Understanding Clock Synchronization

Clock Selection

Configuring external clock synchronization requires making clock selection, quality level, and priority considerations. The clock selection algorithm is used to pick the two best clock sources—primary and secondary—from among the various sources.

The clock selection algorithm is on the basis of the system configuration and execution criteria such as quality level, priority, hardware restrictions, and so on, and is achieved using the following logic and restrictions:

• The following parameters must be configured irrespective of whether the quality level is enabled or not (You can set the quality level with the set chassis synchronization source interfaces external quality-level quality-level configuration command at the [edit] hierarchy level.):

  • Quality level must be configured for nonexternal clocks.

  • In the case of option-1, the quality level must be configured for the external clocks.

  • In the case of option-2, the default quality level for the external clocks is QL_STU.

  The synchronous Ethernet Equipment Clock (EEC) synchronization networking types option-1 and option-2 map to G.813 option 1 (EEC1) and G.812 type IV clock (EEC1) standards, respectively, and can be configured at the [edit chassis synchronization] hierarchy level.

• When the quality-mode-enable statement is included at the [edit chassis synchronization] hierarchy level, the received quality level must be equal to or better than the configured quality level for that particular source, otherwise that source is not considered for clock selection. This is so that a downstream client is guaranteed clock quality of a certain level. (Note that the term certain level here denotes the configured quality level.)

• Starting with Junos OS Release 12.2R1, configuring the quality level for a Synchronous Ethernet interface is optional when the quality-mode-enable and the selection-mode received-quality statements are included at the [edit chassis synchronization] hierarchy level.

  The default quality level value for a Synchronous Ethernet interface is:

  • SEC for the option-1 network type.

  • ST3 for the option-2 network type.

• Configuring the priority statement is optional. When not specified, the external-a interface has a higher default priority than the external-b interface, and the external-b interface has a higher default priority than Ethernet-based sources such as ge or xe clock sources, which have the lowest default priority.

  Note:

  Configured priority is higher than any default priority.

• During clock selection:

  • The active source with the highest quality level is selected.

  • The configured (or default) quality level of the selected clock source is used for Ethernet Synchronization Message Channel (ESMC). In order to receive or transmit ESMC messages out of an interface, at least one logical interface must be configured on that interface.

  • Table 1 explains a few scenarios that must be taken into consideration during clock selection:

    Table 1: Clock Selection Scenarios

    If	Then
    Two or more sources have the same quality level.	The source with highest priority is selected.
    Two or more sources have the same quality level and priority.	The current active source, if any, among these sources is selected.
    Two or more sources have the same quality level and priority, and none of these is currently active.	Any one of these sources is selected.
    Primary clock source is ge|xe-x/y/z, where y is even (0 or 2).	The secondary clock source cannot be ge|xe-x/y/* or ge|xe-x/y + 1/*. For example, if ge-1/2/3 is the primary clock source, then the secondary clock source cannot be ge-1/2/* or ge-1/3/* for an MX80, MX240, MX480, or an MX960 router.
    Primary clock source is ge|xe-x/y/z, where y is odd (1 or 3).	The secondary clock source cannot be ge|xe-x/y/* or ge|xe-x/y - 1/*. For example, if xe-2/3/4 is the primary clock source, then the secondary clock source cannot be xe-2/2/* or xe-2/3/* for an MX80, MX240, MX480, or an MX960 router.
    Primary clock source is ge|xe-x/y/z.	The secondary clock source cannot be ge|xe-x/y/* in the case of 12-port or 16-port 10-Gigabit Ethernet DPC on an MX Series router. For example, if ge-0/1/2 is the primary clock source, then ge-0/1/* cannot be the secondary clock source, but ge-0/0/* can be the secondary clock source.

Starting with Junos OS Release 14.2, note that on PTX Series routers, you can specify the primary and secondary clock sources provided the clock source meets the necessary qualification as set by the clock algorithm. However, in the absence of any user-selected clock source, the clock source with the best quality level is selected by the clock algorithm in the router. Note that the user selection is honored even when better quality level clock sources are available. You can select the clock source with the request chassis synchronization switch clock-source operational mode command. For more information, see request chassis synchronization switch.

Network Option

The clock type or network option is the synchronous Ethernet Equipment Clock (EEC) synchronization networking type. You can set the network option to one of the following values:

• option-1—This option maps to G.813 option 1 (EEC1).

• option-2—This option maps to G.812 type IV clock (EEC1).

To configure the clock type, execute the set chassis synchronization network-option (option-1 | option-2) configuration command at the [edit] hierarchy level.

Clock Mode

You can set the Synchronous Ethernet clock source to one of the following modes:

• free-run—In this mode, the free-running local oscillator is used as a clock source.

  Note:

  For MX80 routers, the free-run clock is provided by the local oscillator.

  For MX240, MX480, and MX960 routers with an SCB, the free-run clock is provided by the MPCs.

  For MX240, MX480, and MX960 routers with an SCBE or an SCBE2, the free-run clock is provided by the local oscillator.

• auto-select—In this mode, the best external clock source is selected.

By default, the auto-select option is selected.

To configure the clock mode, execute the set chassis synchronization clock-mode (free-run | auto-select) configuration command at the [edit] hierarchy level.

Quality Mode

When the quality-mode-enable statement is included at the [edit chassis synchronization] hierarchy level, the system ascertains that the clock selection algorithm uses both quality and priority of the clock sources to select the best clock source for clock synchronization. When the quality-mode-enable statement is not included, only the priority of the clock source is taken into account by the algorithm.

To enable the synchronization quality mode, include the quality-mode-enable statement at the [edit chassis synchronization] hierarchy level.

Selection Mode

You can specify whether the clock source selection must use the configured or the received ESMC or SSM quality level for a qualifying interface. In both selection modes, the interface qualifies for clock source selection only when the received ESMC or SSM quality level on the interface is equal to or greater than the configured ESMC or SSM quality level for the interface.

The selection modes are:

• configured-quality—In this mode, the clock source selection algorithm uses the ESMC or SSM quality level configured for a qualifying interface.

• received-quality—In this mode, the clock source selection algorithm uses the ESMC or SSM quality level received on the qualifying interface.

To configure the clock source algorithm selection mode, execute the set chassis synchronization selection-mode (configured-quality|received-quality) configuration command at the [edit] hierarchy level.

Hold Interval

You can set the chassis synchronization wait time after a change in configuration, the clock selection wait time after reboot of the router, and the switchover wait time after a switchover of SCB before selecting the new clock source. The hold interval options are:

• configuration-change—In this mode, the wait time for clock selection after a change in configuration (clock synchronization configuration) can be set from 15 seconds through 60 seconds.

• restart—In this mode, the wait time for clock selection after reboot of the router can be set from 60 seconds through 180 seconds.

• switchover—In this mode, the switchover wait time after clock recovery can be set from 30 seconds through 60 seconds.

To set the hold interval, execute the set chassis synchronization hold-interval (configuration-change | restart | switchover) seconds configuration command at the [edit] hierarchy level.

Switchover Mode

You can set the switchover mode to switch the clock from a lower quality source to higher quality source or to use the current clock source only. You can configure the switchover mode to one of the following:

• non-revertive—In this mode, the router uses the current clock source as long as it is valid.

• revertive—In this mode, the router automatically switches from a lower to a higher quality clock source whenever the higher clock source becomes available.

The default mode is revertive mode.

To configure the switching mode, execute the set chassis synchronization switchover-mode (revertive | non-revertive) configuration command at the [edit] hierarchy level.

Clock Source

You can specify the parameters that must be considered by the clock selection algorithm while selecting the best clock source. The parameters include the quality level value, the priority of the clock source, the request criteria, and the wait time to restore the interface signal to up state. You must specify these parameters on the external clock interfaces or other qualifying interfaces—which are connected to valid clock sources—to select the best clock source on the basis of the timing messages that are received on these interfaces.

For an SCBE, you can configure only one external interface and configure multiple Ethernet interfaces as needed.

On SCBE2, you can configure two external interfaces—external-0/0 and external-1/0—and configure multiple Ethernet interfaces as needed.

To configure the clock source, execute the set chassis synchronization source interfaces interface-name configuration command. You can also configure the clock source with the set chassis synchronization source interfaces external at the [edit] hierarchy level, where the external option refers to an external clock interface.

To specify the clock source for an interface, you must set the following options:

• priority—You can set the user priority for the selected clock source from 1 through 5.

  To set the synchronization source priority for the selected clock source, execute the set chassis synchronization source interfaces interface-name priority number configuration command or the set chassis synchronization source interfaces external priority number configuration command at the [edit] hierarchy level.

• request—You can set the clock selection request criterion as one of the following:

  • force-switch—With this option, you can force the SCB to switch to a clock source you prefer on a particular interface (that is you can select a clock source on an interface overriding the algorithm), provided the source is enabled and not locked out. Only one configured source can be force-switched.

  • lockout—With this option configured, the clock source is not to be considered by the selection process. Lockout can be configured for any source.

  To configure these options, execute the set chassis synchronization source interfaces interface-name request (force-switch|lockout) configuration command or the set chassis synchronization source interfaces external request (force-switch|lockout) configuration command at the [edit] hierarchy level.

• wait-to-restore—You can set the wait-to-restore time for each interface. When an interface’s signal transitions out of the signal fail state, it must be fault-free for the wait-to-restore time before it is again considered by the clock selection process. You can configure the interface signal upstate time—wait time before opening the interface to receive ESMC messages—from 0 through 12 minutes. The default time is 5 minutes. When the ESMC clock’s EEC quality level (QL) mode is enabled, it sends a signal failure to the clock selection process during the wait-to-restore time. After the wait-to-restore time ends, a new quality level value is sent to the clock selection process.

  To configure the wait-to-restore time, execute the set chassis synchronization source interfaces interface-name wait-to-restore minutes configuration command or the set chassis synchronization source interfaces external wait-to-restore minutes configuration command at the [edit] hierarchy level.

• hold-off-time—Starting with Junos OS Release 14.2, you can configure hold-off time for Synchronous Ethernet interfaces and external clock source interfaces to prevent rapid successive switching between signal fail states. If an interface goes down, hold-off time delays short signal failures from being sent to the clock selection process.

  Note:

  During the hold-off time period, if the clock synchronization process restarts, hold-off time is not considered.

  If you configure hold-off time when the ESMC clock’s EEC QL mode is enabled, the configured quality level is used in the clock selection process during the hold-off time period. During the hold-off time period, the external clock source appears in a locked state until the hold-off time period ends. After the hold-off time period ends, a signal failure is sent to the clock selection process.

  You can configure hold-off time for a range of 300 through 1800 milliseconds. The default hold-off time is 1000 milliseconds.

  To configure hold-off time, execute the set chassis synchronization source interfaces interface-name hold-off-time configuration command at the [edit] hierarchy level.

  Note:

  When a link goes down and comes back up within the configured hold-off time in a clocking hybrid mode configuration (the combined operation of Synchronous Ethernet and Precision Time Protocol) that includes the protocols ptp slave convert-clock-class-to-quality-level configuration statement at the [edit] hierarchy level, the phase might not get locked before the timer expires. This might result in a degradation of clock quality level.

• quality—You can set the ESMC clock’s EEC quality level as prc, prs, sec, smc, ssu-a, ssu-b, st2, st3, st3e, st4, stu, or tnc. Both option I and option II SSM quality levels are supported. Table 2 explains the quality level values.

  Table 2: Quality Levels

  Quality Level	Description
  prc	Timing quality of a primary reference clock (option-1 only).
  prs	Clock traceable to a primary reference source (option-2 only).
  sec	Timing quality of an SDH equipment clock (option-1 only).
  smc	Clock traceable to a self-timed SONET clock (option-2 only).
  ssu-a	Timing quality of a type I or IV client clock (option-1 only).
  ssu-b	Timing quality of a type VI client clock (option-1 only).
  st2	Clock traceable to Stratum 2 (option-2 only).
  st3	Clock traceable to Stratum 3 (option-2 only).
  st3e	Clock traceable to Stratum 3E (option-2 only).
  st4	Clock traceable to Stratum 4 free-run (option-2 only).
  stu	Clock traceable to an unknown quality (option-2 only).
  tnc	Clock traceable to a transit node clock (option-2 only).

  Note:

  When the quality level is not configured and no ESMC messages are received by the clock source, then the quality level is set to DNU for option-1 and DUS for option-2. You can configure the network options, option-1 and option-2 at the [edit chassis synchronization network-option] hierarchy level.

  To avoid source looping on the selected active source—primary or secondary source, whichever is active—even when ESMC transmit is not enabled, a DNU ESMC message is sent out when the network-option statement is configured as option-1, and a DUS ESMC message is sent out when the network-option statement is configured as option-2. This is applicable only for clock sources configured on the Ethernet interfaces.

  To configure the quality level, execute the set chassis synchronization source interfaces interface-name) quality-level (prc | prs |sec | smc | ssu-a | ssu-b | st2 | st3 | st3e | st4 | stu | tnc) configuration command or the set chassis synchronization source interfaces external quality-level (prc | prs |sec | smc | ssu-a | ssu-b | st2 | st3 | st3e | st4 | stu | tnc) configuration command at the [edit] hierarchy level.

ESMC Packet Transmit

You can enable all the interfaces or configure one or more qualifying interfaces on which to permit ESMC transmit messages by executing the set chassis synchronization esmc-transmit interfaces (all |interface-name) configuration command at the [edit] hierarchy level.

Global Wait To Restore

Starting with Junos OS Release 14.2, you can globally configure the time in minutes for source ports to be up before opening the Ethernet Synchronization Message Channel (ESMC) for messages. When a port’s signal transitions out of the signal fail state, it must be fault-free for the global wait-to-restore time before it is again considered by the clock selection process.

To configure the global wait-to-restore time, include the global-wait-to-restore statement at the [edit chassis synchronization] hierarchy level.

To override the global wait-to-restore time on a specific interface, include the wait-to-restore statement at the [edit chassis source interfaces (external-a | external-b | interface interface-name)] hierarchy level.

Maximum Transmit Quality Level

To configure the maximum transmit quality level for SCBE2 as prc, prs, sec, smc, ssu-a, ssu-b, st2, st3, st3e, st4, stu, or tnc, execute the set chassis synchronization max-transmit-quality-level quality-level configuration command at the [edit] hierarchy level.

Interfaces with Upstream Clock Source

You can configure the external interface to operate with a connected router for a clock source. This external interface can be configured for a clock source, which then becomes a candidate for selection as the chassis clock source by the clock source selection algorithm. You can configure several options for the external clock source interface on the SCBE and for the two external clock source interfaces on the SCBE2.

The options include E1 interface options, pulse-per-second option, the signal type for the provided reference clocks, and the T1 interface options at the [edit chassis synchronization interfaces external] hierarchy level.

The following sections explain the clock source interface parameters in detail:

• E1 Interface Options
• Pulse Per Second
• Signal Type
• T1 Interface Options

External Output Interface

You can set several options for the external clock output interface for SCBE or for the two external clock output interfaces for SCBE2.

The options include disabling the holdover mode; configuring a minimum quality threshold; configuring a mode to select a clock source; configuring the transmit quality level to DNU or DUS; and disabling wander filtering at the [edit chassis synchronization output interfaces external] hierarchy level for SCBE or at the [edit chassis synchronization output interfaces (external0-0 | external-1/0)] hierarchy level for SCBE2.

The following sections explain the external output interface parameters in detail:

• Holdover Mode
• Minimum Quality
• Source Mode
• Transmit Quality Level
• Wander Filter

Clock Synchronization Ports

Starting with Junos OS Release 13.3, you can set the time-of-day-format statement as an ASCII string on SCBE and SCBE2 by executing the set chassis synchronization port auxiliary client time-of-day-format ascii string configuration command at the [edit] hierarchy level.

The time of day (TOD) format is specified as a string of ASCII characters. The TOD format string contains information that specifies which ASCII characters to match, which ASCII characters to ignore, and which ASCII characters to translate to particular time units (such as month, day, hour, minute, and so on).

The TOD format string specifies how the incoming string is to be parsed so that the information embedded can be extracted. The format of the TOD option can be executed with the set chassis synchronization port auxiliary time-of-day-format ascii string configuration command at the [edit] hierarchy level, where the format of the data string is $GPRMC,%hh%mm%ss,^,^^^^.^^,^,^^^^^^^^,^,^^^^^,^^^^^,%DD%MM%YY,^^^^^,^*^^.

Table 3 explains pattern-matching characters used in the TOD data string.

There are several patterns that can be received by a router. The following pattern shows an example of a received TOD data string (as defined in the National Marine Electronics Association (NMEA) 0183 standard. The data string is called the Recommended Minimum Specific GPS/Transit Data (RMC) message.) and Table 4 explains it in detail.

MIC-Level Framing Mode

You can configure the LAN framing mode on the 10-Gigabit Ethernet MIC with XFP by executing the set chassis fpc fpc-slot pic pic-slot framing lan at the [edit] hierarchy level.

Note that to operate in LAN framing mode on the 10-Gigabit Ethernet MIC with XFP, you must configure the interface framing mode on the MIC interface. Execute the set interfaces xe-fpc/pic/port framing-mode (lan-phy | wan-phy) configuration command at the [edit] hierarchy level, where the lan-phy option denotes a 802.3ae 10-Gbps LAN-mode interface and the wan-phy option denotes a 802.3ae 10-Gbps WAN-mode interface.