Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 - Manual (Page 114)

96 Brocade ICX 6650 Layer 3 Routing Configuration Guide
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IPv4 point-to-point GRE tunnels
FIGURE 14 GRE encapsulated packet structure
Figure 15 shows the GRE header format.
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.
Delivery Header
GRE Header
Payload Packet
Checksum Ver Protocol Type Reserved0 Checksum
(optional)
Reserved
(optional)