaging the path delays of the transmit and receive messages. This calculation assumes a symmetrical communication path. However, switched networks do not necessarily have symmetrical communication paths because of the buffering process.

PTP provides a method—using transparent clocks—to measure and account for the delay in a time-interval field in network timing packets. This approach makes the switches temporarily transparent to the time source and the time recipient nodes on a network. An end-to-end transparent clock forwards all the messages in a network in the same way that a switch does.

Note

The Cisco PTP implementation supports multicast PTP messages only.

To read a detailed description of synchronization messages, refer to PTP event message sequences. To learn more about how transparent clocks calculate network delays, see PTP clocks.

This figure shows a typical 1588 PTP network that includes time source clocks, switches in boundary clock mode, and intelligent electronic devices (IEDs), such as digital relays or protection devices. In this figure, Time Source 1 is the grandmaster clock. If Time Source 1 becomes unavailable, the time recipient boundary clocks synchronize with Time Source 2.

Figure 1. PTP network
A typical 1588 PTP network diagram illustrating time source clocks, boundary clock switches, and intelligent electronic devices, highlighting the role of Time Source 1 as the grandmaster clock and the failover to Time Source 2 for synchronization.

PTP event message sequences

This section describes the PTP event message sequences that occur during synchronization.

How boundary clocks synchronize

Summary

The key components involved in boundary clock synchronization are:

  • Ordinary clocks: Time recipients that receive synchronization messages

  • Boundary clocks: Time sources that send synchronization messages

  • Event messages: Communication protocol using Sync, Delay_Req, Follow_Up, and Delay_Resp messages

  • Timestamps: Four timing values (t1, t2, t3, t4) used to calculate clock offset and propagation delay

Workflow

The boundary clock synchronization process involves these stages:

  1. The time source sends a Sync message to the time recipient and notes the time (t1) at which it was sent.
  2. The time recipient receives the Sync message and notes the time of reception (t2).
  3. The time source sends the timestamp t1 to the time recipient by embedding it in a Follow_Up message.
  4. The time recipient sends a Delay_Req message to the time source and notes the time (t3) at which it was sent.
  5. The time source receives the Delay_Req message and notes the time of reception (t4).
  6. The time source sends the timestamp t4 to the time recipient by embedding it in a Delay_Resp message.

Result

After this sequence, the time recipient possesses all four timestamps. These timestamps can be used to compute the offset of the time recipient clock relative to the time source, and the mean propagation time of messages between the two clocks.

The offset calculation is based on the assumption that the time for the message to propagate from the time source to the time recipient is the same as the time required from the time recipient to the time source. This assumption is not always valid on an Ethernet network because of asymmetrical packet delay times.

How peer-to-peer transparent clocks synchronize

When a network includes multiple levels of boundary clocks in the hierarchy, with non-PTP-enabled devices between them, synchronization accuracy decreases.

The round-trip time is assumed to be equal to mean_path_delay/2. However, this is not always valid for Ethernet networks. To improve accuracy, the resident time of each intermediary clock is added to the offset in the end-to-end transparent clock. Resident time, however, does not take into consideration the link delay between peers, which is handled by peer-to-peer transparent clocks.

Peer-to-peer transparent clocks measure the link delay between two clock ports implementing the peer delay mechanism. The link delay is used to correct timing information in Sync and Follow_Up messages.

Summary

The key components involved in peer-to-peer transparent clock synchronization are:

  • Port 1: Initiates the synchronization process by generating and receiving timing messages

  • Port 2: Responds to timing messages and provides timestamp information

  • Event messages: Pdelay_Req, Pdelay_Resp, and Pdelay_Resp_Follow_Up messages that carry timing information

  • Timestamps: Four time values (t1, t2, t3, t4) used to calculate mean link delay

Workflow

Figure 2. Detailed Steps—Peer-to-Peer transparent clock synchronization
The image illustrates the detailed steps involved in peer-to-peer transparent clock synchronization, highlighting the timing messages exchanged between ports and the timestamps generated during the process.

These stages describe how peer-to-peer transparent clocks synchronize using event messages:

  1. Port 1 generates timestamp t1 for a Pdelay_Req message.
  2. Port 2 receives and generates timestamp t2 for this message.
  3. Port 2 returns and generates timestamp t3 for a Pdelay_Resp message. To minimize errors caused by frequency offset between the two ports, Port 2 returns the Pdelay_Resp message as quickly as possible after the receipt of the Pdelay_Req message.
  4. Port 2 returns timestamps t2 and t3 in the Pdelay_Resp and Pdelay_Resp_Follow_Up messages respectively.
  5. Port 1 generates timestamp t4 after receiving the Pdelay_Resp message. Port 1 then uses the four timestamps (t1, t2, t3, and t4) to calculate the mean link delay.

Synchronizing the local clock

In a PTP network, drift compensation uses timestamp information and follow-up messages in device hardware to adjust the local clock frequency so it matches the time source clock.

Clock drift in PTP networks

In an ideal PTP network, the time source and the time recipient clocks operate at the same frequency. However, drift might occur on the network. Drift is the frequency difference between the time source and the time recipient clocks.

Best master clock algorithm

The Best Master Clock Algorithm (BMCA) is the basis of PTP functionality that

  • specifies how each clock on the network determines the best time source clock in its subdomain of all the clocks it can see, including itself,

  • runs on the network continuously and quickly adjusts for changes in network configuration, and

  • prevents clock conflicts on the PTP network.

Time source clock selection criteria

The BMCA uses several criteria to determine the best time source clock in the subdomain.

  • Clock quality, for example, GPS is considered the highest quality

  • Clock accuracy of the clock's time base

  • Stability of the local oscillator

  • Closest clock to the grandmaster

The BMCA also prevents clock conflicts on the PTP network by ensuring that:

  • Clocks do not have to negotiate with one another.

  • There is no misconfiguration, such as two time source clocks or no time source clocks, as a result of the time source clock identification process.

PTP clocks

PTP clocks are PTP-enabled network devices that provide time synchronization, consist of different clock types based on their network role, and work together to maintain accurate time across the network.

PTP clock types

A PTP network is made up of PTP-enabled devices and devices that are not using PTP. The PTP-enabled devices typically consist of these clock types.

Grandmaster clock

  • Within a PTP domain, the grandmaster clock (GMC) is the primary source of time for clock synchronization using PTP. The GMC usually uses a highly precise time source, such as GPS or an atomic clock. When the network does not require any external time reference and only needs to be synchronized internally, the GMC can be free-running.

  • The switch can function as a hybrid grandmaster boundary clock using NTP as its source. For more information, see NTP to PTP time conversion.

Ordinary clock

  • An ordinary clock is a PTP clock with a single PTP port. It functions as a node in a PTP network.Ordinary clocks are the most common clock type on a PTP network because they serve as end nodes connected to devices requiring synchronization. Ordinary clocks have various interfaces to external devices.

Boundary clock

  • A boundary clock in a PTP network operates in place of a standard network switch or router. Boundary clocks have more than one PTP port. Each port provides access to a separate PTP communication path. Boundary clocks provide an interface between PTP domains. They intercept and process all PTP messages and pass other network traffic. The boundary clock uses the BMCA to select the best clock seen by any port. The selected port is then configured not to be a time source. The time source port synchronizes the clocks connected downstream, while the port that is not a time source synchronizes with the upstream time source clock.

Transparent clock

  • Transparent clocks in a PTP network update the time interval field included in a PTP event message. This update compensates for switch DELAY and achieves an accuracy within one picosecond.

There are two types of transparent clocks:

  • End-to-end (E2E) transparent clocks measure the PTP event message transit time (also known as resident time) for SYNC and DELAY_REQUEST messages. This measured transit time is added to a data field (correction field) in the corresponding messages:

    • The measured transit time of a SYNC message is added to the correction field of the corresponding SYNC or the FOLLOW_UP message.

    • The measured transit time of a DELAY_REQUEST message is added to the correction field of the corresponding DELAY_RESPONSE message.

    The time recipient uses this information when determining the offset between the time recipient's and the time source's time. E2E transparent clocks do not provide correction for the propagation DELAY of the link itself.

  • Peer-to-peer (P2P) transparent clocks measure PTP event message transit time in the same way E2E transparent clocks do, as described above. In addition, P2P transparent clocks measure the upstream link DELAY. The upstream link DELAY is the estimated packet propagation DELAY between the upstream neighbor P2P transparent clock and the P2P transparent clock under consideration.

    These two times (message transit time and upstream link DELAY time) are both added to the correction field of the PTP event message. The correction field of the message received by the time recipient contains the sum of all link delays. In theory, this is the total end-to-end DELAY (from time source to time recipient) of a SYNC packet.

This figure illustrates PTP clocks in a time source-time recipient hierarchy within a PTP network.

Figure 3. PTP clock hierarchy
PTP clocks are organized in a time source-time recipient hierarchy within a PTP network, illustrating the relationship and flow of time synchronization messages.

PTP profiles

A PTP profile is a set of allowed PTP features applicable to a device that is specific to a particular type of application or environment. It defines configuration values for BMCA options, path delay mechanisms, and transport mechanisms. It also specifies node types and options that are required, permitted, or prohibited.

PTP profile types

The switch supports these PTP profiles:

  • Power Profile

  • Default Profile

The Power Profile is defined in PC37.238-2011 - IEEE Draft Standard Profile for Use of IEEE 1588 Precision Time Protocol in Power System Applications. The Cisco PTP implementation uses the terms Power Profile mode and Default Profile mode when referring to this IEEE 1588 profile and its associated configuration values.

A PTP profile defines these values:

  • BMCA options

  • Configuration management options

  • Path delay mechanisms (peer delay or delay request-response)

  • Range and default values of all PTP configurable attributes and data set members

  • Transport mechanisms that are required, permitted, or prohibited

  • Node types that are required, permitted, or prohibited

  • Options that are required, permitted, or prohibited

Default profile mode

Default profile mode is a PTP profile mode that uses Layer 3 as the PTP transport mode. It supports end-to-end (E2E) as the transparent clock mode and serves as the default PTP profile mode on the switch.

Default profile configuration details

The Default profile uses Layer 3 transport multicast address 224.0.1.129 (MAC address 01-00-5e-00-01-81) for all PTP messages.

Refer to Power profile mode for a list of the configuration values for the switch in Default Profile and Power Profile modes.

Power profile mode

Power profile mode is a PTP configuration mode that uses specific values defined in the IEEE 1588 Power Profile standard for PTP networks in power substations. It includes optimum physical layer settings, higher-level protocols for PTP messages, and preferred BMCA configurations. This setup ensures consistent and reliable network time distribution within substations, between substations, and across wide geographic areas.

Switch optimization for PTP

The switch is optimized for PTP in these ways:

  • Hardware: The switch uses FPGA and PHY for the PTP function. The PHY time-stamps the Fast Ethernet and Gigabit Ethernet ports.

  • Software: In Power Profile mode, the switch uses the configuration values defined in the IEEE 1588 Power Profile standard.

The table compares configuration values defined by the IEEE 1588 Power Profile and the values the switch uses for each PTP profile mode.

Table 1. Configuration values for the IEEE PTP power profile and switch modes

PTP Field

Power Profile Value

Switch Configuration Value

Power Profile Mode

Default Profile Mode

Message transmission

Ethernet 802.3 with Ethertype 0X88F7. PTP messages are sent as 802.1Q- tagged Ethernet frames with a default VLAN 0 and default priority 4.

Access Ports : Untagged Layer 2 packets.

Trunk Ports : 802.1Q tagged Layer 2 packets with native VLAN on the port and default priority value of 4.

Layer 3 packets. By default, 802.1q tagging is disabled.

MAC address : Nonpeer delay messages

01-1B-19-00-00-00.

01-1B-19-00-00-00.

01-00-5e-00-01-81.

MAC address : Peer delay messages

01-80-C2-00-00-0E.

01-80-C2-00-00-0E.

Not applicable to this mode.

Domain number

0.

0.

0.

Path delay calculation

Peer-to-peer transparent clocks.

Peer-to-peer transparent clocks using the peer_delay mechanism.

End-to-end transparent clocks using the delay_request mechanism.

BMCA

Enabled.

Enabled.

Enabled.

Clock type

Two-step clocks are supported.

Two-step.

Two-step.

Time scale

Epoch.1

Epoch.

Epoch.

Grandmaster ID and local time determination

PTP-specific TLV (type, length, value) to indicate Grandmaster ID.

PTP-specific TLV to indicate Grandmaster ID.

PTP-specific type, length, and value to indicate Grandmaster ID.

Time accuracy over network hops

Over 16 hops, the time recipient device synchronization accuracy is within 1 usec (1 microsecond).

Over 16 hops, the time recipient device synchronization accuracy is within 1 usec (1 microsecond).

Not applicable in this mode.

1 Epoch = Elapsed time since epoch start.

Tagging behavior for PTP packets

This table describes the switch tagging behavior in Power Profile and Default Profile modes for PTP packets.

Table 2. Tagging behavior for PTP packets

Switch Port Mode

Configuration

Power Profile Mode

Default Profile Mode

Behavior

Priority

Behavior

Priority

Trunk Port

vlan dot1q tag native enabled

Switch tags packets

7

Switch tags packets

7

Trunk Port

vlan dot1q tag native disabled

PTP software tags packets

4

Untagged

None

Access Port

N/A

Untagged

None

Untagged

None

PTP clock modes supported on the switch

PTP synchronization behavior depends on the PTP clock mode that you configure on the switch. You can configure the switch for one of the global modes described in this reference.

See PTP configuration and mode limitations for guidelines to configure each of the clock modes.

Grandmaster-Boundary clock mode

The switch can function as a hybrid grandmaster boundary clock using NTP as its source. For more information, see NTP to PTP time conversion.

Boundary clock mode

A switch configured for boundary clock mode participates in selecting the best time source clock on the subdomain, selecting from all the clocks it can see, including itself. "If the switch does not detect a more accurate clock than itself, it becomes the time source clock. If the switch detects a more accurate clock, it synchronizes to that clock and becomes a time recipient clock.

After initial synchronization, the switch and the connected devices exchange PTP timing messages to correct the changes caused by clock offsets and network delays.

Forward mode

A switch configured for forward mode passes incoming PTP packets as normal multicast traffic.

E2E transparent clock mode

A switch configured for end-to-end transparent clock mode does not synchronize its clock with the time source clock. A switch in this mode does not participate in time source clock selection and uses the default PTP clock mode on all ports.

P2P transparent clock mode

A switch configured for peer-to-peer transparent clock mode does not synchronize its clock with the time source clock. A switch in this mode does not participate in time source clock selection and uses the default PTP clock mode on all ports.

Configurable boundary clock synchronization algorithm

A configurable boundary clock synchronization algorithm is a PTP mechanism that accommodates various PTP use cases, depending on whether you need to prioritize filtering of input time errors or faster convergence. It provides three synchronization options (feedforward, adaptive, and linear) that comply with IEEE 1588-2008. This approach allows BCs to use different algorithms to control time output based on specific network requirements.

BC synchronization algorithm options

By default, the BC uses a linear feedback controller (that is, a servo) to set the BC's time output to the next clock. The linear servo provides a small amount of PDV filtering and converges in an average amount of time. For improved convergence time, BCs can use the TC feedforward algorithm to measure the delay added by the network elements' forwarding plane (the disturbance). BCs then use that measured delay to control the time output.

While the feedforward BC dramatically speeds up the boundary clock, the feedforward BC does not filter any PDV. The adaptive PDV filter provides high-quality time synchronization when PDV is present over wireless access points (APs) and enterprise switches that do not support PTP and add significant PDV.

A PTP algorithm that filters packet delay variation (PDV) converges more slowly than a PTP algorithm that does not.

Three options are available for BC synchronization:

  • Feedforward—For very fast and accurate convergence; no PDV filtering.

  • Adaptive—Filters as much PDV as possible, given a set of assumptions about the PDV characteristics, the hardware configuration, and the environmental conditions.

    Note
    With the adaptive filter, the switch does not meet the time performance requirements specified in ITU-T G.8261.

  • Linear—Provides simple linear filtering (the default).

Adaptive mode (ptp transfer filter adaptive) is not available in Power Profile mode.

For configuration information, see PTP switch configuration.