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 master, or may synchronize to another clock by being a slave. 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 master, or may synchronize to another clock by being a slave. A boundary clock that is a slave has a single slave port, and transfers timing from that port to the master 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 grand master, PTP client, and PTP-enabled router acting as a Boundary clock.

• Grand Master—An IEEE1588v2 PTP network needs a grand master to provide a precise time source. The most economical way of obtaining the precise time source for the grand master is through a Global Positioning System (GPS) because it provides +/- 100 nanosecond (ns) accuracy. First, the PTP grand master’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 master and PTP slave can act as a PTP boundary clock router. It has two interfaces, one facing the PTP master and another facing the PTP slave. The boundary clock router acts as a slave on the interface facing the PTP master router , and acts as a master on the interface facing the PTP slave router . The PTP boundary clock router is deployed to minimize timing delay in cases where the distance between PTP master router and the PTP slave router is more.

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

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

Figure 1. 372860.eps Functions of a PTP-Enabled Device

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

Figure 2. Clock-Synchronization Process

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

1. The master clock sends a Sync message. The time at which the Sync message leaves the master is time-stamped as t{start subscript}1{end subscript}.
2. The slave clock receives the Sync message and is time-stamped as t{start subscript}2{end subscript}.
3. The slave sends the Delay_Req message, which is time-stamped as t{start subscript}3{end subscript} when it leaves the slave, and as t{start subscript}4{end subscript} when the master receives it.
4. The master 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 master clock and the slave 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 master clock and the slave 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 master sends the Sync or Delay_Resp messages to the slave on the unicast IP address of the slave, and the slave in turn sends the Delay_Req message to the master on the unicast IP address of the master.
• Unicast Negotiation Mode—In unicast negotiation mode, the master does not know of any slave until the slave sends a negotiation message to the master. The unicast negotiation mode is good for scalability purpose because one master can have multiple slaves.

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 PTP slave.
• Maximum Time Interval Error (MTIE) and Time Deviation (TDEV) to verify the PDV.

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

Figure 3. 1PPS Accuracy 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.

Figure 4. Graph Showing PPS Accuracy

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

Figure 5. 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.

Figure 6. Graph to show MTIE and TDEV Measurement

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 PTP master. The GPS device acts as a PTP grand master, 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 master or slave.

Figure 7. Ordinary Clock Scenario with a GPS Device as Grand Master

Perform these tasks to configure an ordinary clock as either master or slave:

You can configure the PTP master and PTP slave in a boundary clock topology as shown in the figure in the same way that you configure a master and slave in ordinary clock mode. This section describes how to configure a Cisco ASR 1002-X Router in boundary clock mode.

Note	Currently, boundary clock supports only unicast negotiation mode.

Figure 8. PTP Boundary Clock Scenario

1.    configure terminal


2.    ptp clock boundary domain domain_number

3.    clock-port name slave

4.    transport ipv4 unicast interface {GigabitEthernet | Loopback} interface-number [negotiation]


5.    clock source ip-address


6.    exit


7.    clock-port name master

8.    transport ipv4 unicast interface {GigabitEthernet | Loopback} interface-number [negotiation]


9.    end

Router# configure terminal

Router(config)# ptp clock boundary domain 0 

Router(config-ptp-clk)# clock-port SLAVE slave 

Router(config-ptp-port)# transport ipv4 unicast interface Loopback11 negotiation

Router(config-ptp-port)# clock source 10.10.10.10

Router(config-ptp-port)# exit

Router(config-ptp-clk)# clock-port MASTER master 

Router(config-ptp-port)# transport ipv4 unicast interface Loopback10 negotiation

Router(config-ptp-port)# end

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: