Brocade Communications Systems ICX 7250 series, Configuration Manual

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FIGURE 8 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
RFC 1191 describes a method for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. When a
FastIron device receives an IP packet that has its Do not Fragment (DF) bit set, and the packet size is greater than the MTU value of the
outbound interface, then the FastIron device returns an ICMP Destination Unreachable message to the source of the packet, with the
code indicating "fragmentation needed and DF set". The ICMP Destination Unreachable message includes the MTU of the outbound
interface. The source host can use this information to help determine the minimum MTU of a path to a destination.
RFC 4459 describes solutions for issues with large packets over a tunnel. The following methods, from RFC 4459, are supported in
Brocade IronWare software:
• If a source attempts to send packets that are larger than the lowest MTU value along the path, Path MTU Discovery (PMTUD)
can signal to the source to send smaller packets. This method is described in Section 3.2 of RFC 4459.
• Inner packets can be fragmented before encapsulation, in such a manner that the encapsulated packet fits in the tunnel path
MTU, which is discovered using PMTUD. This method is described in Section 3.4 of RFC 4459.
By default, PMTUD is enabled.
IPv4 point-to-point GRE tunnels
Brocade FastIron Layer 3 Routing Configuration Guide
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