Jumbo Frames; Enabling Jumbo Frames; Path Mtu Discovery - Extreme Networks ExtremeWare XOS Guide Manual

Configuring Slots and Ports on a Switch
Table 10: Support for Autonegotiation on Various Ports
PHY
1 G fiber
1 G copper at 1000 Mbps
1 G copper at 10/100
Mbps

Jumbo Frames

Jumbo frames are Ethernet frames that are larger than 1522 bytes, including four bytes used for the cyclic
redundancy check (CRC). Extreme products support switching and routing of jumbo frames at
wire-speed on all ports.
Jumbo frames are used between endstations that support larger frame sizes for more efficient transfers
of bulk data. Both endstations involved in the transfer must be capable of supporting jumbo frames.
The switch only performs IP fragmentation, or participates in maximum transmission unit (MTU)
negotiation on behalf of devices that support jumbo frames.

Enabling Jumbo Frames

To enable jumbo frame support, enable jumbo frames on the desired ports. To set the maximum jumbo
frame size, use the following command:
configure jumbo-frame size <number>
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.
Next, enable support on the physical ports that will carry jumbo frames using the following command:
enable jumbo-frame ports [<port_list> | all]
NOTE
Some network interface cards (NICs) have a configured maximum MTU size that does not include the
additional 4 bytes of CRC. Ensure that the NIC maximum MTU size is at or below the maximum MTU
size configured on the switch. Frames that are larger than the MTU size configured on the switch are
dropped at the ingress port.

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:
56
Autonegotiation
On/Off
Not configurable
On/Off
Speed
1 G
1 G
10/100 Mbps
Duplex
Not configurable; Full
duplex
Not configurable
Full/Half duplex
ExtremeWare XOS 10.1 Concepts Guide