Extreme Networks ExtremeWare XOS 10.1, Manual

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Jumbo Frames
ExtremeWare XOS 10.1 Concepts Guide 57
•
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 ipforwarding on the VLAN.
IP Fragmentation within a VLAN
ExtremeWare XOS supports IP fragmentation within a VLAN. This feature does not require you to
configure the MTU size. To use IP fragmentation within a VLAN:
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.
If you leave the MTU size configured to the default value, when you enable jumbo frame support on a
port on the VLAN you will receive a warning that the ip-mtu size for the VLAN is not set at maximum
jumbo frame size. You can ignore this warning if you want IP fragmentation within the VLAN, only.
However, if you do not use jumbo frames, IP fragmentation can only be used for traffic that stays
within the same VLAN. For traffic that is set to other VLANs, to use IP fragmentation, all ports in the
VLAN must be configured for jumbo frame support.