Configuring PTP
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Configuring PTP
This chapter includes the following sections:
- Overview of PTP
- PTP Device Types
- PTP Process
- High Availability
- Licensing Requirements for PTP
- Guidelines and Limitations for PTP
- Default Settings
- Configuring PTP
Overview of PTP
PTP is a time synchronization protocol for nodes distributed across a network. Its hardware timestamp feature provides greater accuracy than other time synchronization protocols such as Network Time Protocol (NTP).
A PTP system can consist of a combination of PTP and non-PTP devices. PTP devices include ordinary clocks, boundary clocks, and transparent clocks. Non-PTP devices include ordinary network switches, routers, and other infrastructure devices.
PTP is a distributed protocol that specifies how real-time PTP clocks in the system synchronize with each other. These clocks are organized into a master-slave synchronization hierarchy with the grandmaster clock, the clock at the top of the hierarchy, determining the reference time for the entire system. Synchronization is achieved by exchanging PTP timing messages, with the members using the timing information to adjust their clocks to the time of their master in the hierarchy. PTP operates within a logical scope called a PTP domain.
PTP Device Types
The following clocks are common PTP devices:
- Ordinary clock
Communicates with the network based on a single physical port, similar to an end host. An ordinary clock can function as a grandmaster clock.
- Boundary clock
Typically has several physical ports, with each port behaving like a port of an ordinary clock. However, each port shares the local clock, and the clock data sets are common to all ports. Each port decides its individual state, either master (synchronizing other ports connected to it) or slave (synchronizing to a downstream port), based on the best clock available to it through all of the other ports on the boundary clock. Messages related to synchronization and establishing the master-slave hierarchy terminate in the protocol engine of a boundary clock and are not forwarded.
- Transparent clock
Forwards all PTP messages like an ordinary switch or router but measures the residence time of a packet in the switch (the time that the packet takes to traverse the transparent clock) and in some cases the link delay of the ingress port for the packet. The ports have no state because the transparent clock does not need to synchronize to the grandmaster clock.
There are two kinds of transparent clocks:
- End-to-end transparent clock
Measures the residence time of a PTP message and accumulates the times in the correction field of the PTP message or an associated follow-up message.
- Peer-to-peer transparent clock
Measures the residence time of a PTP message and computes the link delay between each port and a similarly equipped port on another node that shares the link. For a packet, this incoming link delay is added to the residence time in the correction field of the PTP message or an associated follow-up message.
Note
PTP operates only in boundary clock mode. End-to-end transparent clock and peer-to-peer transparent clock modes are not supported.
PTP Process
The PTP process consists of two phases: establishing the master-slave hierarchy and synchronizing the clocks.
Within a PTP domain, each port of an ordinary or boundary clock follows this process to determine its state:
- Examines the contents of all received announce messages (issued by ports in the master state)
- Compares the data sets of the foreign master (in the announce message) and the local clock for priority, clock class, accuracy, and so on
- Based on this comparison, determines its own state as either master or slave
After the master-slave hierarchy has been established, the clocks are synchronized as follows:
- The master sends a synchronization message to the slave and notes the time it was sent.
- The slave receives the synchronization message and notes the time it was received.
- The slave sends a delay-request message to the master and notes the time it was sent.
- The master receives the delay-request message and notes the time it was received.
- The master sends a delay-response message to the slave.
- The slave uses these timestamps to adjust its clock to the time of its master.
Licensing Requirements for PTP
PTP requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.
Guidelines and Limitations for PTP
PTP has the following configuration guidelines and limitations:
- PTP operates only in boundary clock mode. End-to-end transparent clock and peer-to-peer transparent clock modes are not supported.
- PTP supports transport over User Datagram Protocol (UDP). Transport over Ethernet is not supported.
- PTP supports only multicast communication. Negotiated unicast communication is not supported.
- PTP is limited to a single domain per network.
- All management messages are forwarded on ports on which PTP is enabled. Handling management messages is not supported.
- PTP-capable ports do not identify PTP packets and do not time-stamp or redirect those packets unless you enable PTP on those ports.
- Nexus 3000 should be synced from the neighboring master using a Sync log interval ranging from -3 to -5.
Default Settings
The following table lists the default settings for PTP parameters:
Table 1 Default PTP Parameters Parameters Default PTP
Disabled
PTP domain
0
PTP priority 1 value when advertising the clock
255
PTP priority 2 value when advertising the clock
255
PTP announce interval
1 log second
PTP sync interval
-2 log seconds
PTP announce timeout
3 announce intervals
PTP minimum delay request interval
0 log seconds
PTP VLAN
1
Configuring PTP
Configuring PTP Globally
SUMMARY STEPSYou can enable or disable PTP globally on a device. You can also configure various PTP clock parameters to help determine which clock in the network has the highest priority to be selected as the grandmaster.
1. config t
2. [no] feature ptp
3. [no] ptp source ip-address [vrf vrf]
4. (Optional) [no] ptp domain number
5. (Optional) [no] ptp priority1 value
6. (Optional) [no] ptp priority2 value
7. (Optional) show ptp brief
8. (Optional) show ptp clock
9. (Optional) copy running-config startup-config
DETAILED STEPS
This example shows how to configure PTP globally on the device, specify the source IP address for PTP communications, and configure a preference level for the clock:
switch# config t switch(config)# feature ptp switch(config)# ptp source 10.10.10.1 switch(config)# ptp priority1 1 switch(config)# ptp priority2 1 switch(config)# show ptp brief PTP port status ----------------------- Port State ------- -------------- switch(config)# show ptp clock PTP Device Type: Boundary clock Clock Identity : 0:22:55:ff:ff:79:a4:c1 Clock Domain: 0 Number of PTP ports: 0 Priority1 : 1 Priority2 : 1 Clock Quality: Class : 248 Accuracy : 254 Offset (log variance) : 65535 Offset From Master : 0 Mean Path Delay : 0 Steps removed : 0 Local clock time:Sun Jul 3 14:13:24 2011 switch(config)#Configuring PTP on an Interface
After you globally enable PTP, it is not enabled on all supported interfaces by default. You must enabled PTP on individual interfaces.
Before You BeginSUMMARY STEPSMake sure that you have globally enabled PTP on the switch and configured the source IP address for PTP communication.
1. config t
2. interface ethernet slot/port
3. [no] ptp
4. (Optional) [no] ptp announce {interval log seconds | timeout count}
5. (Optional) [no] ptp delay request minimum interval log seconds
6. (Optional) [no] ptp sync interval log seconds
7. (Optional) [no] ptp vlan vlan-id
8. (Optional) show ptp brief
9. (Optional) show ptp port interface interface slot/port
10. (Optional) copy running-config startup-config
DETAILED STEPS
This example shows how to configure PTP on an interface and configure the intervals for the announce, delay-request, and synchronization messages:
switch# config t switch(config)# interface ethernet 2/1 switch(config-if)# ptp switch(config-if)# ptp announce interval 3 switch(config-if)# ptp announce timeout 2 switch(config-if)# ptp delay-request minimum interval 4 switch(config-if)# ptp sync interval -1 switch(config-if)# show ptp brief PTP port status ----------------------- Port State ------- -------------- Eth2/1 Master switch(config-if)# show ptp port interface ethernet 2/1 PTP Port Dataset: Eth2/1 Port identity: clock identity: 0:22:55:ff:ff:79:a4:c1 Port identity: port number: 1028 PTP version: 2 Port state: Master Delay request interval(log mean): 4 Announce receipt time out: 2 Peer mean path delay: 0 Announce interval(log mean): 3 Sync interval(log mean): -1 Delay Mechanism: End to End Peer delay request interval(log mean): 0 switch(config-if)#Verifying the PTP Configuration
To display the PTP configuration, perform one of the following show commands:
Table 2 PTP Show Commands Command Purpose show ptp brief Displays the PTP status.
show ptp clock Displays the properties of the local clock.
show ptp clocks foreign-masters-record Displays the state of foreign masters known to the PTP process. For each foreign master, the output displays the clock identity, basic clock properties, and whether the clock is being used as a grandmaster.
show ptp corrections Displays the last few PTP corrections.
show ptp parent Displays the properties of the PTP parent.
show ptp port interface ethernet slot/port Displays the status of the PTP port.
show ptp clocks foreign-masters-record interface ethernet slot/port Displays the properties of the PTP clock.
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