не совсем понятно

pre-up command Run command before bringing the interface up. If this command fails then ifup aborts, refraining from marking the interface as config- ured, prints an error message, and exits with status 0. This behav- ior may change in the future.

up command

post-up command Run command after bringing the interface up. If this command fails then ifup aborts, refraining from marking the interface as config- ured (even though it has really been configured), prints an error message, and exits with status 0. This behavior may change in the future.

down command

  Some may prefer to use iptables-save and iptables-restore to save
  rule sets. The deprecated iptables init.d script in included in
  the example section as a reference for a state based init script.
  You can get the same basic functionality by using saving your
  rules with iptables-save and using ifupdown to apply them.

    # sample /etc/network/interfaces lines
    pre-up iptables-restore < /etc/iptables.up.rules
    post-down iptables-restore < /etc/iptables.down.rules

А причем тут маршрутизация? :)