nbsCohpmRunningAveCD is read-only and holds a cumulative statistic since the port was last configured (i.e. since boot-up or the last protocol/FEC/preference change) for the average chromatic dispersion (CD) on this coherent optical interface, i.e. how much the fiber link has spread out the optical signal in time due to wavelength-dependent propagation delay; not-supported is encoded as 0x80000000. Unit measurement: 1 ps/nm. Because it accumulates from the port's last configuration event rather than resetting every interval, a sysadmin uses it as a long-run baseline for this optic, and a value that drifts far from what nbsCohpmCurrentAveCD is showing right now can indicate a slow-developing fiber or transceiver issue.