Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 113)

FIGURE 15 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 (PMTUD) 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,
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.
Configuration considerations for PMTUD support
Consider the following when configuring PMTUD support.
Path MTU Discovery (PMTUD) support
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