By default, the maximum transmission unit
(MTU) size for packets sent on a Layer 3 interface is 1500 bytes.
This size can be manually set to any size from 512 to 1500 bytes
on a per-interface basis. Some configurations require Ethernet frames
with an MTU value greater than 1500 bytes. These are called jumbo
frames.
To use jumbo frames on a firewall you must specifically
enable jumbo frames at the global level. When this is enabled, the
default MTU size for all Layer 3 interfaces is set to a value of
9192 bytes. This default value can then be set to any value in the
range of 512 to 9216 bytes.
After setting a global jumbo frame
size it becomes the default value for all Layer 3 interfaces that have
not explicitly had an MTU value set at the interface configuration
level. This can become a problem if you only want to exchange jumbo
frames on some interfaces. In these situations, you must set the
MTU value at every Layer 3 interface that you do not want to use
the default value.
The following procedure describes how to
enable jumbo frames on a firewall, set the default MTU value for
all Layer 3 interfaces and to then set a different value for a specific
interface.
