Because servers are considered IPv6 hosts, by default their IPv6 addresses are automatically configured by the Neighbor Discovery protocol. However, many servers have multiple network interface cards (NICs) that you might want to swap out for maintenance or replacement. When you replace one NIC, Neighbor Discovery automatically generates a new interface ID for that NIC. This behavior might not be acceptable for a particular server.
Therefore, consider manually configuring the interface ID portion of the IPv6 addresses for each interface of the server. For instructions, refer to How to Configure a User-Specified IPv6 Token. Later, when you need to replace an existing NIC, the already configured IPv6 address is applied to the replacement NIC.