A financial services firm is deploying a SIP Server High Availability (HA) pair across two geographically separate data centers connected by a low-latency MPLS link. During a disaster recovery test, a full network partition occurs between the data centers. Both SIP Server instances, configured as a primary/backup pair, incorrectly assume the 'primary' role, leading to a split-brain scenario. Which configuration parameter is most likely misconfigured, preventing the backup server from correctly detecting the primary's status during the partition?