Extreme Networks ExtremeWare XOS 11.1, Руководство пользователя

Страница: 85 / 496

Jumbo Frames
ExtremeWare XOS 11.1 Concepts Guide
85
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 Aspen 8810 switch does not support fragmentation of any IP packets it forwards.
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 ipforwarding 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 ranges between 1500 and 9216, with 1500 the default.
NOTE
To set the MTU size greater than 1500, all ports in the VLAN must have jumbo frames enabled.