Which of the following protocol data packets can be encapsulated in a VPN using GRE?

GRE is a multiprotocol encapsulation mechanism and can carry all the listed packet types. It inserts a GRE header around the original payload and then places the resulting GRE packet inside a delivery-protocol packet. Because the GRE header contains a Protocol Type field identifying the encapsulated payload, GRE is not restricted to ordinary IPv4 unicast traffic.

IPv6 packets can be transported as GRE payloads when supported by the tunnel endpoints. IP unicast traffic is the most common use case. GRE can also carry IP multicast and broadcast packets, which is one of its major advantages over basic IPsec tunnel selectors that traditionally focus on IP unicast traffic. This enables routing protocols, multicast applications, discovery traffic, and other non-unicast services to operate across a logical point-to-point tunnel.

RFC 2784 defines GRE as a general mechanism for encapsulating an arbitrary network-layer protocol over another network-layer protocol. It also defines the Protocol Type field used to identify the carried payload. Huawei uses GRE as an SD-WAN overlay data-channel option and can additionally secure it using IPsec when confidentiality and integrity are required.

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