PTP employs a hierarchy of clock types to ensure that precise timing and synchronization is maintained between the source and the numerous PTP clients that are distributed throughout the network. A logical grouping of PTP clocks that synchronize with each other using the PTP protocol, but are not necessarily synchronized to the PTP clocks in another domain, is called a PTP domain.

The three PTP clock types are Ordinary clock, Boundary clock, and Transparent clock.

• Ordinary clock —This clock type has a single PTP port in a domain, and maintains the timescale used in the domain. It may serve as a source of time, that is, be a primary, or may synchronize to another clock by being a subordinate. It provides time to an application or to an end device.
• Boundary clock —This clock type has multiple PTP ports in a domain, and maintains the timescale used in the domain. It may serve as a source of time, that is, be a primary, or may synchronize to another clock by being a subordinate. A boundary clock, that is secondary, has a single slave port, and transfers timing from that port to the primary ports.
• Transparent clock —This clock type is a device that measures the time taken for a PTP event message to pass through the device, and provides this information to the clocks receiving this PTP event message.

{start cross reference}Table 13-1{end cross reference} shows the 1588v2 PTP support matrix on a Cisco ASR1000 platform.

The three key components of a PTP-enabled data network are primary reference, PTP client, and PTP-enabled router acting as a Boundary clock.

• Primary Reference —An IEEE1588v2 PTP network needs a primary reference to provide a precise time source. The most economical way of obtaining the precise time source for the primary reference is through a Global Positioning System (GPS) because it provides +/- 100 nanosecond (ns) accuracy. First, the PTP primary reference’s built-in GPS receiver converts the GPS timing information to PTP time information, which is typically Coordinated Universal Time (UTC), and then delivers the UTC time to all the PTP clients.
• PTP client —A PTP client has to be installed on servers, network-monitoring and performance-analysis devices, or other devices that want to use the precise timing information provided by PTP, and it’s mostly an ordinary clock. The two kinds of PTP clients are pure software PTP clients and hardware-assistant PTP clients.
• PTP boundary clock —Any router that is between a PTP primary and PTP secondary can act as a PTP boundary clock router. It has two interfaces, one facing the PTP primary and another facing the PTP secondary. The boundary clock router acts as a secondary on the interface facing the PTP primary router , and acts as a primary on the interface facing the PTP secondary router . The PTP boundary clock router is deployed to minimize timing delay in cases where the distance between PTP primary router and the PTP secondary router is more.

Note	Intermediary nodes between PTP primary and secondary should be a PTP-enabled or transparent clock node.

The following figure shows the functions of a PTP Enabled device.

Clock synchronization is achieved through a series of messages exchanged between the primary clock and the secondary clock as shown in the figure.

After the primary-secondary clock hierarchy is established, the clock synchronization process starts. The message exchange occurs in this sequence:

1. The primary clock sends a Sync message. The time at which the Sync message leaves the primary is time-stamped as t{start subscript}1{end subscript}.
2. The secondary clock receives the Sync message and is time-stamped as t{start subscript}2{end subscript}.
3. The secondary sends the Delay_Req message, which is time-stamped as t{start subscript}3{end subscript} when it leaves the secondary, and as t{start subscript}4{end subscript} when the primary receives it.
4. The primary responds with a Delay_Resp message that contains the time stamp t{start subscript}4{end subscript}.

The clock offset is the difference between the primary clock and the secondary clock, and is calculated as follows:

Offset = t{start subscript}2{end subscript} - t{start subscript}1{end subscript} - meanPathDelay

IEEE1588 assumes that the path delay between the primary clock and the secondary clock is symmetrical, and hence, the mean path delay is calculated as follows:

meanPathDelay = ((t{start subscript}2{end subscript} - t{start subscript}1{end subscript}) + (t{start subscript}4{end subscript} - t{start subscript}3{end subscript}))/2

All PTP communication is performed through message exchange. The two sets of messages defined by IEEE1588v2 are General messages and Event messages.

• General messages —These messages do not require accurate time stamps, and are classified as Announce, Follow_Up, Delay_Resp, Pdelay_Resp_Follow_Up, Management, and Signaling.
• Event messages —These messages require accurate time stamping, and are classified as Sync, Delay_Req, Pdelay_Req, and Pdelay_Resp.

The following are the PTP clocking modes supported on a Cisco ASR 1002-X Router:

• Unicast Mode —In unicast mode, the primary sends the Sync or Delay_Resp messages to the secondary on the unicast IP address of the secondary, and the secondary in turn sends the Delay_Req message to the primary on the unicast IP address of the primary.

• Unicast Negotiation Mode —In unicast negotiation mode, the primary does not know of any secondary until the secondary sends a negotiation message to the primary. The unicast negotiation mode is good for scalability purpose because one primary can have multiple secondary.

Accuracy is an important aspect of PTP implementation on an Ethernet port. For a packet network, Packet Delay Variation (PDV) is one of the key factors that impacts the accuracy of a PTP clock. The Cisco ASR 1002-X Router can handle the PDV of the network with its advanced hardware and software capabilities, such as hardware stamping and special high-priority queue for PTP packets. It can provide around 300 ns accuracy in a scalable deployment scenario.

The two methods used on the same topology to cross-check and verify the results are:

• One-pulse-per-second (1PPS) to verify the secondary PTP.
• Maximum Time Interval Error (MTIE) and Time Deviation (TDEV) to verify the PDV.

The verification topology includes a primary reference with a GPS receiver, a Cisco ASR 1002-X Router, PTP hardware secondary reference clocks with 1PPS output, and a test equipment for the measurement.

The following figure shows the PPS accuracy, with time of day measured using the test equipment as per the topology shown in the following figure. The average PPS accuracy value found is 250 ns.

{start cross reference}Figure 13-5{end cross reference} shows a topology that includes a primary reference with a GPS receiver, a Cisco ASR 1002-X Router, PTP hardware secondary reference clocks, and a test equipment for the MTIE and TDEV measurement.

{start cross reference}Figure 13-6{end cross reference} shows a graph with the MTIE and TDEV measurements to verify the PDV.

IEEE 1588v2 PTP supports these features on a Cisco ASR1002-X Router:

• Two-step Ordinary clock and Boundary clock.
• Hardware-assistant PTP implementation to provide sub-300 ns accuracy.
• PTP operation on all physical onboard Gigabit Ethernet interfaces.
• Supports built-in Gigabit Ethernet links in two-step clock mode.

We recommend that you configure a stable input clock source from a GPS device before configuring primary PTP. The GPS device acts as a PTP primary reference, and the BITS or 10-MHz port of a Cisco ASR 1002-X Router can be used to input or output the network clock. Perform these tasks to configure network clocking on a Cisco ASR 1002-X Router:

You can configure a Cisco ASR 1002-X Router in Ordinary clock mode as either primary or secondary.

Perform these tasks to configure an ordinary clock as either primary or secondary:

A Cisco ASR 1002-X Router can exchange time of day and 1PPS input with an external device, such as a GPS receiver, using the time of day and 1PPS input and output interfaces on the router.

Perform these tasks to configure Time of Day (ToD) messages on the Cisco ASR 1002-X Router:

This example shows how to configure IEEE 1588v2 PTP on a Cisco ASR1002-X Router:

Unicast Negotiation Mode

Master Clock 
ptp clock ordinary domain 1
tod R0 ntp 
input 1pps R0 
clock-port  MASTER master 
transport ipv4 unicast interface loopback 0 negotiation
Slave clock 
ptp clock ordinary domain 1
tod R0 ntp 
output 1pps R0 
clock-port SLAVE slave
transport ipv4 unicast interface loopback 0 negotiation
clock source 10.1.1.1
Boundary clock
 
ptp clock boundary domain 1
clock-port SLAVE slave
transport ipv4 unicast interface loopback 0 negotiation
clock source 10.1.1.1
clock-port  MASTER master 
transport ipv4 unicast interface loopback 1 negotiation

Unicast Mode

Master Clock
ptp clock ordinary domain 1
tod R0 ntp 
input 1pps R0 
clock-port  MASTER master 
transport ipv4 unicast interface loopback 0 
clock destination 20.1.1.1
Slave clock
 
ptp clock ordinary domain 1
tod R0 ntp 
output 1pps R0 
clock-port SLAVE slave
transport ipv4 unicast interface loopback 0 
clock source 10.1.1.1

Use the following commands to verify the IEEE 1588v2 PTP configuration:

• Use the show ptp clock running domain 0 command to display the output:

Router# show ptp clock running domain 0
On the MASTER:
                      PTP Ordinary Clock [Domain 0] 
         State          Ports          Pkts sent      Pkts rcvd      Redundancy Mode
         FREQ_LOCKED    1              31522149       10401171       Hot standby
                               PORT SUMMARY
                                                                        PTP Master
Name   Tx Mode      Role         Transport    State        Sessions     Port Addr
MASTER unicast      master       Lo1          Master       1            -
                             SESSION INFORMATION
MASTER [Lo1] [Sessions 1]
Peer addr          Pkts in    Pkts out   In Errs    Out Errs  
11.11.11.11        10401171   31522149   0          0      
On the SLAVE:
                      PTP Ordinary Clock [Domain 0] 
         State          Ports          Pkts sent      Pkts rcvd      Redundancy Mode
         PHASE_ALIGNED  1              4532802        13357682       Track one
                               PORT SUMMARY
                                                                       PTP Master
Name  Tx Mode      Role         Transport    State        Sessions     Port Addr
SLAVE unicast      slave        Lo20         Slave        1            10.10.10.10
                             SESSION INFORMATION
SLAVE [Lo20] [Sessions 1]
Peer addr          Pkts in    Pkts out   In Errs    Out Errs  
10.10.10.10        13357682   4532802    0          0         

• Use the show platform software ptp tod command to check the time-of-day information:

PTPd ToD information:
Time: 06/24/14 02:06:29

• Use the show platform ptp tod all command to check the time-of- day state:

Router# show platform ptp tod all
On the MASTER
--------------------------------
ToD/1PPS Info for : R0
--------------------------------
RJ45 JACK TYPE        : RS422
ToD CONFIGURED        : YES
ToD FORMAT            : NTPv4
ToD DELAY             : 0
1PPS MODE             : INPUT
1PPS STATE            : UP
ToD STATE             : UP
--------------------------------
On the SLAVE:
--------------------------------
ToD/1PPS Info for : R0
--------------------------------
RJ45 JACK TYPE        : RS422
ToD CONFIGURED        : YES
ToD FORMAT            : NTPv4
ToD DELAY             : 0
1PPS MODE             : OUTPUT
OFFSET                : 0
PULSE WIDTH           : 0
--------------------------------