Frequency Synchronization Timing Concepts
The Cisco IOS XR frequency synchronization infrastructure is used to select between different frequency sources to set the router backplane frequency and time-of-day. There are two important concepts that must be understood with respect to the frequency synchronization implementation.
Sources
A source is a piece of hardware that inputs frequency signals into the system or transmits them out of the system. There are four types of sources:
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Line interfaces. This includes SyncE interfaces.
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Clock interfaces. These are external connectors for connecting other timing signals, such as, GPS, BITS.
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PTP clock. If IEEE 1588 version 2 is configured on the router, a PTP clock may be available to frequency synchronization as a source of the time-of-day and frequency.
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Internal oscillator. This is a free-running internal oscillator chip.
Each timing source has a Quality Level (QL) associated with it which gives the accuracy of the clock. This QL information is transmitted across the network via SSMs over the Ethernet Synchronization Messaging Channel (ESMC) or SSMs contained in the SONET/SDH frames so that devices know the best available source to synchronize to. In order to define a preferred network synchronization flow, and to help prevent timing loops, you can assign priority values to particular timing sources on each router. The combination of QL information and user-assigned priority levels allows each router to choose a timing source to use to clock its SyncE and SONET/SDH interfaces, as described in the ITU standard G.781.
Selection Points
A selection point is any point where a choice is made between several frequency signals, and possibly one or more of them are selected. Selection points form a graph representing the flow of timing signals between the different cards in a router running Cisco IOS XR software. For example, one or multiple selection points select between the different Synchronous Ethernet inputs available on a single line card, and the result of these selection points is forwarded to a selection point on the RSP to select between the selected source from each card.
The input signals to the selection points can be:
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Received directly from a source.
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The output from another selection point on the same card.
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The output from a selection point on a different card.
The output of a selection point can be used in a number of ways:
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Used to drive the signals sent out of a set of sources.
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As input into another selection point on the card.
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As input into a selection point on another card.
Use the show frequency synchronization selection command to see a detailed view of the different selection points within the system.
Restrictions
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The Precision Time Protocol (PTP) session flaps during Route Processor Failover (RPFO).
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