Jumbo Frames Example; Path Mtu Discovery; Ip Fragmentation With Jumbo Frames - Extreme Networks ExtremeWare 7.2e Installation And User Manual

Configuring Ports

Jumbo Frames Example

The following example create two VLANs sw1 and sw2. It adds port 12 to sw1 and port 13 to sw2. It
configures port 12 and 13 for jumbo frames up to 9216 bytes (including CRC). It also configures VLANs
sw1 and sw2 to accept IP packets up to 9194 bytes.
* Summit400-48t:48 # create vlan sw1
* Summit400-48t:49 # create vlan sw2
* Summit400-48t:50 # configure vlan sw1 add port 12
* Summit400-48t:51 # configure vlan sw2 add port 13
* Summit400-48t:52 # configure jumbo-frame size 9216
* Summit400-48t:53 # enable jumbo-frame ports 12,13
* Summit400-48t:54 # configure ip-mtu 9194 vlan vlansw1
* Summit400-48t:55 # configure ip-mtu 9194 vlan vlansw2

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

ExtremeWare 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, follow these steps:
1 Enable jumbo frames on the incoming port.
2 Add the port to a VLAN.
84 ExtremeWare 7.2e Installation and User Guide