With a network card that is on dhcp and used by default route (without any
manual route configuration), set it on manual IP temporarily (set IP, netmask,
gateway).
service network restart

Set the card back to dhcp

service network restart

The default route won't be set anymore, as the gateway specified previously
seems to influence that. Fix: set card to manual, remove gateway, set back to dhcp.

I don't think this is a system-config-network problem, but I don't know what
package handles networking.