Domain ID is one of the first parameter you come across when configuring Cisco Virtual Switching Systems (VSS). At first, it might appear that which value you choose is insignificant and you might even be able to use the same number among different VSS switch pairs. This is true as long as those VSS systems are not connected, whether directly or being Layer-2 adjacent, or the virtual MAC address is not implemented. Otherwise, you might find yourself having connectivity issue in your VSS implementation. In this article, we reveal the cause of the issue when connecting two VSS systems that have identical domain ID and offer various options to rectify. Here, we assume that you have basic knowledge of Cisco VSS.