Serial Interface - Clock Rate
When configuring a Serial interface on a Cisco router, the clock rate must be configured on the device acting as a DCE side of the link. The clock rate must not be configured on the DTE side of the link, otherwise it can create a fault in the syncing of the link communication.
Links:
https://forum.networklessons.com/t/eigrp-static-neighbor/1244/32?u=lagapidis
https://networklessons.com/frame-relay/cisco-frame-relay-switch-configuration#Configuration