Question
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How to add a route that will use a certain Maximum Transmission Unit size (MTU)
Answer
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To add a route with a different mtu value other than interfaces default mtu value you will need to run the “route add” command. You can not add the route via smit or smitty.

Add a network specific route with mtu of 1500.
#route add -net <destination_network> <gateway_address> -mtu 1500

Example: Add route to 192.18.5.0 network using gateway address of 192.19.88.1 with an mtu of 1500.
#route add 192.18.5.0 192.19.88.1 -mtu 1500

Add a host specific route with mtu of 1500.
#route add -host <destination_address> <gateway_address> -mtu 1500

Example: Add a route to 10.55.44.22 host using the gateway of 192.88.5.1 with the mtu of 1500.
#route add -host 10.55.44.22 192.88.5.1 -mtu 1500

To display newly added route:

#pmtu display

Example:
dst gw If pmtu refcnt redisc_t exp
192.18.5.0 192.19.88.1 en0 1500
10.55.44.22 192.88.5.1 en1 1500

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