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External Clock Synchronization Overview for ACX Series Routers

The ACX Series Universal Metro routers support external clock synchronization and automatic clock selection for Synchronous Ethernet, T1 or E1 line timing sources, and external inputs. Configuring external clock synchronization and automatic clock selection requires making clock selection, quality level (QL), and priority considerations. The clock source selection algorithm is used to pick the two best upstream clock sources from among all the various sources, based on system configuration and execution criteria such as QL, priority, and hardware restrictions.

Automatic Clock Selection

With automatic clock selection, the system chooses up to two best upstream clock sources. The system then uses the clock recovered from one of the sources to lock the chassis clock. If an upstream clock with acceptable good quality is not available or if the system is configured in free-run mode, the system uses the internal oscillator. The following automatic clock selection features are supported for Synchronous Ethernet, T1 or E1 line timing sources, and external inputs:

Note:

Automatic clock selection does not apply to the IEEE 1588v2 recovered clock.

Automatic clock selection is supported on the ACX Series routers. Automatic clock selection of the best quality clock source is based on the Ethernet Synchronization Message Channel (ESMC) Synchronization Status Message (SSM) quality level, the configured quality level, and the priority. To configure automatic clock selection, include the auto-select option at the [edit chassis synchronization] hierarchy level. You can also configure the chassis to lock to the free-running local oscillator, which is the Stratum 3E oscillator, by including the free-run option at the [edit chassis synchronization] hierarchy level. The auto-select option enables the clock source selection algorithm to run. The clock source selection algorithm is triggered by the following events:

  • Signal failure detected on the currently selected source

  • Changes in the received Ethernet Synchronization Message Channel (ESMC) Synchronization Status Message (SSM) quality level (QL)

  • Configuration changes. For example, the addition or deletion of a clock source, a change to the QL mode, and so on.

Automatic clock selection supports two modes on the ACX Series router: QL enabled and QL disabled. To configure QL mode, include the quality-mode-enable statement at the [edit chassis synchronization] hierarchy level.

  • QL disabled—The default setting is disable, which means that when the quality-mode-enable statement is not configured, QL is disabled. In this mode, the best clock is selected based on the configured ESMC SSM QL. If the QL of the best clocks are equal, the clock selection is based on the configured priority. If both the configured QL and priority are equal, one of the sources is randomly selected.

  • QL enabled—In this mode, the best clock is selected based on the incoming ESMC SSM QL as long as the incoming QL is at least as good as the source’s configured QL. If the QLs are equal, the clock selection is based on the configured priority. If both the received QL and the priority are equal, one of the sources is selected randomly.

Clock Source Selection Algorithm

The clock source selection algorithm uses the following logic and restrictions:

  • QL must be configured for non-external clocks, whether or not QL is enabled.

  • For network-option option-1, QL must be configured for external clocks (gps or bits) whether or not QL is enabled.

  • In the case of network-option option-2, the default QL for the external clocks is QL_STU, whether or not QL is enabled.

  • Configuring priority is optional. When not specified, gps has a higher default priority than bits, and bits has a higher default priority than Gigabit Ethernet, 10-Gigabit Ethernet, and T1 or E1 clock, which have the lowest default priority.

  • When QL is enabled, the received QL must be equal to or better than the configured QL for that particular source or else that source will not be considered for clock selection. This is so that a downstream client is guaranteed clock quality of a certain level (that “certain level” being the configured QL).

During clock selection:

  • The active source with the highest QL is selected.

  • If QL is the same for two or more sources, then the source with the highest priority is selected.

  • If two or more sources have the same QL and priority, then the currently active source, if any, among these sources is selected.

  • If two or more sources have the same QL and priority, and none of these is currently active, then any one of these may be selected.

  • If selection-mode is configured quality, then the configured (or default) QL of the selected clock source is used for transmitting ESMC. If selection-mode is received quality, then the received QL of the selected clock source is used for ESMC transmit.

  • In order to receive or transmit ESMC messages out of an interface, at least one logical interface should be configured on that interface. If the interface is currently not configured with a logical interface, you may do so using the set interfaces interface-name unit 0 statement at the edit hierarchy level.

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