How much overhead does the GRE header add to an IPv4 packet?

The correct answer indicates that the GRE (Generic Routing Encapsulation) header adds 24 bytes of overhead to an IPv4 packet. The GRE header itself consists of several fields that provide important control information for the encapsulated packets. Specifically, the GRE header is 4 bytes long, and when additional options are included, such as the GRE Key or Sequence Number, they can add up to a total of 24 bytes.

When a GRE packet is encapsulated in an IPv4 packet, the structure includes not just the GRE header but also the required IPv4 headers. Therefore, the 24 bytes consist of the GRE header (which is foundational to the operation of GRE for encapsulating packets) plus the overhead that may arise due to encapsulation requirements and added fields.

This correct understanding combines how GRE operates and its role in providing a tunneling mechanism for various protocols, which makes this choice valid and crucial for those studying networking concepts specifically related to routing and switching.