when portsyncd starts, it first enumerates all front panel ports
and marks them as old interfaces. Then, for new front panel ports
it checks if their indexes exist in previous sets. If yes, it will
treats them as old interfaces and ignore them.
The reason we have this check is because broadcom SAI only removes
front panel ports after sai switch init.
So, if portsyncd starts after orchagent, new interfaces could be
created before portsyncd and treated as old interface.
Signed-off-by: Guohan Lu <lguohan@gmail.com>
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