A customer experienced persistent communication issues between their newly installed Mitel voice server and existing Cisco core switches. Despite involvement from both Mitel and Cisco engineers, the problem remained unresolved with each vendor attributing the fault to the other's equipment. Our team was called in to diagnose and rectify the issue. Through meticulous analysis, we identified mismatched ARP and MTU settings as the root cause. By aligning these parameters, we successfully established seamless communication between the Mitel system and Cisco switches, restoring normal operations and eliminating the need for unnecessary hardware replacements or upgrades.