GRE packet structure and header format
FIGURE 13 GRE encapsulated packet structure
FIGURE 14 GRE header format
The GRE header has the following elds:
• Checksum - 1 bit. This eld is assumed to be zero in this version. If set to 1, this means that the Checksum (optional) and
Reserved (optional) elds are present and the Checksum (optional) eld 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 eld is assumed to be zero in this
version.
• Ver - 3 bits. The GRE protocol version. This eld must be set to zero in this version.
• Protocol Type - 16 bits. The Ethernet protocol type of the packet, as dened in RFC 1700.
• Checksum (optional) - 16 bits. This eld is optional. It contains the IP checksum of the GRE header and the payload packet.
• Reserved (optional) - 16 bits. This eld 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
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04 103
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