1. Field of the Invention
The present invention relates to a network system constituted by a plurality of low-order networks and a high-order network in which the plurality of low-order networks is accommodated.
2. Prior Art
In recent years, popularization of the Internet advances, and users of the Internet sharply increase in number. In this state, the performance of backbone networks of the Internet is required to be improved. As one backbone networks which can realize high performance, a label switching system using an MPLS (Multi Protocol Label Switching) technique is known.
FIG. 17 is a diagram showing a network system using a label switching system. As shown in FIG. 17, the network system is constituted by a core network for realizing the label switching system and a plurality of local area networks (LANs) connected to the core network through routers.
The core network is constituted by a plurality of label switches connected to each other through a communication circuit and a single edge node or a plurality of edge nodes connected to the label switches through the communication circuit. Each of the edge nodes is connected to the LAN through routers.
Each of the label switches acquires routing information (communication route information) in the core network according to known routing protocol such as OSPF (Open Shortest Path First) or BGP4 (Border Gateway Protocol Version 4). Each of the label switches generates information of a path identifier (called a “label”) corresponding to the acquired routing information, and transmits the routing information and the label information to an edge node whose order is lower than the corresponding label switch itself according to label distribution protocol (LDP).
Each of the edge nodes receives the routing information and the label information from the high-order label switch according to the LDP. At this time, each of the edge nodes generates and holds a look-up table in which the routing information is related to the label information.
Thereafter, when each of the edge nodes receives data from a router, the edge node refers to the look-up table, reads a label corresponding to a communication route (transfer route) corresponding to the destination of the data from the look-up table to add the label to the data, and transmits the resultant data to a high-order label switch. When each of the label switches receives data from an edge node or another label switch, the label switch determines an output-path of the data by only referring to a label added to the data, and transmits the data from the determined output-path. When each of the edge nodes receives the data from the label switch, the edge node removes the label added to the data, and transfers the data to a router which should receive the data.
In this manner, in the network system using the label switching system, the label switch of the core network determines the output-path of the data with reference to only the label. For this reason, the core network can repeat data at a high speed.
In a network system using a conventional label switching system, the following problem is posed. If a failure is caused in any label switch in the core network shown in FIG. 17, other label switches acquire routing information representing that the label switch having the failure is bypathed by using OSPF or BGP4, generates label information corresponding to the routing, and gives the new routing information and the label information to an edge node whose order is lower than the label switch. The low-order edge node performs data transmission on the basis of the new routing information and the label information received from the label switch. In this manner, service down of traffic passing through the label switch in which the failure is caused can be prevented.
However, with the process described above, the label switch in which the failure is caused is disconnected from the core network. For this reason, the edge node whose order is lower than that of the label switch cannot communicate with other edge nodes.