The original question was CAN you configure ip directed-broadcast on serial interfaces. The technically correct answer is YES you CAN configure this. But I see very little reason to do so. The directed-broadcast command controls whether the router will take an incoming directed broadcast (packet with destination address of the broadcast address of the local segment) and forward it with a layer 2 broadcast address. On serial interfaces there is not much point in configuring this where there is only one neighbor. (Perhaps a Frame Relay multipoint interface might be an exception)
When I was first learning about Cisco I thought that the directed-broadcast command would enable forwarding a directed broadcast toward some remote destination (and in that context configuring it on serial interfaces makes sense). But I soon learned that this was a mistaken understanding. A directed broadcast toward a remote destination will always be forwarded. The directed-broadcast command only affects broadcasts on the local subnet.
HTH
Rick
HTH
Rick
