nbsCohpmRunningMaxDGD 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 maximum differential group delay (DGD) on this coherent optical interface, i.e. how much delay skew polarization-mode dispersion has introduced between the two polarization axes of the signal; not-supported is encoded as 0xFFFFFFFF. Unit measurement: 1 ps. 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 nbsCohpmCurrentMaxDGD is showing right now can indicate a slow-developing fiber or transceiver issue.