Prepare the Device for Operation
Chapter 3. Configuration
30
When the HA4000 detects an IP payload that exceeds 1460 bytes, the HA4000
notifies the local device of the required MTU size. Note that the PMTU is a
Layer 3-based number, and, therefore, does not include Layer 2 Ethernet
header overhead.
PMTU Modes
The HA4000 has two PMTU modes:
z
Normal mode PMTU is 128 through 2944 bytes.
z
Jumbo mode PMTU is 2945 through 12,160 bytes.
When the PMTU is changed from a normal mode value to a jumbo mode value or
vice versa, reboot the HA4000 for the
pmtu
command to take effect.
Changing the PMTU in the normal mode or jumbo mode range does not require a
reboot. Save the configuration prior to rebooting.
Note:
Jumbo frame processing decreases performance by approximately five
percent. To maximize performance and jumbo frame handling is not required, set
the PMTU to 2944 or less.
Configure the PMTU
At the
config>
prompt, enter this command:
pmtu
<
size_in_bytes
>
For
size-in-bytes
, type a number from 128 through 12,160 bytes.
Example
These commands set the total PMTU size to 1500, save the configuration, and
reboot the HA4000.
config>
pmtu 1500
config>
exit
admin>
copy system:running nvram:config
admin>
reboot
Configure DF Bit Handling
The Don’t Fragment (DF) bit command determines whether packet fragmentation
is allowed over a particular network link. By default, the DF bit is copied from the
original packet to the encapsulating header, and ICMP PMTU messages are
processed.
In most cases, copying the DF bit is appropriate. When fragmenting is desirable,
such as when sending packets over a network with a very small MTU, enter the
dfbit-handling command
with
the
clear
attribute. Use the
set
attribute to
prevent fragmentation and obtain feedback from downstream routers about PMTU
constraints that require fragmentation.
