GRE packet structure and header format
FIGURE 13
GRE encapsulated packet structure
FIGURE 14
GRE header format
The GRE header has the following fields:
•
Checksum - 1 bit. This field is assumed to be zero in this version. If set to 1, this means that the Checksum
(optional)
and
Reserved
(optional)
fields are present and the
Checksum (optional)
field contains valid information.
•
Reserved0 - 12 bits. If bits 1 - 5 are non-zero, then a receiver must discard the packet unless RFC 1701 is implemented. Bits
6 - 12 are reserved for future use and must be set to zero in transmitted packets. This field is assumed to be zero in this
version.
•
Ver - 3 bits. The GRE protocol version. This field must be set to zero in this version.
•
Protocol Type - 16 bits. The Ethernet protocol type of the packet, as defined in RFC 1700.
•
Checksum (optional) - 16 bits. This field is optional. It contains the IP checksum of the GRE header and the payload packet.
•
Reserved (optional) - 16 bits. This field is optional. It is reserved for Brocade internal use.
Path MTU Discovery support
Brocade IronWare software supports the following RFCs for handling large packets over a GRE tunnel:
•
RFC 1191, Path MTU Discovery
•
RFC 4459, MTU and Fragmentation Issues with In-the-Network Tunneling
IPv4 point-to-point GRE tunnels
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