Specifications

Jumbo Frames
ExtremeWare XOS 11.0 Concepts Guide 73
Path MTU Discovery
Using path MTU discovery, a source host assumes that the path MTU is the MTU of the first hop
(which is known). The host sends all datagrams on that path with the “don’t fragment” (DF) bit set,
which restricts fragmentation.
If any of the datagrams must be fragmented by an Extreme Networks switch along the path, the
Extreme switch discards the datagrams and returns an ICMP Destination Unreachable message to the
sending host, with a code meaning “fragmentation needed and DF set”. When the source host receives
the message (sometimes called a “Datagram Too Big” message), the source host reduces its assumed
path MTU and retransmits the datagrams.
The path MTU discovery process ends when one of the following is true:
• The source host sets the path MTU low enough that its datagrams can be delivered without
fragmentation.
• The source host does not set the DF bit in the datagram headers.
If it is willing to have datagrams fragmented, a source host can choose not to set the DF bit in datagram
headers. Normally, the host continues to set DF in all datagrams, so that if the route changes and the
new path MTU is lower, the host can perform path MTU discovery again.
IP Fragmentation with Jumbo Frames
ExtremeWare XOS supports the fragmenting of IP packets. If an IP packet originates in a local network
that allows large packets and those packets traverse a network that limits packets to a smaller size, the
packets are fragmented instead of discarded.
This feature is designed to be used in conjunction with jumbo frames. Frames that are fragmented are
not processed at wire speed within the switch fabric.
NOTE
Jumbo frame-to-jumbo frame fragmentation is not supported. Only jumbo frame-to-normal frame
fragmentation is supported.
To configure VLANs for IP fragmentation:
1 Enable jumbo frames on the incoming port.
2 Add the port to a VLAN.
3 Assign an IP address to the VLAN.
4 Enable IP forwarding on the VLAN.
5 Set the MTU size for the VLAN, using the following command:
configure ip-mtu <mtu> vlan <vlan_name>
The ip-mtu value can be 1500, 9216, or 9194, with 1500 the default.
NOTE
To set the MTU size greater than 1500, all ports in the VLAN must have jumbo frames enabled.