112 113 or 114 is in trouble step s 3 the troubled forwarding engine is suspended step s 4 sequentially for a subordinate forwarding engine to be substituted for the troubled forwarding engine according to the main subordinate relations among the forwarding engines the routing controller 101 controls switching to the corresponding in out interface 121 122 123 or 124 and change the data transmission path to a subordinate forwarding engine of the troubled forwarding engine 111 112 113 or 114 step s 5 each of fig6 and fig7 is a block diagram illustrating data flows when the first forwarding engine is in trouble and is not operating normally referring to fig6 the routing controller 101 controls the first in out interface 121 to be switched to the first direction while the first in out interface 121 is connected with the troubled first forwarding engine 111 in this case the first direction is the switching direction when the first forwarding engine is in trouble additionally the fourth in out interface 114 connected to the fourth forwarding engine 114 as the subordinate forwarding engine performs switching to the third direction the switching direction of subordinate functions therefore the data from the first in out interface 121 do not pass the first forwarding engine 111 but pass through the fourth forwarding engine 114 to be transmitted to the third forwarding engine 113 like the case of normal operations on the contrary as shown in fig7 the data from the second in out interface 122 are transmitted to the second forwarding engine 112.