To check on the connection of the ONTAP Mediator, use the snapmirror mediator show command. If the connection status is unreachable and Site B is unable to reach Site A, you will have an output similar to the one below. Follow the steps in the solution to restore connection

Example:

cluster::*> snapmirror mediator show
Mediator Address Peer Cluster     Connection Status Quorum Status
---------------- ---------------- ----------------- -------------
10.237.86.17     C1_cluster       unreachable       true
SnapMirror consistency group relationship status is out of sync.

C2_cluster::*> snapmirror show -expand
Source            Destination Mirror  Relationship   Total             Last
Path        Type  Path        State   Status         Progress  Healthy Updated
----------- ---- ------------ ------- -------------- --------- ------- --------
vs0:/cg/src_cg_1 XDP vs1:/cg/dst_cg_1 Snapmirrored OutOfSync - false   -
vs0:zrto_cg_655724_188a_RW1 XDP vs1:zrto_cg_655755_188c_DP1 Snapmirrored OutOfSync - false -
vs0:zrto_cg_655733_188a_RW2 XDP vs1:zrto_cg_655762_188c_DP2 Snapmirrored OutOfSync - false -
vs0:zrto_cg_655739_188b_RW1 XDP vs1:zrto_cg_655768_188d_DP1 Snapmirrored OutOfSync - false -
vs0:zrto_cg_655748_188b_RW2 XDP vs1:zrto_cg_655776_188d_DP2 Snapmirrored OutOfSync - false -
5 entries were displayed.

Site B cluster is unable to reach Site A.
C2_cluster::*> cluster peer show
Peer Cluster Name         Cluster Serial Number Availability   Authentication
------------------------- --------------------- -------------- --------------
C1_cluster 			  1-80-000011           Unavailable    ok

Solution

Force a failover to enable I/O from Site B and then establish a zero RTO relationship from Site B to Site A. Complete the following steps to perform a forced failover on Site B.

Unmap all LUNs belonging to the consistency group from Site B.
Delete the SnapMirror consistency group relationship using the force option.
Enter the SnapMirror break command (snapmirror break -destination_path svm:_volume_) on the consistency group constituent volumes to convert volumes from DP to RW, to enable I/O from Site B.

You must issue the SnapMirror break command for each relationship in the consistency group. For example, if there are three volumes in the consistency group, you will issue the command for each volume.
Boot up the Site A nodes to create a zero RTO relationship from Site B to Site A.
Release the consistency group with relationship-info-only on Site A to retain common Snapshot copy and unmap the LUNs belonging to the consistency group.
Convert volumes on Site A from RW to DP by setting up a volume level relationship using either Sync policy or Async policy.
Issue the snapmirror resync command to synchronize the relationships.
Delete the SnapMirror relationships with Sync policy on Site A.
Release the SnapMirror relationships with Sync policy using relationship-info-only true on Site B.
Create a consistency group relationship between Site B to Site A.
From the source cluster, resynchronize the consistency group. Verify the consistency group state is in sync.
Rescan the host LUN I/O paths to restore all paths to the LUNs.