Which statement regarding the LSA header information in AS-External LSA is incorrect?

In the context of the LSA (Link State Advertisement) header information in AS-External LSA (Autonomous System External Link State Advertisement), the statement about the Forwarding Address being always valid is indeed incorrect.

The Forwarding Address field in an AS-External LSA serves a crucial purpose in the OSPF (Open Shortest Path First) routing protocol. It is used to indicate the next hop for reaching an external network. However, the field may not always contain a valid address, depending on certain conditions. For example, in cases where the ASBR (Autonomous System Boundary Router) is not acting as the next hop for reaching the destination network, this field can be set to zero or left undefined. Therefore, stating it is always valid misrepresents its function and potential values.

The other components of the AS-External LSA are correctly defined: the Link State ID reflects the network address of the external route, the Advertising Router identifies the ASBR that originated the LSA, and the Net Mask indicates the subnet mask for the network being advertised. These attributes fulfill specific roles in conveying the necessary details about the external route within the OSPF routing domain, helping routers make informed routing decisions.