We are experiencing a problem that users on remote sites can't logon on
their workstations when link between our NW servers are down.

The scenary:

HeadQuarter - HQ
BranchOffice - BO - We have 6 Bo's.

SERVERHQ1 is on HQ and has has the Master Replica for the root, for the
partition on the HQ, has the Master Replicas for all Partitions on BO1 to
BO5 and the Read-Write Replica for partition on BO6.

SERVERHQ2 is on headquarter and has the Read Write for the Partition on the
headquarter, Read Rite replica for the root and sub-reference replica for
all other BO1 to BO6 partitions.

Each SERVER on BranchOffices has the Read Write replica for it's partition,
except on BO6.

Server on BO6 has the Master replica for it's partition.

Even on BO6 that has it's Master Replica, when link is down between the BO
and HQ they can't log on to their workstations.

How can I investigate what's happening?
Thankyou in advance.