The possible causes are:
The optical fiber is connected incorrectly.
Check that the optical module matches the optical fiber. For example, the multi-mode optical module does not match the single-mode optical fiber.
Check that the optical fiber is correctly connected to the ATM interface. There should be two optical fibers. One sends data and the other receives data. They must be connected correctly; otherwise, the ATM interface cannot be Up.
The clock mode of the interface is set incorrectly.
When two routers are directly connected, check that the internal clock is enabled on one of ATM interfaces. By default, the router uses the line clock. When the routers are directly connected or connected by the WDM device, the internal clock should be enabled on one of the routers through the clock master command.
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