Configuration Guide User guide

FastIron Configuration Guide 1045
53-1002494-02
IPv4 point-to-point GRE tunnels
• 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.
• On FCX devices, only eight different MTU values can be configured over the whole system.
When the SX-FI48GPP module is installed in the FastIron SX device, the maximum number of
different MTU values that can be configured is 16.
• On both FCX devices, and the SX-FI-24GPP, SX-FI48GPP, SX-FI-24HF, SX-FI-2XG, and SX-FI-8XG
modules, PMTUD will not be enabled on the device if the maximum number of MTU values has
already been configured in the system.
• When a new PMTUD value is discovered, and the maximum number of different MTU
values for the system is already configured , the new value will search for the nearest, but
smallest MTU value relative to its own value in the system. For example, in a FCX system,
the new PMTUD value is 800, and the eight different MTU values configured in the system
are 600, 700, 820, 1000, 1100, 1200, 1300, and 1500. The range of MTU values that
can be configured is from 576 through 1500. The new PMTUD value 800 cannot be added
to the system so the nearest, but smallest MTU value is used. In this example, the MTU
value of 700 is considered as the nearest MTU value already configured in the system.