What this means is that the synchronization will run based on loops over
the configuration, rather than over the VPP state. The benefit of this
approach is that no additional API calls have to be made after the
initial VPP configuration is read, making pre-flight checks and diffing
more obvious.
Further, make sync_link_mtu() direction-aware; first we will raise the
link_mtu for interfaces that are growing, and after setting all the
children up/down with sync_mtu_direction() we'll finally shrink
sync_link_mtu() where needed.
We can now do an entire reconciliation run with only one API read at the
beginning!
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