Extreme Networks ExtremeWare XOS 11.3, Manual

Page: 117 / 698

Jumbo Frames
ExtremeWare XOS 11.3 Concepts Guide
117
configure jumbo-frame-size <framesize>
The jumbo frame size range is 1523 to 9216. This value describes the maximum size of the frame in
transit (on the wire), and includes 4 bytes of CRC plus another 4 bytes if 802.1Q tagging is being used.
Set the MTU size for the VLAN, using the following command:
configure ip-mtu <mtu> vlan <vlan_name>
Next, enable support on the physical ports that will carry jumbo frames using the following command:
enable jumbo-frame ports [all | <port_list>]
NOTE
The BlackDiamond 8800 family of switches and the Summit X450 switch enable or disable jumbo frames on the
entire switch; the system returns an error message if you attempt to enable jumbo frames on specified ports.
Path MTU Discovery
NOTE
The BlackDiamond 8800 family of switches and the Summit X450 switch do not support the router specification for
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 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
NOTE
The BlackDiamond 8800 family of switches and the Summit X450 switch do not support fragmentation of any IP
packets they forward.